oxdock_parser/

commands.rs

//! Single-site command registry for all OxDock commands.
//!
//! `declare_commands!` is the sole source of truth. It generates:
//! - StepKind enum : all command + structural AST variants
//! - `pub fn lower_command(name, raw_args)` : name-dispatched lowering
//! - `pub fn all_metadata()` : collects `CommandMeta` from all declarations
//!   plus `all_structural_metadata()` (structural statements are documented
//!   through the same pipeline so reference docs cannot drift).
//!
//! To add a command: add one block inside `declare_commands!`.
//! To add a structural statement: extend the `structural [...]` list,
//! `all_structural_metadata()`, and the `structural_metadata_covers_all_structural_kinds`
//! tripwire below.

use std::fmt;

use crate::ast::{
    Arg, ArgPart, Expr, IoBinding, IoStream, PipeTarget, Step, Value, WorkspaceTarget,
};
use crate::command::{
    ArgSpec, ArgType, CommandMeta, Example, FlagSpec, FlagValueType, IoDirection, Stream,
    split_assignment,
};
use crate::constants::{KEYWORD_EXPORT, KEYWORD_IMPORT};
use crate::error::{ParseError, ParseResult, SpanContext};
use indoc::indoc;

// ── Helpers ────────────────────────────────────────────────────────────────

// Value-parsing helpers (`strip_surrounding_quotes`,
// `split_assignment`, `parse_duration`, `format_duration`) live in
// `crate::command` beside the `ArgType` validators that call them.

/// Reserved words that can never name a `REMOTE` target: the workspace
/// roots plus the block keyword itself. Shared with CLI inventory
/// validation so charset rules cannot drift between parser and host.
pub const REMOTE_RESERVED_WORDS: &[&str] = &["SNAPSHOT", "LOCAL", "CACHE", "SYSTEM", "REMOTE"];

/// Validate a `REMOTE` target name: start alnum, then alnum, `_`, or
/// `-`, max 64 chars. Safe in CLI flags, log lines, tarball names, and
/// muxio stream labels without quoting.
pub fn is_valid_remote_target(target: &str) -> bool {
    if target.is_empty() || target.len() > 64 {
        return false;
    }
    if REMOTE_RESERVED_WORDS.contains(&target) {
        return false;
    }
    let mut chars = target.chars();
    match chars.next() {
        Some(first) if first.is_ascii_alphanumeric() => {}
        _ => return false,
    }
    chars.all(|c| c.is_ascii_alphanumeric() || c == '_' || c == '-')
}

/// Join free-text tail arguments into one value. Single args pass through
/// untouched (preserving `Arg::Expr`); all-`String` tails join exactly like the
/// historical `join_args`; tails containing expressions become `Arg::Parts`
/// with single-space separators so `$x` is never silently dropped.
fn join_value(args: Vec<Arg>, cmd_name: &str) -> ParseResult<Arg> {
    if args.is_empty() {
        return Err(ParseError::validation(
            cmd_name,
            format!("{cmd_name} requires at least one argument"),
            &SpanContext::line_only(0),
        ));
    }
    if args.len() == 1 {
        return Ok(args.into_iter().next().unwrap());
    }
    if args.iter().all(|a| matches!(a, Arg::String(..))) {
        return Ok(Arg::String(
            args.iter()
                .map(|a| a.as_str())
                .collect::<Vec<_>>()
                .join(" "),
            false,
        ));
    }
    let mut parts = Vec::new();
    for (index, arg) in args.into_iter().enumerate() {
        if index > 0 {
            parts.push(ArgPart::Text(" ".to_string(), false));
        }
        match arg {
            Arg::String(text, quoted) => parts.push(ArgPart::Text(text, quoted)),
            Arg::Expr(expr) => parts.push(ArgPart::Expr(expr)),
            Arg::Parts(inner) => parts.extend(inner),
        }
    }
    Ok(Arg::Parts(parts))
}

/// Canonical `lower_command` entry for direct callers holding one pre-joined
/// `KEY=value` token. Script parsing never reaches this : the grammar splits
/// assignments on raw spans first (see `lower_env_command` in parser.rs).
pub fn lower_env_assignment(args: Vec<Arg>) -> ParseResult<StepKind> {
    let arg = args.into_iter().next().ok_or_else(|| {
        ParseError::validation(
            "ENV",
            "ENV requires KEY=value".to_string(),
            &SpanContext::line_only(0),
        )
    })?;
    let Some((key, value)) = split_assignment(arg.as_str())
        .map_err(|e| ParseError::validation("ENV", e.to_string(), &SpanContext::line_only(0)))?
    else {
        return Err(ParseError::validation(
            "ENV",
            "ENV requires KEY=value format".to_string(),
            &SpanContext::line_only(0),
        ));
    };
    Ok(StepKind::Env { key, value })
}

/// Collapse a grammar-classified assignment for commands that take no
/// assignments (`RUN`, `COPY`, ...): canonical `key=<rendered value>` text.
/// Runtime semantics survive intact : `{{ }}` templates stay textual for
/// `expand_string`, and `RUN`'s own post-pass expands bare `$var`.
pub(crate) fn canonical_assignment_arg(key: &str, value: &Arg) -> Arg {
    Arg::String(format!("{key}={}", value.render()), false)
}

/// Render an [`AssertTarget`] for `Display`: stream markers print bare
/// (`stdout` reparses to the marker); values print like other args.
fn fmt_assert_target(target: &AssertTarget) -> String {
    match target {
        AssertTarget::Value(arg) => fmt_value(arg, quote_msg),
        _ => target.render(),
    }
}

/// Render one `Arg` for `Display`: expressions print raw (`$x` must never be
/// quoted or reparsing would literalize them); mixed values print raw unless
/// they hold instruction-boundary characters (`;`, `}`, linebreaks), which
/// force quoting for reparseability.
fn fmt_value(arg: &Arg, quote: fn(&str) -> String) -> String {
    match arg {
        Arg::Expr(_) => arg.render(),
        Arg::String(text, _) => quote(text),
        Arg::Parts(_) => {
            let rendered = arg.render();
            if rendered.contains(';')
                || rendered.contains('}')
                || rendered.contains('\n')
                || rendered.contains('\r')
            {
                quote(&rendered)
            } else {
                rendered
            }
        }
    }
}

fn quote_dsl_string(s: &str) -> String {
    // Single quoting core for Display output: backslash and quote
    // escapes only. The `quote_*`/`fmt_*` helpers below differ in *when*
    // a value needs quoting, never in *how*. No brace or control
    // handling: the parser passes literal contents verbatim, so Display
    // round-trips without them (unlike `escape_dsl_string` in `literal`,
    // which inverts runtime `expand_string` and must also
    // escape `{{` triples and controls).
    format!("\"{}\"", s.replace('\\', "\\\\").replace('"', "\\\""))
}

fn quote_arg(s: &str) -> String {
    let is_safe = s.chars().all(|c| c.is_ascii_alphanumeric() || c == '_')
        && !s.starts_with(|c: char| c.is_ascii_digit() || c == '-' || c == '/' || c == '.')
        && !crate::Command::is_statement_keyword(s);
    if is_safe && !s.is_empty() {
        s.to_string()
    } else {
        quote_dsl_string(s)
    }
}

fn quote_msg(s: &str) -> String {
    let safe = s.chars().all(|c| c.is_ascii_alphanumeric() || c == '_')
        && !s.starts_with(|c: char| c.is_ascii_digit())
        && !crate::Command::is_statement_keyword(s);
    if safe && !s.is_empty() {
        s.to_string()
    } else {
        quote_dsl_string(s)
    }
}

fn quote_run(s: &str) -> String {
    if s.is_empty() || s.chars().any(|c| c == ';' || c == '\n') || s.contains("//") {
        return quote_dsl_string(s);
    }
    s.split(' ')
        .map(|w| {
            if w.starts_with(|c: char| c.is_ascii_digit())
                || w.starts_with(['/', '.', '-', ':', '='])
            {
                quote_dsl_string(w)
            } else {
                w.to_string()
            }
        })
        .collect::<Vec<_>>()
        .join(" ")
}

/// Render one exec-form (`RUN [...]`) argv element for `Display`:
/// string literals print JSON-quoted; typed expressions (`$var`,
/// `F()`, ints, bools, nested lists) print raw via `render` so
/// reparsing yields the same typed element; mixed values print raw
/// unless they hold instruction-boundary characters.
fn fmt_exec_arg(arg: &Arg) -> String {
    match arg {
        Arg::String(text, _) => quote_dsl_string(text),
        Arg::Expr(_) => arg.render(),
        Arg::Parts(_) => {
            let rendered = arg.render();
            if rendered.contains(';')
                || rendered.contains('}')
                || rendered.contains('\n')
                || rendered.contains('\r')
            {
                quote_dsl_string(&rendered)
            } else {
                rendered
            }
        }
    }
}

/// Render an [`Arg`] for `Display`: the quoted flag drives quoting (not
/// content sniffing : digit-leading values like `10s` or `0` must stay
/// bare to reparse with the same flag).
fn fmt_raw_arg(arg: &Arg) -> String {
    match arg {
        Arg::String(s, true) => quote_dsl_string(s),
        _ => arg.render(),
    }
}

fn fmt_io(b: &IoBinding) -> String {
    let s = match b.stream {
        IoStream::Stdin => "stdin",
        IoStream::Stdout => "stdout",
        IoStream::Stderr => "stderr",
    };
    match &b.pipe {
        Some(PipeTarget::Var(v)) => format!("{}=${}", s, v),
        None => s.to_string(),
    }
}

// ── declare_commands! ──────────────────────────────────────────────────────

// Keywords parsed by PEG rules rather than plain-command lowering (`WITH_IO`,
// `AWAIT`, ...). When a line starts with one of these but fails to parse as
// such, lowering falls through here : report a committed syntax error instead
// of an unknown command.
pub(crate) fn is_known_command(name: &str) -> bool {
    if name == "ELSE" {
        return true;
    }
    all_metadata().iter().any(|meta| meta.name == name)
}

pub(crate) fn invalid_syntax_error(name: &str, raw_args: &[Arg]) -> ParseError {
    let received = raw_args
        .iter()
        .map(Arg::render)
        .collect::<Vec<_>>()
        .join(" ");
    let got = if received.is_empty() {
        "nothing".to_string()
    } else {
        format!("`{received}`")
    };
    let found = if received.is_empty() {
        None
    } else {
        Some(received.clone())
    };
    let expected = all_metadata()
        .iter()
        .find(|meta| meta.name == name)
        .map(|meta| vec![meta.syntax.to_string()])
        .unwrap_or_default();
    let ctx = SpanContext::line_only(0);
    match structural_hint(name, &received) {
        Some(hint) => ParseError::invalid_syntax(
            name,
            format!("invalid syntax for command {name}: {hint}"),
            found,
            expected,
            Some(hint),
            &ctx,
        ),
        None => ParseError::invalid_syntax(
            name,
            format!("invalid syntax for command {name}: got {got}."),
            found,
            expected,
            None,
            &ctx,
        ),
    }
}

fn unknown_command_error(name: &str, raw_args: &[Arg]) -> ParseError {
    let received = raw_args
        .iter()
        .map(Arg::render)
        .collect::<Vec<_>>()
        .join(" ");
    let hint = structural_hint(name, &received).or_else(|| case_hint(name));
    let ctx = SpanContext::line_only(0);
    match hint {
        Some(hint) => ParseError::unknown_command(
            name,
            format!("unknown command: {name}\n{hint}"),
            Some(hint),
            &ctx,
        ),
        None => ParseError::unknown_command(name, format!("unknown command: {name}"), None, &ctx),
    }
}

/// Single decision function for the lowering fallback: keyword led lines
/// (structural statements, `ELSE`, every registered command) are committed
/// syntax errors, never unknown commands. Only truly unknown names fall
/// through to `unknown_command_error`. Callers enrich the result with the
/// token span via `ParseError::with_span`.
pub(crate) fn classify(name: &str, raw_args: &[Arg]) -> ParseError {
    if is_known_command(name) {
        invalid_syntax_error(name, raw_args)
    } else {
        unknown_command_error(name, raw_args)
    }
}

fn structural_hint(name: &str, received: &str) -> Option<String> {
    let got = if received.is_empty() {
        "nothing".to_string()
    } else {
        format!("`{received}`")
    };
    match name {
        "WITH_IO" => Some(with_io_hint(&got, received)),
        "AWAIT" => Some(format!(
            "AWAIT waits for a background task variable, e.g. `LET $t: HANDLE = ASYNC ECHO hi` then `AWAIT $t`; got {got}."
        )),
        "CANCEL" => Some(format!(
            "CANCEL stops a background task variable, e.g. `CANCEL $t` (from `LET $t: HANDLE = ASYNC ...`); got {got}."
        )),
        "ASYNC" => Some(format!(
            "ASYNC runs a command in the background, e.g. `ASYNC RUN ...`, `ASYNC {{ ... }}`, or `LET $t: HANDLE = ASYNC ...`; got {got}."
        )),
        "FOR" => Some(format!(
            "FOR loops need `FOR $item: TYPE IN <expr> {{ ... }}` (or `FOR $key: STRING, $value: TYPE IN <expr> {{ ... }}`); got {got}."
        )),
        "IF" => Some(format!(
            "IF needs a condition and a block, e.g. `IF true {{ ECHO yes }}`; got {got}."
        )),
        "ELSE" => Some(format!(
            "ELSE must directly follow an `IF ... {{ ... }}` block, e.g. `IF true {{ ECHO yes }} ELSE {{ ECHO no }}`; got {got}."
        )),
        "LET" => Some(format!(
            "LET assigns a variable, e.g. `LET $name: STRING = <expr>`, `LET $t: HANDLE = ASYNC ...`, `LET $out: STRING = <command>` (capture), `LET $out: STRING = AWAIT $t`, or `LET $var: TYPE = {{ ... }}` (inline block); got {got}."
        )),
        "SET" => Some(
            "`SET` is not a keyword; mutate a declared variable with `$var = <expr>`, e.g. `$count = 2`.".to_string(),
        ),
        "TIMEOUT" => Some(format!(
            "TIMEOUT needs a duration and a command or block, e.g. `TIMEOUT 30s RUN ...`; got {got}."
        )),
        "REMOTE" => Some(format!(
            "REMOTE needs a target and a braced block, e.g. `REMOTE prod [$version, env:DEPLOY_ENV] {{ ... }}`; got {got}."
        )),
        "FUNC" => Some(format!(
            "FUNC defines a function, e.g. `FUNC GREET($name: STRING) {{ RETURN $name }}`; got {got}."
        )),
        "RETURN" => Some(format!(
            "RETURN ends the nearest function, ASYNC task, or inline LET block with a value, e.g. `RETURN $x`; got {got}."
        )),
        "WHILE" => Some(format!(
            "WHILE needs a Bool condition and a block, e.g. `WHILE !$done {{ ... }}`; got {got}."
        )),
        "BREAK" => Some(
            "`BREAK` exits the innermost enclosing FOR/WHILE loop; it must appear inside a loop.".to_string(),
        ),
        "CONTINUE" => Some(
            "`CONTINUE` skips to the next iteration of the innermost enclosing FOR/WHILE loop; it must appear inside a loop.".to_string(),
        ),
        "INHERIT_ENV" => Some(format!(
            "INHERIT_ENV takes a key list, e.g. `INHERIT_ENV [HOME, PATH]`; got {got}."
        )),
        name if name == KEYWORD_IMPORT => Some(format!(
            "IMPORT brings module functions into bare-call scope, e.g. `IMPORT [STD]` or `IMPORT [STD, MOCK]`; got {got}."
        )),
        name if name == KEYWORD_EXPORT => Some(
            "`EXPORT` is reserved for future script-module support and cannot be used yet."
                .to_string(),
        ),
        _ => None,
    }
}

/// Diagnose a `WITH_IO` line that failed to parse: most often a malformed
/// binding list (bindings are bare streams or `<stream>=$var`).
fn with_io_hint(got: &str, received: &str) -> String {
    const SYNTAX: &str =
        "WITH_IO needs `WITH_IO [bindings] <command>` or `WITH_IO [bindings] { <commands> }`";
    const BINDINGS: &str = "bindings are `stdin`, `stdout`, `stderr`, or `<stream>=$var` with a PIPE-typed variable (e.g. `[stdout=$p]`, `[stdin=$p]`)";
    if let Some(after_open) = received.strip_prefix('[') {
        match after_open.split_once(']') {
            None => {
                return format!("{SYNTAX}: missing closing `]` in the binding list; got {got}.");
            }
            Some((bindings, _)) => {
                for part in bindings.split(',') {
                    let part = part.trim();
                    if part.is_empty() {
                        continue;
                    }
                    let (stream, binding) = match part.split_once('=') {
                        Some((stream, binding)) => (stream.trim(), Some(binding.trim())),
                        None => (part, None),
                    };
                    if !matches!(stream, "stdin" | "stdout" | "stderr") {
                        return format!(
                            "{SYNTAX}: invalid stream `{stream}`; expected `stdin`, `stdout`, or `stderr`; got {got}."
                        );
                    }
                    let valid = match binding {
                        None => true,
                        Some(value) => value.strip_prefix('$').is_some_and(|var| {
                            !var.trim().is_empty()
                                && var.chars().all(|c| c.is_ascii_alphanumeric() || c == '_')
                        }),
                    };
                    if !valid {
                        return format!(
                            "{SYNTAX}: invalid binding `{part}`; {BINDINGS}; got {got}."
                        );
                    }
                }
            }
        }
    }
    format!("{SYNTAX}; got {got}. {BINDINGS}.")
}

/// `echo hi` is almost certainly `ECHO hi`: commands are uppercase.
fn case_hint(name: &str) -> Option<String> {
    let upper = name.to_ascii_uppercase();
    if upper != name
        && all_metadata()
            .iter()
            .any(|meta| meta.name == upper.as_str())
    {
        return Some(format!("did you mean `{upper}`? commands are uppercase."));
    }
    None
}

macro_rules! declare_commands {
    (
        structural [
            $( $sname:ident $( { $( $sfname:ident : $sftype:ty ),* $(,)? } )? ),* $(,)?
        ]

        $(
            $cmd_ident:ident => [
                name: $name:expr,
                variant: $vname:ident $( { $( $vfname:ident : $vftype:ty ),* $(,)? } )? $( ( $( $ttuple:ty ),* $(,)? ) )?,
                syntax: $syntax:expr,
                summary: $summary:expr,
                description: $desc:expr,
                args: $args:expr,
                flags: $flags:expr,
                default_output: $out:expr,
                examples: $examples:expr,
                lower: $lower:expr,
            ]
        ),* $(,)?
    ) => {
        #[derive(Debug, Clone, PartialEq)]
        pub enum StepKind {
            $( $vname $( { $( $vfname : $vftype ),* } )? $( ( $( $ttuple ),* ) )?, )*
            $( $sname $( { $( $sfname : $sftype ),* } )?, )*
        }

        pub fn lower_command(name: &str, raw_args: Vec<Arg>) -> ParseResult<StepKind> {
            match name {
                $(
                    s if s == $name => {
                        let meta = CommandMeta {
                            name: $name, syntax: $syntax, summary: $summary,
                            description: $desc, args: $args, flags: $flags,
                            default_output: $out, examples: $examples,
                        };
                        let (flags, positional) = crate::strip_flags(raw_args, &meta)?;
                        crate::command::validate_positionals_against_meta(
                            s,
                            &meta.args,
                            &positional,
                        )?;
                        let lower_fn: fn(Vec<(String, Arg)>, Vec<Arg>) -> ParseResult<StepKind> = $lower;
                        lower_fn(flags, positional)
                    }
                )*
                _ => {
                    Err(classify(name, &raw_args))
                }
            }
        }

        pub fn all_metadata() -> Vec<CommandMeta> {
            let mut out = vec![
                $( CommandMeta {
                    name: $name, syntax: $syntax, summary: $summary,
                    description: $desc, args: $args, flags: $flags,
                    default_output: $out, examples: $examples,
                }, )*
            ];
            // Structural statements are registered separately (see
            // all_structural_metadata) but documented through the same
            // pipeline so docs-gen never drifts from the parser.
            out.extend(all_structural_metadata());
            out
        }
    };
}

impl StepKind {
    /// Visit every immediate child expression (arguments, conditions,
    /// durations, initializers). Boxed sub-kinds and bodies are NOT
    /// descended: pair with [`walk_child_kinds`](Self::walk_child_kinds)
    /// or use [`walk_step_exprs`](Self::walk_step_exprs) for the full
    /// tree. Exhaustive: a new variant fails compilation here, so AST
    /// passes built on this visitor can never silently miss a command.
    pub fn walk_exprs(&self, f: &mut impl FnMut(&Expr)) {
        fn walk_arg(arg: &Arg, f: &mut impl FnMut(&Expr)) {
            match arg {
                Arg::Expr(expr) => f(expr),
                Arg::Parts(parts) => {
                    for part in parts {
                        if let ArgPart::Expr(expr) = part {
                            f(expr);
                        }
                    }
                }
                Arg::String(..) => {}
            }
        }
        fn walk_opt(arg: &Option<Arg>, f: &mut impl FnMut(&Expr)) {
            if let Some(arg) = arg {
                walk_arg(arg, f);
            }
        }
        fn walk_target(target: &AssertTarget, f: &mut impl FnMut(&Expr)) {
            if let AssertTarget::Value(arg) = target {
                walk_arg(arg, f);
            }
        }
        match self {
            StepKind::Assign { expr, .. } | StepKind::Set { expr, .. } => f(expr),
            StepKind::Return { expr } => f(expr),
            StepKind::Call { args, .. } => {
                for arg in args {
                    f(arg);
                }
            }
            StepKind::For { in_expr, .. } => f(in_expr),
            StepKind::If { cond, else_ifs, .. } => {
                f(cond);
                for (cond, _) in else_ifs {
                    f(cond);
                }
            }
            StepKind::While { cond, .. } => f(cond),
            StepKind::Timeout { duration, .. } | StepKind::Sleep { duration } => {
                walk_arg(duration, f);
            }
            StepKind::Workdir(arg)
            | StepKind::Echo(arg)
            | StepKind::Run(arg)
            | StepKind::Mkdir(arg)
            | StepKind::HashSha256 { path: arg }
            | StepKind::Exit(arg) => walk_arg(arg, f),
            StepKind::Env { value, .. } => walk_arg(value, f),
            StepKind::Copy { from, to, .. } | StepKind::Symlink { from, to, .. } => {
                walk_arg(from, f);
                walk_arg(to, f);
            }
            StepKind::CopyGit { rev, from, to, .. } => {
                walk_arg(rev, f);
                walk_arg(from, f);
                walk_arg(to, f);
            }
            StepKind::Ls(arg) | StepKind::Read(arg) => walk_opt(arg, f),
            StepKind::Write { path, contents } | StepKind::Append { path, contents } => {
                walk_arg(path, f);
                walk_opt(contents, f);
            }
            StepKind::Expand { path, overrides } => {
                walk_opt(path, f);
                for (_, value) in overrides {
                    walk_arg(value, f);
                }
            }
            StepKind::AssertEq {
                actual, expected, ..
            } => {
                walk_target(actual, f);
                walk_opt(expected, f);
            }
            StepKind::AssertContains { haystack, needle } => {
                walk_target(haystack, f);
                walk_arg(needle, f);
            }
            StepKind::ListAppend { item, .. } => walk_arg(item, f),
            StepKind::RunExec { argv } => {
                for arg in argv {
                    walk_arg(arg, f);
                }
            }
            StepKind::WithIo { .. }
            | StepKind::WithIoBlock { .. }
            | StepKind::TypeAlias { .. }
            | StepKind::Workspace(_)
            | StepKind::InheritEnv { .. }
            | StepKind::Cwd
            | StepKind::ReadLine { .. }
            | StepKind::AssignCapture { .. }
            | StepKind::AwaitCapture { .. }
            | StepKind::AsyncBlock { .. }
            | StepKind::AssignAsync { .. }
            | StepKind::Await { .. }
            | StepKind::Cancel { .. }
            | StepKind::RemoteBlock { .. }
            | StepKind::FuncDef { .. }
            | StepKind::Break
            | StepKind::Continue => {}
        }
    }

    /// Visit every immediate child kind: boxed sub-commands and the kinds
    /// of every body step. Exhaustive like [`walk_exprs`](Self::walk_exprs):
    /// new block shapes fail compilation here.
    pub fn walk_child_kinds(&self, f: &mut impl FnMut(&StepKind)) {
        fn walk_body(body: &[Step], f: &mut impl FnMut(&StepKind)) {
            for step in body {
                f(&step.kind);
            }
        }
        match self {
            StepKind::WithIo { cmd, .. } => f(cmd),
            StepKind::AssignCapture { cmd, .. } => f(cmd),
            StepKind::For { body, .. }
            | StepKind::While { body, .. }
            | StepKind::FuncDef { body, .. }
            | StepKind::Timeout { body, .. }
            | StepKind::AssignAsync { body, .. }
            | StepKind::AsyncBlock { body }
            | StepKind::RemoteBlock { body, .. } => walk_body(body, f),
            StepKind::If {
                then_body,
                else_ifs,
                else_body,
                ..
            } => {
                walk_body(then_body, f);
                for (_, body) in else_ifs {
                    walk_body(body, f);
                }
                if let Some(body) = else_body {
                    walk_body(body, f);
                }
            }
            StepKind::InheritEnv { .. }
            | StepKind::TypeAlias { .. }
            | StepKind::Workdir(_)
            | StepKind::Workspace(_)
            | StepKind::Env { .. }
            | StepKind::Echo(_)
            | StepKind::Run(_)
            | StepKind::RunExec { .. }
            | StepKind::Copy { .. }
            | StepKind::CopyGit { .. }
            | StepKind::Symlink { .. }
            | StepKind::Mkdir(_)
            | StepKind::Ls(_)
            | StepKind::Cwd
            | StepKind::Read(_)
            | StepKind::ReadLine { .. }
            | StepKind::Write { .. }
            | StepKind::Append { .. }
            | StepKind::Expand { .. }
            | StepKind::AssertEq { .. }
            | StepKind::AssertContains { .. }
            | StepKind::HashSha256 { .. }
            | StepKind::Exit(_)
            | StepKind::WithIoBlock { .. }
            | StepKind::Assign { .. }
            | StepKind::Set { .. }
            | StepKind::AwaitCapture { .. }
            | StepKind::Await { .. }
            | StepKind::Cancel { .. }
            | StepKind::Sleep { .. }
            | StepKind::ListAppend { .. }
            | StepKind::Call { .. }
            | StepKind::Return { .. }
            | StepKind::Break
            | StepKind::Continue => {}
        }
    }

    /// Visit every descendant expression in the full subtree: immediate
    /// expressions (deep via [`Expr::walk`](crate::ast::Expr::walk)) plus
    /// every nested body and boxed sub-command. The single traversal every
    /// AST inspection pass should use.
    pub fn walk_step_exprs(&self, f: &mut impl FnMut(&Expr)) {
        self.walk_exprs(&mut |expr| expr.walk(f));
        self.walk_child_kinds(&mut |kind| kind.walk_step_exprs(f));
    }
}

/// First-argument target for `ASSERT_EQ` / `ASSERT_CONTAINS`.
///
/// Values (`Arg`) evaluate in memory and never touch disk. The `Stdout`
/// and `Stderr` markers observe stream buffers. Pipes are asserted through
/// plain variables: a `$var` holding a `PIPE` peeks its backend bytes at
/// runtime, so no pipe marker variant exists. Bare `stdout` / `stderr`
/// spellings lower to markers; quoted spellings stay literal string
/// values, so quoting remains interchangeable everywhere.
#[derive(Debug, Clone, PartialEq)]
pub enum AssertTarget {
    Value(Arg),
    Stdout,
    Stderr,
}

impl AssertTarget {
    pub fn render(&self) -> String {
        match self {
            AssertTarget::Value(arg) => arg.render(),
            AssertTarget::Stdout => "stdout".to_string(),
            AssertTarget::Stderr => "stderr".to_string(),
        }
    }
}

/// Lower the first positional of `ASSERT_EQ` / `ASSERT_CONTAINS`.
///
/// `Arg::Expr` (variables, key-paths, calls) is always a value : a `$var`
/// holding a `PIPE` peeks its backend bytes at runtime. Bare (unquoted)
/// `stdout` / `stderr` spellings become stream markers; every other
/// spelling, quoted or not, stays a literal value. In particular a `$var`
/// holding a path never reads disk, and quoted `"stdout"` names the
/// seven-character string, not the stream.
fn lower_assert_target(arg: Arg) -> ParseResult<AssertTarget> {
    match arg {
        Arg::Expr(_) => Ok(AssertTarget::Value(arg)),
        Arg::String(text, quoted) if !quoted => match text.as_str() {
            "stdout" => Ok(AssertTarget::Stdout),
            "stderr" => Ok(AssertTarget::Stderr),
            _ => Ok(AssertTarget::Value(lower_assert_operand(Arg::String(
                text, false,
            )))),
        },
        other => Ok(AssertTarget::Value(lower_assert_operand(other))),
    }
}

/// Give bare (unquoted, template-free) assertion operands the same typing
/// they carry in expression positions, so `ASSERT_EQ $status 200` compares
/// `Int(200)` rather than the string `"200"`. Signed integers (`-5` in
/// first position), decimals (`3.5`), and `true`/`false` all convert;
/// everything else, including quoted strings, stays a string. Note a
/// grammar property, not a limitation of this helper: `$x -5` in argument
/// position parses as subtraction (`expr_add_sub`), so negative expected
/// values must be bound first (`LET $e: INT = 0 - 5`).
fn lower_assert_operand(arg: Arg) -> Arg {
    match arg {
        Arg::String(text, false) => {
            if let Ok(i) = text.parse::<i64>() {
                Arg::Expr(Expr::Literal(Value::int(i)))
            } else if text.contains('.') && text.parse::<f64>().is_ok() {
                Arg::Expr(Expr::Literal(Value::float(
                    text.parse::<f64>().unwrap_or(f64::NAN),
                )))
            } else if text == "true" {
                Arg::Expr(Expr::Literal(Value::bool(true)))
            } else if text == "false" {
                Arg::Expr(Expr::Literal(Value::bool(false)))
            } else {
                Arg::String(text, false)
            }
        }
        other => other,
    }
}

declare_commands! {
    structural [
        WithIo { bindings: Vec<IoBinding>, cmd: Box<StepKind> },
        WithIoBlock { bindings: Vec<IoBinding> },
        TypeAlias { name: String, target: String },
        For { key_var: Option<String>, key_type: Option<String>, var: String, var_type: String, in_expr: Expr, body: Vec<Step> },
        If { cond: Box<Expr>, then_body: Vec<Step>, else_ifs: Vec<(Box<Expr>, Vec<Step>)>, else_body: Option<Vec<Step>> },
        Assign { var: String, decl_type: String, expr: Expr },
        Set { var: String, expr: Expr },
        AssignCapture { var: String, decl_type: String, cmd: Box<StepKind> },
        AwaitCapture { out_var: String, out_type: String, task_var: String },
        AsyncBlock { body: Vec<Step> },
        AssignAsync { var: String, decl_type: String, body: Vec<Step> },
        Await { var: String },
        Cancel { var: String },
        Timeout { duration: Arg, body: Vec<Step> },
        RemoteBlock { target: String, vars: Vec<String>, env: Vec<String>, body: Vec<Step> },
        RunExec { argv: Vec<Arg> },
        FuncDef { name: String, params: Vec<(String, String)>, body: Vec<Step> },
        Call { name: String, args: Vec<Expr> },
        Return { expr: Box<Expr> },
        While { cond: Box<Expr>, body: Vec<Step> },
        Break,
        Continue,
    ]

    Workdir => [
        name: "WORKDIR",
        variant: Workdir(Arg),
        syntax: "WORKDIR <path>",
        summary: "Change the working directory.",
        description: indoc! {r#"
            Sets the current working directory.

            Relative paths resolve against the current directory; `/` resets to
            the workspace root. Paths cannot escape the workspace.
        "#},
        args: &[ ArgSpec { name: "path", arg_type: ArgType::Path, description: "Directory to change to", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "change working directory", fence_meta: None, code: indoc! {r#"
            # Later relative paths resolve under the new directory.
            WORKDIR project/src
            WRITE generated.txt generated-under-workdir

            LET $body: STRING = READ generated.txt
            ASSERT_EQ $body "generated-under-workdir"
        "#} },             Example { name: "workdir in a scoped block", fence_meta: None, code: indoc! {r#"
            # The block reverts to the starting directory on exit.
            LET $outside: STRING = CWD
            MKDIR project

            [bool:true] {
                WORKDIR project
                WRITE inner.txt inner
            }

            LET $back: STRING = CWD
            ASSERT_EQ $back $outside
            LET $body: STRING = READ project/inner.txt
            ASSERT_EQ $body "inner"
        "#} } ],
        lower: |_flags, args| {
            let path = args.into_iter().next().ok_or_else(|| ParseError::validation("WORKDIR", "WORKDIR requires a path".to_string(), &SpanContext::line_only(0)))?;
            Ok(StepKind::Workdir(path))
        },
    ],

    Workspace => [
        name: "WORKSPACE",
        variant: Workspace(WorkspaceTarget),
        syntax: "WORKSPACE (SNAPSHOT|LOCAL|CACHE|SYSTEM) [--local]",
        summary: "Switch workspace roots.",
        description: indoc! {r#"
            Switches the workspace root. The selection reverts at scope
            exit like `WORKDIR`.

            - `SNAPSHOT`: the materialized build snapshot (the default).
            - `LOCAL`: the local workspace directory.
            - `CACHE`: a persistent per-project directory shared across
              runs, never evicted. It lives under the OS user cache
              (`OXDOCK_CACHE_DIR` pins an exact directory);
              `WORKSPACE CACHE --local` keeps it in
              `<project>/.cache/workspace` instead.
            - `SYSTEM`: full filesystem access. Scripts using it are not
              hermetic.
        "#},
        args: &[ ArgSpec { name: "target", arg_type: ArgType::OneOf(&["SNAPSHOT", "LOCAL", "CACHE", "SYSTEM"]), description: "Target root", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[ FlagSpec { name: "local", long: "--local", value_type: FlagValueType::Flag, required: false, description: "Use the project-local cache directory instead of the OS user cache (CACHE only)" } ],
        default_output: None,
        examples: &[ Example { name: "switch roots", fence_meta: None, code: indoc! {r#"
            IMPORT [STD]
            WORKSPACE LOCAL

            LET $t: STRING = PATH_TYPE(".")
            ASSERT_EQ $t "dir"
        "#} }, Example { name: "workspace cache in a scoped block", fence_meta: None, code: indoc! {r#"
            [bool:true] {
                WORKSPACE CACHE
                WRITE cached.txt cached-content
            }

            COPY --from-workspace CACHE cached.txt restored.txt
            LET $body: STRING = READ restored.txt
            ASSERT_EQ $body "cached-content"
        "#} } ],
        lower: |flags, args| {
            let local = flags.iter().any(|(k, _)| k == "local");
            let target = args.into_iter().next().ok_or_else(|| ParseError::validation("WORKSPACE", "WORKSPACE requires a target".to_string(), &SpanContext::line_only(0)))?;
            match target.as_str() {
                "SNAPSHOT" | "LOCAL" | "SYSTEM" if local => Err(ParseError::validation("WORKSPACE", "WORKSPACE --local requires CACHE".to_string(), &SpanContext::line_only(0))),
                "SNAPSHOT" => Ok(StepKind::Workspace(WorkspaceTarget::Snapshot)),
                "LOCAL" => Ok(StepKind::Workspace(WorkspaceTarget::Local)),
                "CACHE" => Ok(StepKind::Workspace(WorkspaceTarget::Cache { local })),
                "SYSTEM" => Ok(StepKind::Workspace(WorkspaceTarget::System)),
                other => Err(ParseError::validation("WORKSPACE", format!("unknown workspace target: {other}"), &SpanContext::line_only(0))),
            }
        },
    ],

    Env => [
        name: "ENV",
        variant: Env { key: String, value: Arg },
        syntax: "ENV KEY=value",
        summary: "Set an environment variable.",
        description: indoc! {r#"
            Inserts or updates an env var.

            The value uses the unified string-value rules shared by every command:
            `"..."` or `'...'` quotes keep exact bytes (spaces, tabs), a lone `$var`
            evaluates that variable, `{{ ... }}` placeholders interpolate, unquoted
            words join with single spaces, and the first `=` splits key from value
            (`KEY=a=b` stores `a=b`).

            A `$var` inside larger text stays literal — write `{{ $var }}` to
            interpolate there.
        "#},
        args: &[ ArgSpec { name: "assignment", arg_type: ArgType::String, description: "KEY=value pair; the value resolves as STRING", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[
            Example { name: "set env", fence_meta: None, code: indoc! {r#"
                ENV APP_MODE=production
                LET $mode: STRING = env:APP_MODE
                ASSERT_EQ $mode "production"
            "#} },
            Example { name: "quoted value with spaces", fence_meta: None, code: indoc! {r#"
                # Quotes keep the space: SET_FORTH stores `outer scope`.
                ENV SET_FORTH="outer scope"
                WRITE out.txt "{{ env:SET_FORTH }}"

                LET $body: STRING = READ out.txt
                ASSERT_EQ $body "outer scope"
            "#} },
            Example { name: "variable value", fence_meta: None, code: indoc! {r#"
                # A lone $var evaluates, like ECHO $var.
                LET $who: STRING = "Alice"
                ENV GREETING=$who
                WRITE out.txt "{{ env:GREETING }}"

                LET $body: STRING = READ out.txt
                ASSERT_EQ $body "Alice"
            "#} },
            Example { name: "all value forms agree", fence_meta: None, code: indoc! {r#"
                # A bare variable, a quoted literal, and a template all
                # store plain strings through the same value rules.
                LET $x: STRING = "Ada"
                ENV A=$x
                ENV B="hello world"
                ENV C="{{ $x }} concatenated"
                WRITE check.txt "{{ env:A }}|{{ env:B }}|{{ env:C }}"

                LET $body: STRING = READ check.txt
                ASSERT_EQ $body "Ada|hello world|Ada concatenated"
            "#} },
            Example { name: "scoped env reverts", fence_meta: None, code: indoc! {r#"
                # ENV inside a braced block reverts when the block exits
                ENV MODE=production

                [bool:true] {
                    ENV MODE=staging
                    WRITE inner.txt "{{ env:MODE }}"
                }

                WRITE outer.txt "{{ env:MODE }}"

                LET $inner_body: STRING = READ inner.txt
                ASSERT_EQ $inner_body "staging"

                LET $outer_body: STRING = READ outer.txt
                ASSERT_EQ $outer_body "production"
            "#} },
            Example { name: "shell reads env per platform", fence_meta: None, code: indoc! {r#"
                # A shell command reads its own environment, with
                # per-platform spelling: quoted "$VAR" passes the parser
                # through untouched on unix ...
                ENV PROXY_PORT=23791

                [family:unix] LET $o: STRING = RUN echo serving on "$PROXY_PORT"

                # ... while cmd expands %VAR% on Windows.
                [family:windows] LET $o: STRING = RUN echo serving on %PROXY_PORT%

                ASSERT_CONTAINS $o "23791"
            "#} },
        ],
        lower: |_flags, args| lower_env_assignment(args),
    ],

    InheritEnv => [
        name: "INHERIT_ENV",
        variant: InheritEnv { keys: Vec<String> },
        syntax: "INHERIT_ENV [<key>, ...]",
        summary: "Inherit env vars from host.",
        description: indoc! {r#"
            Declares which host environment variables to inherit into the script.

            Must appear before any other commands and at most once. Without this
            directive, the script starts with an empty environment.
        "#},
        args: &[ ArgSpec { name: "keys", arg_type: ArgType::Rest(&ArgType::String), description: "Host variables to inherit", io: IoDirection::Read, index: 0, required: false, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "inherit env", fence_meta: None, code: indoc! {r#"
            INHERIT_ENV [PATH, HOME]
            LET $path: STRING = env:PATH
            ASSERT_CONTAINS $path ":"
        "#} } ],
        lower: |_flags, args| {
            let keys = args.into_iter().map(|a| a.as_str().to_string()).collect();
            Ok(StepKind::InheritEnv { keys })
        },
    ],

    Echo => [
        name: "ECHO",
        variant: Echo(Arg),
        syntax: "ECHO <message>",
        summary: "Print to stdout.",
        description: "Outputs message to stdout.",
        args: &[ ArgSpec { name: "message", arg_type: ArgType::Rest(&ArgType::String), description: "Text", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: Some(Stream::Stdout),
        examples: &[
            Example { name: "echo", fence_meta: None, code: indoc! {r#"
                ECHO build-complete
                ASSERT_CONTAINS stdout "build-complete"
            "#} },
            Example { name: "variables", fence_meta: None, code: indoc! {r#"
                # {{ }} interpolates inside text; a lone $var evaluates on its own.
                LET $x: STRING = "World"
                ECHO "braced:{{ $x }}"
                ECHO $x
                ASSERT_EQ stdout "braced:World\nWorld\n"
            "#} },
        ],
        lower: |_flags, args| Ok(StepKind::Echo(join_value(args, "ECHO")?)),
    ],

    Run => [
        name: "RUN",
        variant: Run(Arg),
        syntax: "RUN <command...> | RUN [\"exe\", \"arg\", ...]",
        summary: "Execute shell command or direct executable.",
        description: indoc! {r#"
            Shell form (`RUN <command...>`) runs the joined command string in the
            system shell (`$SHELL -c` / `COMSPEC /C`).

            Exec form (`RUN ["exe", "arg", ...]`) spawns the executable directly
            with no shell, so there is no shell expansion, globbing, redirection,
            or pipes; use it for portable commands.

            Guards and wrappers (`ASYNC`, `TIMEOUT`, `WITH_IO`, `LET`) apply to
            both forms.
        "#},
        args: &[ ArgSpec { name: "command", arg_type: ArgType::Rest(&ArgType::String), description: "Command", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "run", fence_meta: None, code: indoc! {r#"
            RUN echo hello

            # Captured runs prove the output, not just the exit status.
            LET $o: STRING = RUN echo hello
            ASSERT_CONTAINS $o "hello"
        "#}         }, Example { name: "run exec form", fence_meta: None, code: indoc! {r#"
            # No shell: `>` stays a literal argument, so no file is created.
            IMPORT [STD]
            RUN ["cargo", "--version", ">", "x.txt"]
            ASSERT_CONTAINS stdout "cargo"

            LET $t: STRING = PATH_TYPE("x.txt")
            ASSERT_EQ $t "absent"
        "#} } ],
        lower: |_flags, args| match args.as_slice() {
            [Arg::Expr(Expr::List(elems))] if elems.is_empty() => {
                Err(ParseError::validation("RUN", "RUN requires at least one argument".to_string(), &SpanContext::line_only(0)))
            }
            [Arg::Expr(Expr::List(elems))] => Ok(StepKind::RunExec {
                argv: elems.iter().cloned().map(Arg::Expr).collect(),
            }),
            _ => Ok(StepKind::Run(join_value(args, "RUN")?)),
        },
    ],

    Copy => [
        name: "COPY",
        variant: Copy { from_workspace: Option<WorkspaceTarget>, from_host: bool, to_host: bool, from: Arg, to: Arg },
        syntax: "COPY [--from-workspace SNAPSHOT|LOCAL|CACHE|SYSTEM] [--from-host | --to-host] <from> <to>",
        summary: "Copy file into workspace.",
        description: "Copies from host (the source is never moved or modified). Docker destination semantics: a file copied onto a directory (an existing one, or a trailing-slash spell like `out/`) is duplicated inside it under its own basename; a directory source duplicates its contents into the destination; any other destination path is created holding the copied bytes.",
        args: &[
            ArgSpec { name: "from", arg_type: ArgType::Path, description: "Source", io: IoDirection::Read, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "to", arg_type: ArgType::Path, description: "Dest", io: IoDirection::Write, index: 1, required: true, fallback_stream: None },
        ],
        flags: &[ FlagSpec { name: "from_workspace", long: "--from-workspace", value_type: FlagValueType::String, required: false, description: "Copy from the given workspace root instead of the build context" }, FlagSpec { name: "from_host", long: "--from-host", value_type: FlagValueType::Flag, required: false, description: "Fetch from the host workspace into the guest (REMOTE bodies only)" }, FlagSpec { name: "to_host", long: "--to-host", value_type: FlagValueType::Flag, required: false, description: "Push from the guest workspace back to the host (REMOTE bodies only)" } ],
        default_output: None,
        examples: &[ Example { name: "copy", fence_meta: Some("roots:unified"), code: indoc! {r#"
            # Copy to a new name, then read back.
            WRITE src.txt content
            COPY src.txt dst.txt

            LET $body: STRING = READ dst.txt
            ASSERT_EQ $body "content"
        "#} }, Example { name: "copy from workspace", fence_meta: None, code: indoc! {r#"
            # Same name, different contents per root: only LOCAL has ws-content.
            WRITE shared.txt from-snapshot
            WORKSPACE LOCAL
            WRITE shared.txt ws-content

            WORKSPACE SNAPSHOT
            COPY --from-workspace LOCAL shared.txt ws-copy.txt

            LET $body: STRING = READ ws-copy.txt
            ASSERT_EQ $body "ws-content"
        "#} } ],
        lower: |flags, args| {
            let from_workspace = flags
                .iter()
                .find(|(k, _)| k == "from_workspace")
                .map(|(_, v)| match v.as_str() {
                    "SNAPSHOT" => Ok(WorkspaceTarget::Snapshot),
                    "LOCAL" => Ok(WorkspaceTarget::Local),
                    "CACHE" => Ok(WorkspaceTarget::Cache { local: false }),
                    "SYSTEM" => Ok(WorkspaceTarget::System),
                    other => Err(ParseError::validation("COPY", format!("unknown workspace source: {other}"), &SpanContext::line_only(0))),
                })
                .transpose()?;
            let mut it = args.into_iter();
            let from = it.next().ok_or_else(|| ParseError::validation("COPY", "COPY requires a source".to_string(), &SpanContext::line_only(0)))?;
            let to = it.next().ok_or_else(|| ParseError::validation("COPY", "COPY requires a destination".to_string(), &SpanContext::line_only(0)))?;
            let from_host = flags.iter().any(|(k, _)| k == "from_host");
            let to_host = flags.iter().any(|(k, _)| k == "to_host");
            if from_host && to_host {
                return Err(ParseError::validation("COPY", "COPY --from-host and --to-host are mutually exclusive".to_string(), &SpanContext::line_only(0)));
            }
            Ok(StepKind::Copy { from_workspace, from_host, to_host, from, to })
        },
    ],

    CopyGit => [
        name: "COPY_GIT",
        variant: CopyGit { rev: Arg, from: Arg, to: Arg, include_dirty: bool },
        syntax: "COPY_GIT [--include-dirty] <rev> <src> <dst>",
        summary: "Copy from git revision.",
        description: "Checkout and copy.",
        args: &[
            ArgSpec { name: "rev", arg_type: ArgType::String, description: "Rev", io: IoDirection::Read, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "src", arg_type: ArgType::Path, description: "Src", io: IoDirection::Read, index: 1, required: true, fallback_stream: None },
            ArgSpec { name: "dst", arg_type: ArgType::Path, description: "Dst", io: IoDirection::Write, index: 2, required: true, fallback_stream: None },
        ],
        flags: &[ FlagSpec { name: "dirty", long: "--include-dirty", value_type: FlagValueType::Flag, required: false, description: "Include dirty" } ],
        default_output: None,
        examples: &[ Example { name: "git copy missing source errors", fence_meta: Some("expect_error:\"COPY source missing\""), code: indoc! {r#"COPY_GIT HEAD src.txt dst.txt"#} } ],
        lower: |flags, args| {
            let include_dirty = flags.iter().any(|(k, _)| k == "dirty");
            let mut it = args.into_iter();
            let rev = it.next().ok_or_else(|| ParseError::validation("COPY_GIT", "COPY_GIT requires a revision".to_string(), &SpanContext::line_only(0)))?;
            let from = it.next().ok_or_else(|| ParseError::validation("COPY_GIT", "COPY_GIT requires a source".to_string(), &SpanContext::line_only(0)))?;
            let to = it.next().ok_or_else(|| ParseError::validation("COPY_GIT", "COPY_GIT requires a destination".to_string(), &SpanContext::line_only(0)))?;
            Ok(StepKind::CopyGit { rev, from, to, include_dirty })
        },
    ],

    Symlink => [
        name: "SYMLINK",
        variant: Symlink { from_workspace: Option<WorkspaceTarget>, from: Arg, to: Arg },
        syntax: "SYMLINK [--from-workspace SNAPSHOT|LOCAL|CACHE|SYSTEM] <from> <to>",
        summary: "Create symlink.",
        description: "Creates symlink. A directory destination (existing, or a trailing-slash spell) receives the link under the source basename.",
        args: &[
            ArgSpec { name: "from", arg_type: ArgType::Path, description: "Target", io: IoDirection::Read, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "to", arg_type: ArgType::Path, description: "Link", io: IoDirection::Write, index: 1, required: true, fallback_stream: None },
        ],
        flags: &[ FlagSpec { name: "from_workspace", long: "--from-workspace", value_type: FlagValueType::String, required: false, description: "Symlink from the given workspace root instead of the build context" } ],
        default_output: None,
        examples: &[ Example { name: "symlink", fence_meta: Some("roots:unified"), code: indoc! {r#"
            # A symlink reads like its target.
            WRITE original.txt content
            SYMLINK original.txt link.txt

            LET $body: STRING = READ link.txt
            ASSERT_EQ $body "content"
        "#} }, Example { name: "symlink from workspace", fence_meta: None, code: indoc! {r#"
            # Same name, different contents per root: only LOCAL has ws-content.
            WRITE shared.txt from-snapshot
            WORKSPACE LOCAL
            WRITE shared.txt ws-content

            WORKSPACE SNAPSHOT
            SYMLINK --from-workspace LOCAL shared.txt ws-link.txt

            LET $body: STRING = READ ws-link.txt
            ASSERT_EQ $body "ws-content"
        "#} } ],
        lower: |flags, args| {
            let from_workspace = flags
                .iter()
                .find(|(k, _)| k == "from_workspace")
                .map(|(_, v)| match v.as_str() {
                    "SNAPSHOT" => Ok(WorkspaceTarget::Snapshot),
                    "LOCAL" => Ok(WorkspaceTarget::Local),
                    "CACHE" => Ok(WorkspaceTarget::Cache { local: false }),
                    "SYSTEM" => Ok(WorkspaceTarget::System),
                    other => Err(ParseError::validation("SYMLINK", format!("unknown workspace source: {other}"), &SpanContext::line_only(0))),
                })
                .transpose()?;
            let mut it = args.into_iter();
            let from = it.next().ok_or_else(|| ParseError::validation("SYMLINK", "SYMLINK requires a source".to_string(), &SpanContext::line_only(0)))?;
            let to = it.next().ok_or_else(|| ParseError::validation("SYMLINK", "SYMLINK requires a target".to_string(), &SpanContext::line_only(0)))?;
            Ok(StepKind::Symlink { from_workspace, from, to })
        },
    ],

    Mkdir => [
        name: "MKDIR",
        variant: Mkdir(Arg),
        syntax: "MKDIR <path>",
        summary: "Create directory.",
        description: "Creates dir with parents.",
        args: &[ ArgSpec { name: "path", arg_type: ArgType::Path, description: "Dir path", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "mkdir", fence_meta: None, code: indoc! {r#"
            IMPORT [STD]
            MKDIR deeply/nested/tree

            LET $t: STRING = PATH_TYPE("deeply/nested/tree")
            ASSERT_EQ $t "dir"
        "#} } ],
        lower: |_flags, args| Ok(StepKind::Mkdir(args.into_iter().next().ok_or_else(|| ParseError::validation("MKDIR", "MKDIR requires a path".to_string(), &SpanContext::line_only(0)))?)),
    ],

    Ls => [
        name: "LS",
        variant: Ls(Option<Arg>),
        syntax: "LS [<path>]",
        summary: "List directory.",
        description: "Lists entries.",
        args: &[ ArgSpec { name: "path", arg_type: ArgType::Path, description: "Dir", io: IoDirection::Read, index: 0, required: false, fallback_stream: None } ],
        flags: &[],
        default_output: Some(Stream::Stdout),
        examples: &[ Example { name: "ls", fence_meta: None, code: indoc! {r#"
            MKDIR inventory
            WRITE inventory/a.txt a
            LS inventory
            ASSERT_CONTAINS stdout "a.txt"
        "#} } ],
        lower: |_flags, args| Ok(StepKind::Ls(args.into_iter().next())),
    ],

    Cwd => [
        name: "CWD",
        variant: Cwd,
        syntax: "CWD",
        summary: "Print working directory.",
        description: "Outputs cwd.",
        args: &[],
        flags: &[],
        default_output: Some(Stream::Stdout),
        examples: &[ Example { name: "cwd", fence_meta: None, code: indoc! {r#"
            CWD

            # CWD tracks WORKDIR: the listing names the new directory.
            MKDIR sub
            WORKDIR sub
            LET $c: STRING = CWD
            ASSERT_CONTAINS $c "sub"
        "#} } ],
        lower: |_flags, _args| Ok(StepKind::Cwd),
    ],

    Read => [
        name: "READ",
        variant: Read(Option<Arg>),
        syntax: "READ [<path>]",
        summary: "Read file to stdout.",
        description: "Outputs file contents.",
        args: &[ ArgSpec { name: "path", arg_type: ArgType::Path, description: "File", io: IoDirection::Read, index: 0, required: false, fallback_stream: None } ],
        flags: &[],
        default_output: Some(Stream::Stdout),
        examples: &[ Example { name: "read", fence_meta: None, code: indoc! {r#"
            WRITE note.txt "hello"
            READ note.txt

            LET $body: STRING = READ note.txt
            ASSERT_EQ $body "hello"
        "#} } ],
        lower: |_flags, args| Ok(StepKind::Read(args.into_iter().next())),
    ],

    ReadLine => [
        name: "READ_LINE",
        variant: ReadLine { var: String },
        syntax: "READ_LINE $var",
        summary: "Read one line from stdin into a variable.",
        description: indoc! {r#"
            Reads bytes until newline without waiting for EOF, leaving the pipe open.

            Trailing newline is stripped (shell-read parity). On premature EOF
            assigns accumulated bytes and returns.
        "#},
        args: &[ ArgSpec { name: "var", arg_type: ArgType::String, description: "Target variable (`$name`); the line binds as STRING", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "read line", fence_meta: None, code: indoc! {r#"
            # The trailing newline is stripped: the variable holds exactly `first`.
            LET $lines: PIPE
            WITH_IO [stdout=$lines] ECHO "first"
            WITH_IO [stdin=$lines] READ_LINE $reply
            ASSERT_EQ $reply "first"
        "#} } ],
        lower: |_flags, args| {
            let arg = args.into_iter().next().ok_or_else(|| ParseError::validation("READ_LINE", "READ_LINE requires a variable".to_string(), &SpanContext::line_only(0)))?;
            let var = match arg {
                Arg::Expr(Expr::Var(name)) => name,
                // Quoted "$var" keeps its sigil through the generic string
                // path; templates defer untouched exactly as before.
                Arg::String(s, _) if s.starts_with('$') || s.contains("{{") => s.trim_start_matches('$').to_string(),
                other => return Err(ParseError::validation("READ_LINE", format!("READ_LINE requires a $variable, found {:?}", other), &SpanContext::line_only(0))),
            };
            if var.is_empty() {
                return Err(ParseError::validation("READ_LINE", "READ_LINE requires a variable".to_string(), &SpanContext::line_only(0)))
            }
            Ok(StepKind::ReadLine { var })
        },
    ],

    Write => [
        name: "WRITE",
        variant: Write { path: Arg, contents: Option<Arg> },
        syntax: "WRITE <path> [<contents>]",
        summary: "Write to file.",
        description: "Writes contents.",
        args: &[
            ArgSpec { name: "path", arg_type: ArgType::Path, description: "File", io: IoDirection::Write, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "contents", arg_type: ArgType::Rest(&ArgType::String), description: "Content", io: IoDirection::Write, index: 1, required: false, fallback_stream: Some(Stream::Stdin) },
        ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "write", fence_meta: None, code: indoc! {r#"
            WRITE output.txt hello-world
            LET $body: STRING = READ output.txt
            ASSERT_EQ $body "hello-world"
        "#} } ],
        lower: |_flags, args| {
            let mut it = args.into_iter();
            let path = it.next().ok_or_else(|| ParseError::validation("WRITE", "WRITE requires a path".to_string(), &SpanContext::line_only(0)))?;
            let remaining: Vec<Arg> = it.collect();
            let contents = if remaining.is_empty() { None } else { Some(join_value(remaining, "WRITE")?) };
            Ok(StepKind::Write { path, contents })
        },
    ],

    Append => [
        name: "APPEND",
        variant: Append { path: Arg, contents: Option<Arg> },
        syntax: "APPEND <path> [<contents>]",
        summary: "Append to file.",
        description: "Appends contents.",
        args: &[
            ArgSpec { name: "path", arg_type: ArgType::Path, description: "File", io: IoDirection::Write, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "contents", arg_type: ArgType::Rest(&ArgType::String), description: "Content", io: IoDirection::Write, index: 1, required: false, fallback_stream: Some(Stream::Stdin) },
        ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "append", fence_meta: None, code: indoc! {r#"
            WRITE log.txt line1
            APPEND log.txt line2

            # APPEND concatenates with no separator.
            LET $all: STRING = READ log.txt
            ASSERT_EQ $all "line1line2"
        "#} } ],
        lower: |_flags, args| {
            let mut it = args.into_iter();
            let path = it.next().ok_or_else(|| ParseError::validation("APPEND", "APPEND requires a path".to_string(), &SpanContext::line_only(0)))?;
            let remaining: Vec<Arg> = it.collect();
            let contents = if remaining.is_empty() { None } else { Some(join_value(remaining, "APPEND")?) };
            Ok(StepKind::Append { path, contents })
        },
    ],

    Expand => [
        name: "EXPAND",
        variant: Expand { path: Option<Arg>, overrides: Vec<(String, Arg)> },
        syntax: "EXPAND [<path>] [<KEY=val> ...]",
        summary: "Expand a template file (or stdin) to stdout.",
        description: indoc! {r#"
            A template is any text file — or piped stdin when no path is given —
            containing `{{ ... }}` placeholders. EXPAND replaces each placeholder
            and prints the result to stdout.

            Placeholders: `{{ NAME }}` reads a `KEY=val` override passed on this
            command; `{{ env:NAME }}` reads an override, falling back to the
            environment; `{{ $var }}` reads a script variable (dotted paths allowed).
            A missing key is an error, never a silent empty.

            Substitution runs in a single pass. EXPAND is not recursive and does not
            expand nested placeholders: a value that itself contains `{{ ... }}` is
            inserted verbatim and never expanded again.

            A bare `$var` argument is a template path; `KEY=val` arguments are
            overrides whose values follow the unified string-value rules (same as
            `ENV`: quotes keep exact bytes, a lone `$var` evaluates,
            `{{ ... }}` interpolates).

            NOTE: `WRITE` interpolates `{{ ... }}` while writing, so escape it
            (`\{{ ... }}`) when writing a template file for a later `EXPAND`.

            With no path, the template arrives on stdin through a pipe. When piping
            from a shell, single-quote the template (`echo '{{ $x }}'`): double
            quotes let the shell swallow `$x`, so oxdock receives an empty `{{ }}`
            placeholder and errors.
        "#},
        args: &[
            ArgSpec { name: "path", arg_type: ArgType::Path, description: "Template file to expand; omit to expand stdin", io: IoDirection::Read, index: 0, required: false, fallback_stream: None },
            ArgSpec { name: "overrides", arg_type: ArgType::Rest(&ArgType::String), description: "Template overrides shadowing that key (unified string values)", io: IoDirection::Read, index: 1, required: false, fallback_stream: None },
        ],
        flags: &[],
        default_output: Some(Stream::Stdout),
        examples: &[
            Example { name: "expand", fence_meta: None, code: indoc! {r#"
                # Placeholders read overrides first, then the environment.
                ENV NAME="Alice"
                WRITE template.md "Hello {{ env:NAME }}!"
                EXPAND template.md

                ASSERT_CONTAINS stdout "Hello Alice!"
            "#} },
            Example { name: "override with spaces", fence_meta: None, code: indoc! {r#"
                # WRITE would interpolate {{ }} right away, so escape it.
                # The file must literally contain {{ env:NAME }} for EXPAND.
                WRITE template.md "Hello \{{ env:NAME }}!"
                EXPAND template.md NAME="Alice Smith"

                ASSERT_CONTAINS stdout "Hello Alice Smith!"
            "#} },
            Example { name: "variable override", fence_meta: None, code: indoc! {r#"
                # Same escaping: keep the placeholder literal until EXPAND.
                # A lone $who evaluates, like ECHO $who.
                LET $who: STRING = "Bob"
                WRITE template.md "Hi \{{ env:WHO }}!"
                EXPAND template.md WHO=$who

                ASSERT_CONTAINS stdout "Hi Bob!"
            "#} },
            Example { name: "override forms agree", fence_meta: None, code: indoc! {r#"
                # A bare variable and a template-with-tail expand identically.
                LET $x: STRING = "Ada"
                WRITE template.md "Hi \{{ env:NAME }} and \{{ env:NAME2 }}!"
                EXPAND template.md NAME=$x NAME2="{{ $x }} concatenated"

                ASSERT_CONTAINS stdout "Hi Ada and Ada concatenated!"
            "#} },
            Example { name: "expand stdin", fence_meta: None, code: indoc! {r#"
                # No path: the template arrives on stdin through a pipe.
                LET $tpl: PIPE
                WITH_IO [stdout=$tpl] ECHO "Hello \{{ env:NAME }}!"
                WITH_IO [stdin=$tpl] EXPAND NAME=Alice

                ASSERT_CONTAINS stdout "Hello Alice!"
            "#} },
            Example { name: "override does not leak", fence_meta: None, code: indoc! {r#"
                # KEY=val overrides shadow env for that EXPAND only.
                # They never update the environment itself.
                ENV NAME="Alice"
                WRITE template.md "Hi \{{ env:NAME }}!"

                EXPAND template.md NAME="Bob"
                ASSERT_CONTAINS stdout "Hi Bob!"

                EXPAND template.md
                ASSERT_CONTAINS stdout "Hi Alice!"
            "#} },
        ],
        lower: |_flags, args| {
            let mut path = None;
            let mut overrides = Vec::new();
            for arg in args {
                let text = arg.as_str();
                if let Some((key, value)) = split_assignment(text).map_err(|e| ParseError::validation("EXPAND", e.to_string(), &SpanContext::line_only(0)))? {
                    overrides.push((key, value));
                } else if path.is_none() { path = Some(arg); }
                else { return Err(ParseError::validation("EXPAND", "EXPAND accepts at most one path".to_string(), &SpanContext::line_only(0))) }
            }
            Ok(StepKind::Expand { path, overrides })
        },
    ],

    AssertEq => [
        name: "ASSERT_EQ",
        variant: AssertEq { hash: Option<String>, actual: AssertTarget, expected: Option<Arg> },
        syntax: "ASSERT_EQ <actual> <expected> | ASSERT_EQ --hash <sha256> <actual>",
        summary: "Assert strict equality.",
        description: indoc! {r#"
            Compares two evaluated values with typed equality (no coercion:
            `INT(42)` never equals `STRING("42")`), aborting the pipeline
            with a step-numbered error showing expected vs actual otherwise.

            Both sides are values: `$var`, literals, templates, and calls
            evaluate in memory and never touch disk. Read files explicitly
            first (`LET $text: STRING = READ "out.txt"`, then
            `ASSERT_EQ $text ...`).
            Bare `stdout` / `stderr` observe stream buffers; a `$var`
            holding a `PIPE` observes its backend bytes. `--hash` compares
            the SHA-256 of a string, pipe, or captured-stdout actual
            instead of the raw bytes (`stderr` is unsupported).
        "#},
        args: &[
            ArgSpec { name: "actual", arg_type: ArgType::Any, description: "Value, stdout, stderr, or a $var holding a PIPE", io: IoDirection::Read, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "expected", arg_type: ArgType::Rest(&ArgType::Any), description: "Expected (required unless --hash)", io: IoDirection::Read, index: 1, required: false, fallback_stream: None },
        ],
        flags: &[ FlagSpec { name: "hash", long: "--hash", value_type: FlagValueType::String, required: false, description: "SHA-256" } ],
        default_output: None,
        examples: &[ Example { name: "assert eq", fence_meta: None, code: indoc! {r#"
            LET $status: INT = 200
            ASSERT_EQ $status 200
        "#} },
        Example { name: "assert eq file", fence_meta: None, code: indoc! {r#"
            WRITE payload.bin stable-content
            LET $body: STRING = READ payload.bin
            ASSERT_EQ $body "stable-content"
        "#} },
        Example { name: "assert eq hash", fence_meta: None, code: indoc! {r#"
            # --hash compares the SHA-256 digest instead of raw bytes.
            WRITE payload.bin stable-content
            LET $body: STRING = READ payload.bin
            ASSERT_EQ --hash 08135c1b6349b0e4f894c36221952f0de00e6b4d82f80895abf359755e77103c $body
        "#} } ],
        lower: |flags, args| {
            let hash = flags.iter().find(|(k, _)| k == "hash").map(|(_, v)| v.as_str().to_string());
            let mut it = args.into_iter();
            let actual = lower_assert_target(it.next().ok_or_else(|| ParseError::validation("ASSERT_EQ", "ASSERT_EQ requires a value".to_string(), &SpanContext::line_only(0)))?)?;
            let remaining: Vec<Arg> = it
                .map(lower_assert_operand)
                .collect::<Vec<Arg>>();
            // Exactly two operands, except --hash carries its expectation
            // in the flag and takes none positionally.
            let expected = if remaining.is_empty() {
                if hash.is_some() {
                    None
                } else {
                    return Err(ParseError::validation("ASSERT_EQ", "ASSERT_EQ requires an expected value".to_string(), &SpanContext::line_only(0)))
                }
            } else {
                Some(join_value(remaining, "ASSERT_EQ")?)
            };
            Ok(StepKind::AssertEq { hash, actual, expected })
        },
    ],

    AssertContains => [
        name: "ASSERT_CONTAINS",
        variant: AssertContains { haystack: AssertTarget, needle: Arg },
        syntax: "ASSERT_CONTAINS <haystack> <needle>",
        summary: "Assert containment.",
        description: indoc! {r#"
            Checks containment and aborts the pipeline with a step-numbered
            error otherwise: substring for strings, element match for lists,
            key presence for maps, substring over stream and pipe buffers.

            Like `ASSERT_EQ`, both sides are values read without implicit
            I/O; read files explicitly first
            (`LET $text: STRING = READ "cfg.txt"`).
            Bare `stdout` / `stderr` observe stream buffers; a `$var`
            holding a `PIPE` observes its backend bytes.
        "#},
        args: &[
            ArgSpec { name: "haystack", arg_type: ArgType::Any, description: "Value, stdout, stderr, or a $var holding a PIPE", io: IoDirection::Read, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "needle", arg_type: ArgType::Rest(&ArgType::Any), description: "Substring, element, or key", io: IoDirection::Read, index: 1, required: true, fallback_stream: None },
        ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "assert contains", fence_meta: None, code: indoc! {r#"
            ECHO build-complete
            ASSERT_CONTAINS stdout "build-complete"
        "#} } ],
        lower: |flags, args| {
            let _ = flags;
            let mut it = args.into_iter();
            let haystack = lower_assert_target(it.next().ok_or_else(|| ParseError::validation("ASSERT_CONTAINS", "ASSERT_CONTAINS requires a value".to_string(), &SpanContext::line_only(0)))?)?;
            let remaining: Vec<Arg> = it
                .map(lower_assert_operand)
                .collect::<Vec<Arg>>();
            if remaining.is_empty() {
                return Err(ParseError::validation("ASSERT_CONTAINS", "ASSERT_CONTAINS requires a needle".to_string(), &SpanContext::line_only(0)))
            }
            let needle = join_value(remaining, "ASSERT_CONTAINS")?;
            Ok(StepKind::AssertContains { haystack, needle })
        },
    ],

    HashSha256 => [
        name: "HASH_SHA256",
        variant: HashSha256 { path: Arg },
        syntax: "HASH_SHA256 <path>",
        summary: "Print SHA-256.",
        description: "Computes digest.",
        args: &[ ArgSpec { name: "path", arg_type: ArgType::Path, description: "File", io: IoDirection::Read, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: Some(Stream::Stdout),
        examples: &[ Example { name: "hash", fence_meta: None, code: indoc! {r#"
            WRITE payload.txt hello
            HASH_SHA256 payload.txt

            LET $digest: STRING = HASH_SHA256 payload.txt
            ASSERT_EQ $digest "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824\n"
        "#} } ],
        lower: |_flags, args| Ok(StepKind::HashSha256 { path: args.into_iter().next().ok_or_else(|| ParseError::validation("HASH_SHA256", "HASH_SHA256 requires a path".to_string(), &SpanContext::line_only(0)))? }),
    ],

    Exit => [
        name: "EXIT",
        variant: Exit(Arg),
        syntax: "EXIT <code>",
        summary: "Exit pipeline.",
        description: indoc! {r#"
            Stops the pipeline immediately with an `EXIT requested with code <code>`
            error; steps after it never run, at any nesting depth.

            Enclosing blocks still unwind their LET/ENV/WORKDIR/WORKSPACE state,
            anonymous background tasks are killed synchronously, and files written
            before the EXIT persist.

            The code reaches the OS: the CLI exits with `<code>` clamped to
            0..=255, and a guest EXIT relays through `REMOTE` the same way.
        "#},
        args: &[ ArgSpec { name: "code", arg_type: ArgType::Int, description: "Code", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "exit", fence_meta: Some("expect_error:\"EXIT requested with code 0\""), code: indoc! {r#"EXIT 0"#} } ],
        lower: |_flags, args| {
            // Static literals were already Int-checked by the central
            // validator; dynamics resolve (and validate) at runtime.
            let code = args.into_iter().next().ok_or_else(|| ParseError::validation("EXIT", "EXIT requires a code".to_string(), &SpanContext::line_only(0)))?;
            Ok(StepKind::Exit(code))
        },
    ],

    Sleep => [
        name: "SLEEP",
        variant: Sleep { duration: Arg },
        syntax: "SLEEP <duration>",
        summary: "Pause execution for a duration.",
        description: indoc! {r#"
            Parks the step for the duration (e.g. 500ms, 10s, 2m).

            Cooperative: checks for cancellation so an enclosing TIMEOUT or task
            teardown interrupts the sleep. Cross-platform alternative to shell sleep
            for testing time boundaries.
        "#},
        args: &[ ArgSpec { name: "duration", arg_type: ArgType::Duration, description: "How long to sleep", io: IoDirection::Write, index: 0, required: true, fallback_stream: None } ],
        flags: &[],
        default_output: None,
        examples: &[
            Example { name: "sleep", fence_meta: None, code: indoc! {r#"SLEEP 100ms"#} },
            Example {
                name: "sleep variable duration",
                fence_meta: None,
                code: indoc! {r#"
                # Durations resolve at runtime, so variables work too:
                # quoted or bare, both bind the same string.
                LET $pause: STRING = "100ms"
                SLEEP $pause

                LET $bare: STRING = 100ms
                SLEEP $bare
            "#},
            },
        ],
        lower: |_flags, args| {
            let mut it = args.into_iter();
            let raw = it
                .next()
                .ok_or_else(|| ParseError::validation("SLEEP", "SLEEP requires a duration (e.g. SLEEP 500ms)".to_string(), &SpanContext::line_only(0)))?;
            if it.next().is_some() {
                return Err(ParseError::validation("SLEEP", "SLEEP takes exactly one duration argument".to_string(), &SpanContext::line_only(0)))
            }
            // Static literals were Duration-checked by the central
            // validator; dynamics ($var, templates) resolve at runtime.
            Ok(StepKind::Sleep { duration: raw })
        },
    ],

    ListAppend => [
        name: "LIST_APPEND",
        variant: ListAppend { list: String, item: Arg },
        syntax: "LIST_APPEND $list <item>",
        summary: "Append an item to a LIST variable in place.",
        description: indoc! {r#"
            Appends the item to the LIST variable in place.

            When the binding holds the only reference the push runs in
            amortized constant time. Aliased buffers detach first, so
            other holders keep their contents.
        "#},
        args: &[
            ArgSpec { name: "list", arg_type: ArgType::List, description: "Target LIST variable (`$name`)", io: IoDirection::Write, index: 0, required: true, fallback_stream: None },
            ArgSpec { name: "item", arg_type: ArgType::Any, description: "Item to append (any value)", io: IoDirection::Write, index: 1, required: true, fallback_stream: None },
        ],
        flags: &[],
        default_output: None,
        examples: &[ Example { name: "list append", fence_meta: None, code: indoc! {r#"
            # Appends accumulate in order.
            LET $items: LIST<STRING> = []
            LIST_APPEND $items "first"
            LIST_APPEND $items "second"

            LET $want: LIST<STRING> = ["first", "second"]
            ASSERT_EQ $items $want
        "#} } ],
        lower: |_flags, args| {
            let mut it = args.into_iter();
            let raw_list = it
                .next()
                .ok_or_else(|| ParseError::validation("LIST_APPEND", "LIST_APPEND requires a LIST variable (e.g. LIST_APPEND $items $x)".to_string(), &SpanContext::line_only(0)))?;
            let list = match raw_list {
                Arg::Expr(Expr::Var(name)) => name,
                Arg::String(s, _) => s.trim_start_matches('$').to_string(),
                other => return Err(ParseError::validation("LIST_APPEND", format!("LIST_APPEND requires a $variable, found {:?}", other), &SpanContext::line_only(0))),
            };
            if list.is_empty() {
                return Err(ParseError::validation("LIST_APPEND", "LIST_APPEND requires a LIST variable (e.g. LIST_APPEND $items $x)".to_string(), &SpanContext::line_only(0)))
            }
            let item = it
                .next()
                .ok_or_else(|| ParseError::validation("LIST_APPEND", "LIST_APPEND requires an item to append (e.g. LIST_APPEND $items $x)".to_string(), &SpanContext::line_only(0)))?;
            if it.next().is_some() {
                return Err(ParseError::validation("LIST_APPEND", "LIST_APPEND takes exactly two arguments: LIST_APPEND $list <item>".to_string(), &SpanContext::line_only(0)))
            }
            Ok(StepKind::ListAppend { list, item })
        },
    ],
}

// ── Structural metadata ──────────────────────────────────────────────────
// Single source of truth for structural-statement documentation (TIMEOUT,
// ASYNC, AWAIT, WITH_IO, IF, FOR, ...). These constructs are parsed by PEG
// rules rather than `declare_commands!`, so their reference docs live here
// instead of `crates/docs-gen/src/command_ref.rs` : adding a structural
// StepKind without registering it here fails `structural_metadata_covers_all_structural_kinds`
// below, and docs-gen renders these entries dynamically (no hardcoded copy).
pub fn all_structural_metadata() -> Vec<CommandMeta> {
    vec![
        CommandMeta {
            name: "WITH_IO",
            syntax: "WITH_IO [<stream>[=$var], ...] <command> | WITH_IO [bindings] { <commands> }",
            summary: "Reroute standard streams.",
            description: indoc! {r#"
                Reroutes the standard streams of the next command or, in block form,
                of every enclosed command.

                Bindings map streams (`stdin`, `stdout`, `stderr`) to a PIPE-typed
                variable (`stdout=$p`, `stdin=$p`), resolved from the variable
                when the step runs. Both stdout and stderr pipes capture output
                the same way. Declare the handle first with `LET $p: PIPE`.

                Pipes hold bytes in memory and spill to a temp file above 8 MiB, so a
                producer can finish before the consumer starts.

                If WITH_IO wraps an ASYNC block whose body is a single RUN, guarded or
                not, the pipe is a zero copy OS kernel pipe instead: pair it with a
                consumer that runs while the producer is alive, since output past the
                64 KiB kernel buffer stalls until drained. That promotion never crosses
                a function boundary: pipes created, bound, or passed by variable inside FUNC
                bodies are always script pipes, even when the surrounding task would
                otherwise promote.

                A second producer or consumer on a live handle is an explicit
                error. A handle bound as output can later feed another
                command's `stdin`, connecting commands without touching the
                terminal. Binding `stdout` and `stderr` to the same live
                handle fails deterministically. Merge streams in shell
                via `2>&1` instead.

                Nested blocks stack defaults; inline bindings override inherited ones for
                their command only; closing a block restores previous wiring.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "with_io block",
                    fence_meta: None,
                    code: indoc! {r#"
                LET $log: PIPE
                WITH_IO [stdout=$log] {
                  ECHO first
                  ECHO second
                }
                WITH_IO [stdin=$log] WRITE captured.txt

                # The piped bytes landed in the file.
                LET $body: STRING = READ captured.txt
                ASSERT_CONTAINS $body "first"
                ASSERT_CONTAINS $body "second"
            "#},
                },
                Example {
                    name: "variable pipe binding",
                    fence_meta: None,
                    code: indoc! {r#"
                # Declare the pipe first: `LET $p: PIPE` mints a fresh
                # backend without touching a stream. A plain string here
                # would be a TypeMismatch.
                LET $p: PIPE
                WITH_IO [stdout=$p] ECHO hello
                WITH_IO [stdin=$p] READ_LINE $line
                ASSERT_EQ $line "hello"
            "#},
                },
            ],
        },
        CommandMeta {
            name: "FOR",
            syntax: "FOR $item: TYPE IN <expr> { <commands> } | FOR $key: STRING, $value: TYPE IN <expr> { <commands> }",
            summary: "Iterate over a list or map.",
            description: indoc! {r#"
                The loop variable receives each element (lists) or value (maps); with
                two variables, the first receives the key.

                Loop variables are declared with explicit types and scoped per iteration;
                they do not leak outward. The body may be a braced block
                or a single-line `{ ... }` command.

                `GLOB("...")` patterns must be quoted (`*` is not a bare word, so
                `GLOB(*)` is a parse error); GLOB returns a root-relative sorted list,
                empty when nothing matches, and rejects `..` escapes.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "for loop",
                    fence_meta: None,
                    code: indoc! {r#"
                # Each element binds in turn; the loop body sees every one.
                LET $items: LIST<STRING> = ["a", "b"]
                FOR $item: STRING IN $items {
                  ECHO $item
                }
                ASSERT_CONTAINS stdout "a"
                ASSERT_CONTAINS stdout "b"

                # Key and value bind together for maps.
                LET $map: MAP<x: INT> = {"x": 1}
                FOR $k: STRING, $v: INT IN $map {
                  ECHO "{{ $k }}={{ $v }}"
                }
                ASSERT_CONTAINS stdout "x=1"
            "#},
                },
                Example {
                    name: "expand every match",
                    fence_meta: None,
                    code: indoc! {r#"
                # Single-line body; $x is a template path, WHO an override.
                IMPORT [STD]
                WRITE a.txt "hi \{{ env:WHO }}!"
                FOR $x: STRING IN GLOB("*.txt") { EXPAND $x WHO=World }

                ASSERT_CONTAINS stdout "hi World!"
            "#},
                },
            ],
        },
        CommandMeta {
            name: "IF",
            syntax: "IF <expr> { <commands> } [ELSE IF <expr> { <commands> } ...] [ELSE { <commands> }]",
            summary: "Conditional execution.",
            description: indoc! {r#"
                The condition is evaluated as a boolean expression.

                Prefix `!` negates (`IF !false`); `&&` binds tighter than
                `||`, and both short-circuit, so `IF true || $missing`
                never evaluates the right side. Only Bool values are
                accepted as conditions.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "if else",
                    fence_meta: None,
                    code: indoc! {r#"
                IMPORT [STD]

                # True branch runs; the false branch is skipped.
                IF true {
                  WRITE yes.txt taken
                } ELSE {
                  WRITE yes.txt skipped
                }

                # ELSE IF selects the first true branch.
                IF false {
                  WRITE skipped.txt no
                } ELSE IF true {
                  WRITE fallback.txt taken
                }

                # !false evaluates to true, so this branch runs.
                IF !false {
                  WRITE negated.txt taken
                }

                LET $yes_body: STRING = READ yes.txt
                LET $fallback_body: STRING = READ fallback.txt
                LET $negated_body: STRING = READ negated.txt
                ASSERT_EQ $yes_body "taken"
                ASSERT_EQ $fallback_body "taken"
                ASSERT_EQ $negated_body "taken"
                LET $t: STRING = PATH_TYPE("skipped.txt")
                ASSERT_EQ $t "absent"
            "#},
                },
                Example {
                    name: "logical condition composition",
                    fence_meta: None,
                    code: indoc! {r#"
                IMPORT [STD]
                LET $role: STRING = "admin"
                LET $level: INT = 3

                # || is true when either side holds; && needs both.
                IF $role == "owner" || $level >= 5 {
                    WRITE unexpected.txt no
                } ELSE {
                    WRITE fallback.txt or-false
                }

                LET $fb: STRING = READ fallback.txt
                ASSERT_EQ $fb "or-false"
                LET $t1: STRING = PATH_TYPE("unexpected.txt")
                ASSERT_EQ $t1 "absent"

                IF $role == "admin" || $level >= 5 {
                    WRITE chosen.txt or-true
                }

                LET $ch: STRING = READ chosen.txt
                ASSERT_EQ $ch "or-true"

                IF $role == "admin" && $level >= 5 {
                    WRITE unexpected-too.txt no
                } ELSE {
                    WRITE and.txt and-false
                }

                LET $an: STRING = READ and.txt
                ASSERT_EQ $an "and-false"
                LET $t2: STRING = PATH_TYPE("unexpected-too.txt")
                ASSERT_EQ $t2 "absent"
            "#},
                },
            ],
        },
        CommandMeta {
            name: "LET",
            syntax: "LET $var: TYPE = <expr> | LET $p: PIPE | LET $var: TYPE = ASYNC { <commands> } | LET $var: TYPE = <command> | LET $var: TYPE = AWAIT $task | LET $var: TYPE = { <commands> }",
            summary: "Bind script-local variables.",
            description: indoc! {r#"
                Declares a script-local variable with an explicit type (STRING, INT,
                FLOAT, BOOL, PIPE, LIST, MAP, HANDLE, DURATION, PATH). Duplicate LET
                in the same scope frame is a redeclaration error; mutate with
                `$var = <expr>`.

                Variables are usable in templates (`{{ $var }}`), guards, and
                expressions. With `ASYNC`, spawns a background task and stores its
                handle (see ASYNC). The `$` sigil on the name is mandatory.

                No hoisting: a variable exists only after its LET runs, in
                execution order. Reading `$var` before its LET (or after the
                block that declared it exits) fails with
                `undefined variable $var`. Scopes are a stack of frames and
                resolution walks innermost outward, so nothing pre-declares
                names. Function bodies read outer variables through the same
                walk, but their own LETs never leak out (see FUNC).

                The right-hand side is always an expression — literals, lists, maps,
                arithmetic (`+ - * /` with `*`/`/` binding tighter, unary `-`,
                parentheses), comparisons (`< <= > >=` binding tighter than
                `== !=`), logical `&&` (tighter) and `||` with short-circuit,
                `!` negation, `env:KEY` reads, `INSPECT($var)` snapshots,
                `GLOB("*.md")`, `INT(x)` / `FLOAT(x)` conversions — never a
                `{{ ... }}` template; interpolation happens in string values,
                not here.
                The one exception is pipes: `LET $p: PIPE` with no `=`
                and no initializer mints a fresh anonymous backend,
                lazily materialized at first binding, so two declarations
                never share a channel.

                Numbers are numeric literals: `42` binds `INT`, `3.14` binds
                `FLOAT`. `Int x Int` stays `INT` (checked, integer division,
                so `7 / 2` is `3`); any `Float` operand promotes to `FLOAT`.
                Division by zero, overflow, and non-finite results are errors.
                Both numeric sides compare numerically (`1 == 1.0` is true);
                otherwise `==`/`!=` compare rendered strings and ordering on
                non-numerics is a Type Error. Constant subtrees fold at parse
                time and dynamic arithmetic compiles to flat RPN with
                identical semantics.

                Float equality is exact with no epsilon. Floats store decimals
                in binary, so a value is exact only when its reduced fraction
                has a power-of-2 denominator: 0.5 (1/2), 0.25 (1/4), 0.75
                (3/4) are exact, while 0.1 (1/10), 0.2 (1/5), 0.3 (3/10)
                repeat forever in binary (like 1/3 in decimal) and truncate,
                so `0.1 + 0.2 == 0.3` is false (the sum is
                `0.30000000000000004`). Rule of thumb: endings .5, .25, .75,
                .125, .625, .875 are exact; .1, .2, .3 and similar are
                approximations. Bound approximations instead of comparing
                them: `IF $sum > 0.299999 && $sum < 0.300001`.

                Comparisons do not chain: `a < b < c` is a parse error, not
                `(a < b) < c`. Chaining would compare a `BOOL` against a
                number (a runtime Type Error in C-style parsing) or evaluate
                the middle term twice (Python-style chaining), so the grammar
                accepts exactly one comparison operator per level. Write the
                conjunction explicitly: `$a < $b && $b < $c`. The same holds
                for equality (`$a == $b == $c` is rejected).

                Captured command output is a string, so convert before math:
                `LET $total: INT = $total + INT($size_str)` (`INT` trims ASCII
                whitespace; `FLOAT` accepts int strings and rejects
                non-finite).

                Bare words need no quotes: `LET $d: STRING = 30s` binds the same string
                as quoted.

                When the right-hand side is a synchronous command
                (`LET $out: STRING = ECHO hi`), the command runs to completion and its
                exact stdout bytes are captured into the variable as a string (no newline
                stripping; commands with no stdout capture as `""`; non-UTF8 stdout is
                an error). Combining capture with an explicit
                `WITH_IO [stdout=$var]` is a parse error.

                Coming from Bash, the capture line looks familiar but behaves
                strictly:

                | | Bash `output=$(...)` | OxDock `LET $out: STRING = ...` |
                | --- | --- | --- |
                | Trailing newlines | Stripped (all of them) | Preserved byte-exact |
                | Variable type | Always an untyped string | Declared: STRING, INT, FLOAT, ... |
                | Math on output | Implicit: `$((var + 1))` | Explicit: `INT($out) + 1` |
                | Failing command | Continues with empty output unless `set -e` | Step fails immediately, binds nothing |

                `LET $out: TYPE = AWAIT $var` binds the background task's
                explicit `RETURN` value instead (tasks stream their stdout
                live, so there is no output left to capture); a task that
                succeeded without `RETURN` yields `INT` 0, like a process
                exit status.

                An inline block (`LET $var: TYPE = { <commands> }`) runs its
                steps in a fresh scope and binds the nearest `RETURN` value,
                like a zero-arg function body: fallthrough without `RETURN`
                binds `""`, and `BREAK`/`CONTINUE` escaping the block are
                errors. The block reads outer variables but its own LETs
                never leak out. A `{k: v}` shape still parses as a map
                literal; anything else in braces is a block.

                The split is deliberate: synchronous commands capture
                stdout because they run inline to completion on the same
                thread; background tasks never capture stdout because
                concurrent output has no well-defined value. Task results
                travel only through `RETURN` (or `INT` 0 for void tasks).

                `LET $e: STRING = env:FOO` reads the script environment into a plain
                string.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "let",
                    fence_meta: None,
                    code: indoc! {r#"
                LET $name: STRING = "world"
                ECHO "hello, {{ $name }}"
                ASSERT_CONTAINS stdout "hello, world"

                LET $items: LIST<STRING> = ["a", "b"]
                ASSERT_CONTAINS $items "a"
                ASSERT_CONTAINS $items "b"

                LET $count: INT = 42
                ASSERT_EQ $count 42
            "#},
                },
                Example {
                    name: "no hoisting",
                    fence_meta: Some("expect_error:\"undefined variable\""),
                    code: indoc! {r#"
                # Reading before the LET runs is an error, not an empty value.
                ECHO $too_early
                LET $too_early: STRING = "too late"
            "#},
                },
                Example {
                    name: "glob binding",
                    fence_meta: None,
                    code: indoc! {r#"
                # The RHS is an expression: GLOB(...) runs and binds a list.
                IMPORT [STD]
                WRITE a.txt "x"
                LET $files: LIST<STRING> = GLOB("*.txt")
                FOR $f: STRING IN $files { ECHO $f }

                ASSERT_CONTAINS stdout "a.txt"
            "#},
                },
                Example {
                    name: "scoped variable reverts",
                    fence_meta: None,
                    code: indoc! {r#"
                # LET inside a braced block reverts when the block exits.
                LET $a: STRING = "outer"

                [bool:true] {
                    LET $a: STRING = "inner"
                    WRITE inner.txt "{{ $a }}"
                }

                WRITE outer.txt "{{ $a }}"

                LET $in_body: STRING = READ inner.txt
                ASSERT_EQ $in_body "inner"

                LET $out_body: STRING = READ outer.txt
                ASSERT_EQ $out_body "outer"
            "#},
                },
                Example {
                    name: "capture command output",
                    fence_meta: None,
                    code: indoc! {r#"
                # Capture keeps the trailing newline.
                LET $out: STRING = ECHO hi
                ASSERT_EQ $out "hi\n"
            "#},
                },
                Example {
                    name: "inline block",
                    fence_meta: None,
                    code: indoc! {r#"
                LET $who: STRING = "ada"

                # An inline block binds its RETURN value like a function body.
                LET $res: STRING = {
                    LET $loud: STRING = "{{ $who }}!"
                    RETURN $loud
                }
                ASSERT_EQ $res "ada!"

                # Any declared type works: the block value checks like any RHS.
                LET $n: INT = {
                    RETURN 40 + 2
                }
                ASSERT_EQ $n 42
            "#},
                },
                Example {
                    name: "arithmetic over captured output",
                    fence_meta: None,
                    code: indoc! {r#"
                # Captured output converts explicitly: INT() then arithmetic.
                IMPORT [STD]
                LET $size_str: STRING = ECHO 41
                LET $total: INT = INT($size_str) + 1
                ASSERT_EQ $total 42

                # FLOAT() promotes instead of truncating.
                LET $ratio: FLOAT = 1 + 2.5
                ASSERT_EQ $ratio 3.5

                # Int x Int stays INT: integer division truncates.
                LET $half: INT = 7 / 2
                ASSERT_EQ $half 3
            "#},
                },
                Example {
                    name: "float equality is exact",
                    fence_meta: None,
                    code: indoc! {r#"
                # Binary fractions compare cleanly; decimal fractions may not:
                # 0.1 + 0.2 is 0.30000000000000004, so == is false.
                IMPORT [STD]
                LET $exact: BOOL = 0.5 + 0.25 == 0.75
                LET $decimal: BOOL = 0.1 + 0.2 == 0.3
                IF $exact {
                    WRITE exact.txt yes
                }
                IF $decimal {
                    WRITE unexpected.txt no
                }

                LET $ok: STRING = READ exact.txt
                ASSERT_EQ $ok "yes"

                LET $t: STRING = PATH_TYPE("unexpected.txt")
                ASSERT_EQ $t "absent"
            "#},
                },
                Example {
                    name: "bound inexact decimals",
                    fence_meta: None,
                    code: indoc! {r#"
                # Never test inexact decimals for equality; bound them.
                LET $sum: FLOAT = 0.1 + 0.2
                IF $sum > 0.299999 && $sum < 0.300001 {
                    WRITE bounded.txt yes
                }

                LET $ok: STRING = READ bounded.txt
                ASSERT_EQ $ok "yes"
            "#},
                },
                Example {
                    name: "inspect a variable",
                    fence_meta: None,
                    code: indoc! {r#"
                # INSPECT($var) snapshots a variable into a MAP: declared
                # type plus live details (pipe backend stats here), so
                # scripts can branch on engine state.
                IMPORT [STD]
                LET $p: PIPE
                WITH_IO [stdout=$p] ECHO hello
                LET $info: MAP<ANY> = INSPECT($p)
                IF $info.is_os_pipe {
                    WRITE unexpected.txt "should be a script pipe"
                }

                ASSERT_EQ $info.type "PIPE"
                LET $t: STRING = PATH_TYPE("unexpected.txt")
                ASSERT_EQ $t "absent"
            "#},
                },
            ],
        },
        CommandMeta {
            name: "MUTATION",
            syntax: "$var = <expr>",
            summary: "Mutate a declared variable.",
            description: indoc! {r#"
                Reassigns an existing variable, converting the new value to
                the type declared at LET time. The explicit annotation is
                what authorizes string-to-number conversion here (`$n = "42"`
                binds 42 for an INT); a non-numeric string is an error.
                Expressions never convert: `"100" + 1` is a Type Error, use
                `INT()` / `FLOAT()` to cross that boundary explicitly.

                The leading `$` distinguishes mutation from `KEY=value` command
                assignments. Assigning an undeclared variable or a mismatched type is
                an error.

                Mutation writes through to the scope where the variable was
                declared, so it survives block exit: `LET $x` outside a block
                followed by `$x = ...` inside still reads back the new value
                afterwards, for every type. This is the counterpart to LET
                shadowing, where `LET $x` *inside* the block declares a
                separate inner variable that reverts on exit.

                Reassignment rebinds the variable to a new value: values
                captured earlier keep the old word (see the rebinding
                example below).
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "mutate",
                    fence_meta: None,
                    code: indoc! {r#"
                # Mutation writes through: the binding holds the new value.
                LET $count: INT = 1
                $count = 2
                ASSERT_EQ $count 2
            "#},
                },
                Example {
                    name: "rebind keeps earlier captures",
                    fence_meta: None,
                    code: indoc! {r#"
                # Reassignment rebinds: a map built from `$s` still
                # reads the original string after `$s` moves on.
                LET $s: STRING = "test"
                LET $m: MAP<ANY> = {a: $s}
                $s = "changed"
                ASSERT_EQ $m.a "test"

                # Appending to one list binding never touches its clones.
                LET $a: LIST<STRING> = ["1"]
                LET $b: LIST<STRING> = $a
                LIST_APPEND $a "2"
                LET $want: LIST<STRING> = ["1"]
                ASSERT_EQ $b $want
            "#},
                },
                Example {
                    name: "convert before math",
                    fence_meta: None,
                    code: indoc! {r#"
                # Captured output is a string: `"100" + 1` is a Type Error.
                # Convert explicitly, then mutate with arithmetic.
                IMPORT [STD]
                LET $raw: STRING = ECHO 100
                LET $n: INT = INT($raw)
                $n = $n + 1

                # The declared type also converts plain strings on assignment.
                $n = "42"
                ASSERT_EQ $n 42

                # Same crossing for decimals via FLOAT().
                LET $frac_str: STRING = ECHO 2.5
                LET $f: FLOAT = FLOAT($frac_str) + 0.25
                ASSERT_EQ $f 2.75
            "#},
                },
            ],
        },
        CommandMeta {
            name: "ASYNC",
            syntax: "ASYNC <command...> | ASYNC { <commands> } | LET $var: HANDLE = ASYNC { <commands> }",
            summary: "Run steps in a background thread.",
            description: indoc! {r#"
                Runs a command or block of commands in a background thread with
                subshell isolation.

                Mutations (ENV, WORKDIR) stay within the block. With `LET`, stores a
                task handle for `AWAIT`. Task output streams live to the parent
                stdout; a task publishes a value with an explicit `RETURN`,
                which `LET $out: TYPE = AWAIT $task` binds.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "async",
                    fence_meta: None,
                    code: indoc! {r#"
                # Inline and block forms both run in the background; AWAIT joins them.
                ASYNC ECHO "warming-up"
                LET $a: HANDLE = ASYNC ECHO "first"
                LET $b: HANDLE = ASYNC {
                    ECHO "second"
                }
                AWAIT $a
                AWAIT $b
                ASSERT_CONTAINS stdout "first"
                ASSERT_CONTAINS stdout "second"
                "#},
                },
                Example {
                    name: "async task handle",
                    fence_meta: None,
                    code: indoc! {r#"
                    LET $task: HANDLE = ASYNC {
                        ECHO "built"
                    }
                    AWAIT $task
                    ASSERT_CONTAINS stdout "built"
                "#},
                },
            ],
        },
        CommandMeta {
            name: "AWAIT",
            syntax: "AWAIT $var | LET $out: STRING = AWAIT $var",
            summary: "Join a background task.",
            description: indoc! {r#"
                Blocks until the named task completes. Propagates errors if the task failed.

                Task output streams live during the run; joining binds nothing by
                itself. `LET $out: TYPE = AWAIT $var` binds the task's explicit
                `RETURN` value instead, or `INT` 0 when the task succeeded
                without one (add `RETURN <expr>` to the task body to yield
                a value).
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "await",
                    fence_meta: None,
                    code: indoc! {r#"
                LET $task: HANDLE = ASYNC ECHO "done"
                AWAIT $task
                ASSERT_CONTAINS stdout "done"
            "#},
                },
                Example {
                    name: "await capture",
                    fence_meta: None,
                    code: indoc! {r#"
                LET $task: HANDLE = ASYNC {
                    ECHO "logged"
                    RETURN "returned"
                }

                # AWAIT binds the RETURN value, not the streamed output.
                LET $out: STRING = AWAIT $task
                ASSERT_EQ $out "returned"
            "#},
                },
            ],
        },
        CommandMeta {
            name: "CANCEL",
            syntax: "CANCEL $var",
            summary: "Synchronously cancel a background task.",
            description: indoc! {r#"
                Kills the named background task spawned via LET $var: HANDLE = ASYNC ....

                Blocking: returns only after the task thread has been joined and its OS
                process reaped, so no residual filesystem or stream mutation follows. A
                later AWAIT $var reports cancellation. Only named tasks can be cancelled.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "cancel",
                    fence_meta: None,
                    code: indoc! {r#"
                LET $task: HANDLE = ASYNC SLEEP 30s
                CANCEL $task
            "#},
                },
                Example {
                    name: "await after cancel reports cancellation",
                    fence_meta: Some("expect_error:\"was cancelled\""),
                    code: indoc! {r#"
                # A cancelled task stays cancelled: joining it reports.
                LET $task: HANDLE = ASYNC SLEEP 30s
                CANCEL $task
                AWAIT $task
            "#},
                },
            ],
        },
        CommandMeta {
            name: "TIMEOUT",
            syntax: "TIMEOUT <duration> <command...> | TIMEOUT <duration> { <commands> } | TIMEOUT <duration> AWAIT $var",
            summary: "Enforce an execution deadline.",
            description: indoc! {r#"
                Aborts the wrapped step or block with a deadline error if it exceeds the
                duration (e.g. 500ms, 10s, 2m; a bare number means seconds).

                A blocking foreground process is killed.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "timeout",
                    fence_meta: None,
                    code: indoc! {r#"
                    TIMEOUT 30s WRITE heartbeat.txt alive
                    LET $beat: STRING = READ heartbeat.txt
                    ASSERT_EQ $beat "alive"
                "#},
                },
                Example {
                    name: "timeout block",
                    fence_meta: None,
                    code: indoc! {r#"
                    TIMEOUT 30s {
                        WRITE a.txt one
                        WRITE b.txt two
                    }
                    LET $a: STRING = READ a.txt
                    LET $b: STRING = READ b.txt
                    ASSERT_EQ $a "one"
                    ASSERT_EQ $b "two"
                "#},
                },
                Example {
                    name: "deadline aborts the step",
                    fence_meta: Some("expect_error:\"TIMEOUT after\""),
                    code: indoc! {r#"
                    # 50ms expires long before the sleep does: the step dies
                    # with a deadline error instead of running out the clock.
                    TIMEOUT 50ms SLEEP 30s
                "#},
                },
                Example {
                    name: "timeout variable duration",
                    fence_meta: None,
                    code: indoc! {r#"
                    # Durations resolve at runtime, so variables work too.
                    LET $budget: DURATION = "30s"
                    TIMEOUT $budget WRITE heartbeat.txt alive

                    LET $beat: STRING = READ heartbeat.txt
                    ASSERT_EQ $beat "alive"
                "#},
                },
            ],
        },
        CommandMeta {
            name: "REMOTE",
            syntax: "REMOTE <target> [[$var, ...] [env:NAME, ...]] { <commands> }",
            summary: "Run a sealed block on a remote target.",
            description: indoc! {r#"
                Runs the braced block on the named remote target, which the
                host binds at startup (`--remote`). The block is sealed:
                only header-listed `$var` and `env:NAME` entries cross, the
                guest starts empty, and results return through declared
                transfers or `WITH_IO` pipes. The target is a static
                literal, never a variable. Unknown targets fail before
                execution starts. Bodies check twice: the host static
                pass validates the body before anything ships, and the
                guest re-validates on arrival through the same entry
                point, so a body type error fails locally, never mid-run
                on the remote. A failing guest fails the host step: EXIT
                in the body exits the guest, and the host relays the code
                to its own process exit status (see EXIT).
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "sealed header values cross, outer scope does not",
                    fence_meta: Some("mock_remote:prod"),
                    code: indoc! {r#"
                    # Only header-listed names arrive: `$version` is visible
                    # on the guest, `$secret` was never sent.
                    LET $version: STRING = "1.2.3"
                    LET $secret: STRING = "host-only"
                    REMOTE prod [$version] {
                        WRITE version.txt "{{ $version }}"
                        COPY --to-host ./version.txt back.txt
                    }

                    # The pushed file proves the guest saw the injected value.
                    LET $back: STRING = READ back.txt
                    ASSERT_EQ $back "1.2.3"
                "#},
                },
                Example {
                    name: "background tasks stream through shared pipes",
                    fence_meta: Some("mock_remote:prod"),
                    code: indoc! {r#"
                    # Two background REMOTEs share one capture pipe while a
                    # WITH_IO block holds the write end open.
                    IMPORT [STD]
                    LET $cap: PIPE
                    WITH_IO [stdout=$cap] {
                        LET $a: HANDLE = ASYNC REMOTE prod {
                            ECHO "from-a"
                        }
                        LET $b: HANDLE = ASYNC REMOTE prod {
                            ECHO "from-b"
                        }
                        AWAIT $a
                        AWAIT $b
                    }

                    # EOF ends the drain: no sentinel line, no baked-in count.
                    LET $n: INT = 0
                    WHILE !EOF($cap) {
                        WITH_IO [stdin=$cap] READ_LINE $line
                        $n = $n + 1
                    }
                    ASSERT_EQ $n 2
                "#},
                },
                Example {
                    name: "declared files cross both directions",
                    fence_meta: Some("mock_remote:prod roots:unified"),
                    code: indoc! {r#"
                    # Fetches ride in, pushes ride back. Anything undeclared
                    # never leaves its side.
                    WRITE send.txt "payload"
                    REMOTE prod {
                        COPY --from-host ./send.txt got.txt
                        LET $v: STRING = READ got.txt
                        ASSERT_EQ $v "payload"
                        WRITE back.txt "returned"
                        COPY --to-host ./back.txt back.txt
                    }

                    # The push landed on the host; guest staging is gone.
                    LET $b: STRING = READ back.txt
                    ASSERT_EQ $b "returned"
                "#},
                },
                Example {
                    name: "guest workspace selection stays on the guest",
                    fence_meta: Some("mock_remote:prod"),
                    code: indoc! {r#"
                    # WORKSPACE switches inside the block resolve against the
                    # guest staging dir, which serves as both the SNAPSHOT
                    # root and the LOCAL build context. The host selection
                    # restores when the block exits.
                    REMOTE prod {
                        WORKSPACE SNAPSHOT
                        WRITE out.txt "from-snapshot"
                        COPY --to-host ./out.txt snap.txt
                    }

                    LET $s: STRING = READ snap.txt
                    ASSERT_EQ $s "from-snapshot"
                "#},
                },
                Example {
                    name: "guest cache persists across execs",
                    fence_meta: Some("mock_remote:prod"),
                    code: indoc! {r#"
                    # WORKSPACE CACHE addresses guest-local persistent
                    # storage: the second block reads what the first wrote,
                    # while the guest staging starts empty every time.
                    REMOTE prod {
                        WORKSPACE CACHE
                        WRITE marker.txt "run-marker"
                    }
                    REMOTE prod {
                        WORKSPACE CACHE
                        LET $m: STRING = READ marker.txt
                        ASSERT_EQ $m "run-marker"
                    }
                "#},
                },
                Example {
                    name: "pushes outside staging fail closed",
                    fence_meta: Some("mock_remote:prod expect_error:\"escapes staging\""),
                    code: indoc! {r#"
                    # WORKSPACE SYSTEM exposes the whole guest machine, but
                    # transfers stay staging scoped: a push source outside
                    # staging fails loudly instead of crossing.
                    REMOTE prod {
                        WORKSPACE SYSTEM
                        COPY --to-host /no/such/file.txt got.txt
                    }
                "#},
                },
                Example {
                    name: "guest exit fails the step, host survives",
                    fence_meta: Some(
                        "mock_remote:prod expect_error:\"REMOTE 'prod' exited with code 3\"",
                    ),
                    code: indoc! {r#"
                    # EXIT in the body exits the guest with code 3. The
                    # host is unaffected: it keeps executing after the
                    # guest is already dead, and the failure surfaces as
                    # a host step error carrying the same code. The CLI
                    # then exits with it, so the guest code reaches the OS.
                    LET $t: HANDLE = ASYNC REMOTE prod {
                        EXIT 3
                    }
                    WRITE host-alive.txt "still here"
                    AWAIT $t
                "#},
                },
                Example {
                    name: "remote block without a runner",
                    fence_meta: Some("expect_error:\"has no registered session\""),
                    code: indoc! {r#"
                    # Without a bound session, the block bails instead of
                    # executing anywhere unexpected.
                    REMOTE prod {
                        ECHO "do something on remote"
                    }
                "#},
                },
            ],
        },
        CommandMeta {
            name: "FUNC",
            syntax: "FUNC NAME([$param: TYPE, ...]) { <commands> }",
            summary: "Define a user function.",
            description: indoc! {r#"
                Defines a user function with UPPERCASE name and explicitly typed
                parameters.

                Params bind by position, converting each argument to its
                declared parameter type before the body runs.
                Bodies run in a fresh variable scope; LETs inside do not leak. A nested
                FUNC definition is scoped to its block and reverts on exit. Names share
                one namespace with native and host-registered functions, which a FUNC
                may never shadow.

                Functions resolve like variables: a name is visible from its
                definition line, so recursion works but mutual recursion does
                not (the second name does not exist while the first body
                lowers). Calls name their module (`STD::GLOB(...)`) unless
                imported; see IMPORT.

                Invoke any function with one syntax: `NAME(...)` as a statement
                (discarding the value) or `LET $var: TYPE = NAME(...)` to capture
                the RETURN value (fallthrough without RETURN captures as "").
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[
                Example {
                    name: "func def call",
                    fence_meta: None,
                    code: indoc! {r#"
                FUNC GREET($name: STRING) {
                  RETURN $name
                }

                LET $res: STRING = GREET("ada")
                ASSERT_EQ $res "ada"

                # Statement form: parens stay, the value drops.
                GREET("bex")
            "#},
                },
                Example {
                    name: "call with pipes",
                    fence_meta: None,
                    code: indoc! {r#"
                # A pipe handle travels into a function as a typed argument
                # and is usable as a binding target in both directions.
                # `LET $p: PIPE` mints the handle; `$p` passes it on.
                FUNC DRAIN($q: PIPE) {
                  WITH_IO [stdin=$q] READ_LINE $line
                  RETURN $line
                }

                LET $p: PIPE
                WITH_IO [stdout=$p] ECHO "payload"

                LET $got: STRING = DRAIN($p)
                ASSERT_EQ $got "payload"
            "#},
                },
            ],
        },
        CommandMeta {
            name: "RETURN",
            syntax: "RETURN [<expr>]",
            summary: "Return a value from a function, task, or inline block.",
            description: indoc! {r#"
                Ends the nearest enclosing boundary with a value: a function
                call, an `ASYNC` task (bound by `LET $o = AWAIT $t`), or an
                inline `LET` block. Bare `RETURN` with no expression yields
                `""`.

                Falling off the end without RETURN yields "". RETURN with no
                enclosing boundary (including at top level) is an error; use
                EXIT or ECHO there.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[Example {
                name: "return",
                fence_meta: None,
                code: indoc! {r#"
                FUNC PICK($flag: BOOL) {
                  IF $flag {
                    RETURN "yes"
                  }
                  RETURN "no"
                }

                LET $res: STRING = PICK(true)
                ASSERT_EQ $res "yes"

                # Fallthrough without RETURN yields its own value.
                LET $no: STRING = PICK(false)
                ASSERT_EQ $no "no"
            "#},
            }],
        },
        CommandMeta {
            name: "WHILE",
            syntax: "WHILE <bool-expr> { <commands> }",
            summary: "Loop while a condition holds.",
            description: indoc! {r#"
                Re-evaluates a Bool condition each iteration (same is_truthy rule as IF;
                non-Bool is a type error).

                Each iteration runs in a fresh scope; mutate outer state with $var = ...
                so the next check observes it. BREAK exits the loop; CONTINUE skips to
                the next check.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[Example {
                name: "while loop",
                fence_meta: None,
                code: indoc! {r#"
                # The condition re-evaluates every iteration: three passes, then stop.
                LET $n: INT = 0
                WHILE $n < 3 {
                  WRITE tick.txt "{{ $n }}"
                  $n = $n + 1
                }

                ASSERT_EQ $n 3
                LET $tick: STRING = READ tick.txt
                ASSERT_EQ $tick "2"
            "#},
            }],
        },
        CommandMeta {
            name: "BREAK",
            syntax: "BREAK",
            summary: "Exit the innermost loop.",
            description: indoc! {r#"
                Exits the innermost enclosing FOR or WHILE loop.

                BREAK outside a loop, or across a FUNC or ASYNC boundary, is an error.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[Example {
                name: "break",
                fence_meta: None,
                code: indoc! {r#"
                # BREAK leaves after the first pass: only "a" is written.
                FOR $x: STRING IN ["a", "b"] {
                  WRITE picked.txt "{{ $x }}"
                  BREAK
                }

                LET $body: STRING = READ picked.txt
                ASSERT_EQ $body "a"
            "#},
            }],
        },
        CommandMeta {
            name: "CONTINUE",
            syntax: "CONTINUE",
            summary: "Skip to the next loop iteration.",
            description: indoc! {r#"
                Skips the rest of the innermost enclosing FOR or WHILE body and starts
                the next iteration.

                CONTINUE outside a loop, or across a FUNC or ASYNC boundary, is an error.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[Example {
                name: "continue",
                fence_meta: None,
                code: indoc! {r#"
                # CONTINUE skips the write on "a": only "b" lands.
                FOR $x: STRING IN ["a", "b"] {
                  IF $x == "a" {
                    CONTINUE
                  }
                  WRITE picked.txt "{{ $x }}"
                }

                LET $body: STRING = READ picked.txt
                ASSERT_EQ $body "b"
            "#},
            }],
        },
        CommandMeta {
            name: KEYWORD_IMPORT,
            syntax: "IMPORT [<module>, ...] | IMPORT <module>",
            summary: "Bring module functions into bare-call scope.",
            description: indoc! {r#"
                Every function call names its module (`STD::GLOB(...)`,
                `MOCK::READ_CSV(...)`) unless the module is imported:
                `IMPORT [STD]` lets the rest of the scope call `GLOB(...)`
                bare. Calls resolve at parse time against `SCRIPT`
                definitions first, then imported modules; unknown modules,
                unknown functions, and unimported bare calls are parse
                errors, never runtime surprises.

                IMPORT is a lowering directive, not a step: it applies from
                its line to the enclosing block exit, then reverts, exactly
                like `LET` scoping but with no runtime footprint. Guards do
                not apply to it. Two imported modules exporting one name is
                an ambiguity error: qualify the call instead.

                `EXPORT` is reserved for future script-module support and
                cannot be used yet.
            "#},
            args: &[],
            flags: &[],
            default_output: None,
            examples: &[Example {
                name: "import",
                fence_meta: None,
                code: indoc! {r#"
                # Calls name their module (STD::GLOB); IMPORT [STD] drops the prefix.
                WRITE a.txt "hi \{{ env:WHO }}!"
                IMPORT [STD]
                FOR $x: STRING IN GLOB("*.txt") { EXPAND $x WHO=World }
                ASSERT_CONTAINS stdout "hi World!"
            "#},
            }],
        },
    ]
}

// ── Display ────────────────────────────────────────────────────────────────

impl fmt::Display for StepKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            StepKind::InheritEnv { keys } => write!(f, "INHERIT_ENV [{}]", keys.join(", ")),
            StepKind::Workdir(a) => write!(f, "WORKDIR {}", fmt_value(a, quote_arg)),
            StepKind::Workspace(t) => write!(f, "WORKSPACE {}", t),
            StepKind::Env { key, value } => {
                write!(f, "ENV {}={}", key, fmt_value(value, quote_arg))
            }
            StepKind::Run(c) => write!(f, "RUN {}", fmt_value(c, quote_run)),
            StepKind::RunExec { argv } => {
                let parts: Vec<String> = argv.iter().map(fmt_exec_arg).collect();
                write!(f, "RUN [{}]", parts.join(", "))
            }
            StepKind::Echo(m) => write!(f, "ECHO {}", fmt_value(m, quote_msg)),
            StepKind::Copy {
                from_workspace,
                from_host,
                to_host,
                from,
                to,
            } => {
                let mut head = String::from("COPY");
                if let Some(target) = from_workspace {
                    head.push_str(&format!(" --from-workspace {target}"));
                }
                if *from_host {
                    head.push_str(" --from-host");
                }
                if *to_host {
                    head.push_str(" --to-host");
                }
                write!(
                    f,
                    "{head} {} {}",
                    fmt_value(from, quote_arg),
                    fmt_value(to, quote_arg)
                )
            }
            StepKind::Symlink {
                from_workspace,
                from,
                to,
            } => {
                if let Some(target) = from_workspace {
                    write!(
                        f,
                        "SYMLINK --from-workspace {} {} {}",
                        target,
                        fmt_value(from, quote_arg),
                        fmt_value(to, quote_arg)
                    )
                } else {
                    write!(
                        f,
                        "SYMLINK {} {}",
                        fmt_value(from, quote_arg),
                        fmt_value(to, quote_arg)
                    )
                }
            }
            StepKind::Mkdir(a) => write!(f, "MKDIR {}", fmt_value(a, quote_arg)),
            StepKind::Ls(a) => {
                write!(f, "LS")?;
                if let Some(x) = a {
                    write!(f, " {}", fmt_value(x, quote_arg))?;
                }
                Ok(())
            }
            StepKind::Cwd => write!(f, "CWD"),
            StepKind::Read(a) => {
                write!(f, "READ")?;
                if let Some(x) = a {
                    write!(f, " {}", fmt_value(x, quote_arg))?;
                }
                Ok(())
            }
            StepKind::ReadLine { var } => write!(f, "READ_LINE ${}", var),
            StepKind::Write { path, contents } => {
                write!(f, "WRITE {}", fmt_value(path, quote_arg))?;
                if let Some(b) = contents {
                    write!(f, " {}", fmt_value(b, quote_msg))?;
                }
                Ok(())
            }
            StepKind::Append { path, contents } => {
                write!(f, "APPEND {}", fmt_value(path, quote_arg))?;
                if let Some(b) = contents {
                    write!(f, " {}", fmt_value(b, quote_msg))?;
                }
                Ok(())
            }
            StepKind::Expand { path, overrides } => {
                write!(f, "EXPAND")?;
                if let Some(p) = path {
                    write!(f, " {}", fmt_value(p, quote_arg))?;
                }
                for (k, v) in overrides {
                    write!(f, " {}={}", k, fmt_value(v, quote_arg))?;
                }
                Ok(())
            }
            StepKind::AssertEq {
                hash,
                actual,
                expected,
            } => {
                if let Some(d) = hash {
                    write!(f, "ASSERT_EQ --hash {d} {}", fmt_assert_target(actual))?;
                } else {
                    write!(
                        f,
                        "ASSERT_EQ {} {}",
                        fmt_assert_target(actual),
                        fmt_value(
                            expected
                                .as_ref()
                                .expect("Display of ASSERT_EQ without --hash needs expected"),
                            quote_msg
                        )
                    )?;
                }
                Ok(())
            }
            StepKind::AssertContains { haystack, needle } => write!(
                f,
                "ASSERT_CONTAINS {} {}",
                fmt_assert_target(haystack),
                fmt_value(needle, quote_msg)
            ),
            StepKind::WithIo { bindings, cmd } => {
                let p: Vec<String> = bindings.iter().map(fmt_io).collect();
                write!(f, "WITH_IO [{}] {}", p.join(", "), cmd)
            }
            StepKind::WithIoBlock { bindings } => {
                let p: Vec<String> = bindings.iter().map(fmt_io).collect();
                write!(f, "WITH_IO [{}] {{...}}", p.join(", "))
            }
            StepKind::CopyGit {
                rev,
                from,
                to,
                include_dirty,
            } => {
                if *include_dirty {
                    write!(
                        f,
                        "COPY_GIT --include-dirty {} {} {}",
                        fmt_value(rev, quote_arg),
                        fmt_value(from, quote_arg),
                        fmt_value(to, quote_arg)
                    )
                } else {
                    write!(
                        f,
                        "COPY_GIT {} {} {}",
                        fmt_value(rev, quote_arg),
                        fmt_value(from, quote_arg),
                        fmt_value(to, quote_arg)
                    )
                }
            }
            StepKind::HashSha256 { path } => {
                write!(f, "HASH_SHA256 {}", fmt_value(path, quote_arg))
            }
            StepKind::Exit(code) => write!(f, "EXIT {}", fmt_raw_arg(code)),
            StepKind::Sleep { duration } => write!(f, "SLEEP {}", fmt_raw_arg(duration)),
            StepKind::ListAppend { list, item } => {
                write!(f, "LIST_APPEND ${} {}", list, fmt_raw_arg(item))
            }
            StepKind::For {
                key_var,
                key_type,
                var,
                var_type,
                in_expr,
                body,
            } => {
                match key_var {
                    Some(k) => {
                        let kt = key_type.as_deref().unwrap_or("STRING");
                        write!(
                            f,
                            "FOR ${}: {}, ${}: {} IN {} {{",
                            k, kt, var, var_type, in_expr
                        )?
                    }
                    None => write!(f, "FOR ${}: {} IN {} {{", var, var_type, in_expr)?,
                }
                for s in body {
                    write!(f, "\n    {}", s)?;
                }
                write!(f, "\n}}")
            }
            StepKind::If {
                cond,
                then_body,
                else_ifs,
                else_body,
            } => {
                write!(f, "IF {} {{", cond)?;
                for s in then_body {
                    write!(f, "\n    {}", s)?;
                }
                write!(f, " }}")?;
                for (c, b) in else_ifs {
                    write!(f, " ELSE IF {} {{", c)?;
                    for s in b {
                        write!(f, "\n    {}", s)?;
                    }
                    write!(f, " }}")?;
                }
                if let Some(b) = else_body {
                    write!(f, " ELSE {{")?;
                    for s in b {
                        write!(f, "\n    {}", s)?;
                    }
                    write!(f, " }}")?;
                }
                Ok(())
            }
            StepKind::Assign {
                var,
                decl_type,
                expr,
            } => {
                // Bare pipe declarations round-trip without an initializer.
                if matches!(expr, Expr::FreshPipe) {
                    write!(f, "LET ${}: {}", var, decl_type)
                } else {
                    write!(f, "LET ${}: {} = {}", var, decl_type, expr)
                }
            }
            StepKind::Set { var, expr } => write!(f, "${} = {}", var, expr),
            StepKind::AssignCapture {
                var,
                decl_type,
                cmd,
            } => {
                write!(f, "LET ${}: {} = {}", var, decl_type, cmd)
            }
            StepKind::AsyncBlock { body } => {
                write!(f, "ASYNC {{")?;
                for s in body {
                    write!(f, "\n    {}", s)?;
                }
                write!(f, "\n}}")
            }
            StepKind::AssignAsync {
                var,
                decl_type,
                body,
            } => {
                write!(f, "LET ${}: {} = ASYNC {{", var, decl_type)?;
                for s in body {
                    write!(f, "\n    {}", s)?;
                }
                write!(f, "\n}}")
            }
            StepKind::Await { var } => write!(f, "AWAIT ${}", var),
            StepKind::AwaitCapture {
                out_var,
                out_type,
                task_var,
            } => {
                write!(f, "LET ${}: {} = AWAIT ${}", out_var, out_type, task_var)
            }
            StepKind::Cancel { var } => write!(f, "CANCEL ${}", var),
            StepKind::Timeout { duration, body } => {
                let budget = fmt_raw_arg(duration);
                if body.len() == 1 {
                    write!(f, "TIMEOUT {} {}", budget, body[0].kind)
                } else {
                    write!(f, "TIMEOUT {} {{", budget)?;
                    for s in body {
                        write!(f, "\n    {}", s)?;
                    }
                    write!(f, "\n}}")
                }
            }
            StepKind::RemoteBlock {
                target,
                vars,
                env,
                body,
            } => {
                let mut entries: Vec<String> = vars.iter().map(|v| format!("${v}")).collect();
                entries.extend(env.iter().map(|e| format!("env:{e}")));
                if entries.is_empty() {
                    write!(f, "REMOTE {target} {{")?;
                } else {
                    write!(f, "REMOTE {target} [{}] {{", entries.join(", "))?;
                }
                for s in body {
                    write!(f, "\n    {}", s)?;
                }
                write!(f, "\n}}")
            }
            StepKind::FuncDef { name, params, body } => {
                let ps: Vec<String> = params
                    .iter()
                    .map(|(p, t)| format!("${}: {}", p, t))
                    .collect();
                write!(f, "FUNC {}({}) {{", name, ps.join(", "))?;
                for s in body {
                    write!(f, "\n    {}", s)?;
                }
                write!(f, "\n}}")
            }
            StepKind::Call { name, args } => {
                let ps: Vec<String> = args.iter().map(|a| format!("{}", a)).collect();
                write!(f, "{}({})", name, ps.join(", "))
            }
            StepKind::Return { expr } => write!(f, "RETURN {}", expr),
            StepKind::TypeAlias { name, target } => write!(f, "TYPE {} = {}", name, target),
            StepKind::While { cond, body } => {
                write!(f, "WHILE {} {{", cond)?;
                for s in body {
                    write!(f, "\n    {}", s)?;
                }
                write!(f, "\n}}")
            }
            StepKind::Break => write!(f, "BREAK"),
            StepKind::Continue => write!(f, "CONTINUE"),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::command::{format_duration, parse_duration};
    use crate::parser::parse_script;

    fn parse_err(script: &str) -> String {
        parse_script(script, lower_command)
            .expect_err("script must fail to parse")
            .to_string()
    }

    #[test]
    fn malformed_with_io_binding_names_the_bad_binding() {
        let err = parse_err("WITH_IO [stdout=discard] ECHO \"test\"\n");
        assert!(err.contains("invalid syntax for command WITH_IO"), "{err}");
        assert!(!err.contains("unknown command"), "{err}");
        assert!(err.contains("stdout=discard"), "{err}");
        assert!(err.contains("[stdout=$p]"), "{err}");
    }

    #[test]
    fn connect_is_unknown_command() {
        // `CONNECT` left core for the NET plugin: the builtin name no
        // longer resolves. Use `NET_CONNECT` with explicit pipes.
        let err = parse_err("CONNECT 127.0.0.1:8080\n");
        assert!(err.contains("unknown command"), "{err}");
        assert!(err.contains("CONNECT"), "{err}");
    }

    #[test]
    fn listen_is_unknown_command() {
        // `LISTEN` left core for the NET plugin: the builtin name no
        // longer resolves. Use `NET_LISTEN` for a listener handle.
        let err = parse_err("LISTEN 127.0.0.1:8080\n");
        assert!(err.contains("unknown command"), "{err}");
        assert!(err.contains("LISTEN"), "{err}");
    }

    #[test]
    fn await_without_task_variable_points_at_syntax() {
        let err = parse_err("AWAIT ECHO \"test\"\n");
        assert!(err.contains("invalid syntax for command AWAIT"), "{err}");
        assert!(!err.contains("unknown command"), "{err}");
        assert!(err.contains("AWAIT $t"), "{err}");
        assert!(err.contains("ECHO"), "{err}");
    }

    #[test]
    fn bare_let_without_type_points_at_typed_syntax() {
        let err = parse_err("LET $x = 1\n");
        assert!(err.contains("invalid syntax for command LET"), "{err}");
        assert!(err.contains("LET $name: STRING = <expr>"), "{err}");
    }

    #[test]
    fn workspace_accepts_all_four_targets_uppercase_only() {
        // WORKSPACE targets are uppercase-only, like every other DSL
        // keyword argument: lowercase spellings are rejected.
        for (spelling, target) in [
            ("SNAPSHOT", WorkspaceTarget::Snapshot),
            ("LOCAL", WorkspaceTarget::Local),
            ("CACHE", WorkspaceTarget::Cache { local: false }),
            ("SYSTEM", WorkspaceTarget::System),
        ] {
            let steps =
                parse_script(&format!("WORKSPACE {spelling}\n"), lower_command).expect("parses");
            assert_eq!(steps.len(), 1);
            assert_eq!(steps[0].kind, StepKind::Workspace(target.clone()));
            assert_eq!(steps[0].kind.to_string(), format!("WORKSPACE {target}"));
        }
        for spelling in ["snapshot", "local", "cache", "system"] {
            let err = parse_err(&format!("WORKSPACE {spelling}\n"));
            assert!(
                err.contains("expected one of SNAPSHOT|LOCAL|CACHE|SYSTEM"),
                "{spelling}: {err}"
            );
        }
        let err = parse_err("WORKSPACE REMOTE\n");
        assert!(
            err.contains("expected one of SNAPSHOT|LOCAL|CACHE|SYSTEM"),
            "{err}"
        );

        // `--local` selects the project-tree cache and round-trips
        // through Display; anywhere else it is rejected.
        let steps = parse_script("WORKSPACE CACHE --local\n", lower_command).expect("parses");
        assert_eq!(
            steps[0].kind,
            StepKind::Workspace(WorkspaceTarget::Cache { local: true })
        );
        assert_eq!(steps[0].kind.to_string(), "WORKSPACE CACHE --local");
        for bad in [
            "WORKSPACE SNAPSHOT --local\n",
            "WORKSPACE LOCAL --local\n",
            "WORKSPACE SYSTEM --local\n",
        ] {
            let err = parse_err(bad);
            assert!(err.contains("--local requires CACHE"), "{bad}: {err}");
        }
    }

    #[test]
    fn copy_from_workspace_selects_source_root() {
        for (spelling, target) in [
            ("SNAPSHOT", WorkspaceTarget::Snapshot),
            ("LOCAL", WorkspaceTarget::Local),
            ("CACHE", WorkspaceTarget::Cache { local: false }),
            ("SYSTEM", WorkspaceTarget::System),
        ] {
            let steps = parse_script(
                &format!("COPY --from-workspace {spelling} a.txt b.txt\n"),
                lower_command,
            )
            .expect("parses");
            assert!(
                matches!(&steps[0].kind, StepKind::Copy { from_workspace: Some(t), .. } if *t == target),
                "unexpected lowering for {spelling}: {:?}",
                steps[0].kind
            );
        }
        // The `=` form carries the value inline on one token: `--`
        // tokens never match `assignment`, so `strip_flags` splits it.
        for script in [
            "COPY --from-workspace=CACHE a.txt b.txt\n",
            "COPY --from-workspace=\"CACHE\" a.txt b.txt\n",
        ] {
            let steps = parse_script(script, lower_command).expect("parses");
            assert!(
                matches!(
                    &steps[0].kind,
                    StepKind::Copy {
                        from_workspace: Some(WorkspaceTarget::Cache { local: false }),
                        ..
                    }
                ),
                "unexpected lowering for {script:?}: {:?}",
                steps[0].kind
            );
        }
        // An absent flag means the build-context default.
        let steps = parse_script("COPY a.txt b.txt\n", lower_command).expect("parses");
        assert!(
            matches!(
                &steps[0].kind,
                StepKind::Copy {
                    from_workspace: None,
                    ..
                }
            ),
            "unexpected lowering: {:?}",
            steps[0].kind
        );
        // Unknown and lowercase values are rejected uppercase-only.
        for bad in ["REMOTE", "local"] {
            let err = parse_err(&format!("COPY --from-workspace {bad} a.txt b.txt\n"));
            assert!(err.contains("unknown workspace source"), "{bad}: {err}");
        }
    }

    #[test]
    fn symlink_from_workspace_selects_source_root() {
        for (spelling, target) in [
            ("SNAPSHOT", WorkspaceTarget::Snapshot),
            ("LOCAL", WorkspaceTarget::Local),
            ("CACHE", WorkspaceTarget::Cache { local: false }),
            ("SYSTEM", WorkspaceTarget::System),
        ] {
            let steps = parse_script(
                &format!("SYMLINK --from-workspace {spelling} a.txt b.txt\n"),
                lower_command,
            )
            .expect("parses");
            assert!(
                matches!(&steps[0].kind, StepKind::Symlink { from_workspace: Some(t), .. } if *t == target),
                "unexpected lowering for {spelling}: {:?}",
                steps[0].kind
            );
            let roundtrip = steps[0].kind.to_string();
            assert!(
                roundtrip.contains("--from-workspace"),
                "display should round-trip the flag: {roundtrip}"
            );
        }
        let steps = parse_script("SYMLINK a.txt b.txt\n", lower_command).expect("parses");
        assert!(
            matches!(
                &steps[0].kind,
                StepKind::Symlink {
                    from_workspace: None,
                    ..
                }
            ),
            "unexpected lowering: {:?}",
            steps[0].kind
        );
        for bad in ["REMOTE", "local"] {
            let err = parse_err(&format!("SYMLINK --from-workspace {bad} a.txt b.txt\n"));
            assert!(err.contains("unknown workspace source"), "{bad}: {err}");
        }
        // The `=` form carries the value inline, like COPY.
        let steps = parse_script(
            "SYMLINK --from-workspace=LOCAL a.txt b.txt\n",
            lower_command,
        )
        .expect("parses");
        assert!(
            matches!(
                &steps[0].kind,
                StepKind::Symlink {
                    from_workspace: Some(WorkspaceTarget::Local),
                    ..
                }
            ),
            "unexpected lowering: {:?}",
            steps[0].kind
        );
    }

    #[test]
    fn dash_dash_equals_tokens_bypass_assignment() {
        // `--` tokens never match `assignment`: flags keep their `=`
        // form as one argument, while other commands see the same text.
        let steps = parse_script("ENV --foo=bar\n", lower_command).expect("parses");
        let StepKind::Env { key, value } = &steps[0].kind else {
            panic!("expected Env, got {:?}", steps[0].kind);
        };
        assert_eq!(key, "--foo");
        assert_eq!(value.as_str(), "bar");

        let steps = parse_script("RUN echo --foo=bar\n", lower_command).expect("parses");
        let StepKind::Run(cmd) = &steps[0].kind else {
            panic!("expected Run, got {:?}", steps[0].kind);
        };
        assert!(
            cmd.as_str().contains("--foo=bar"),
            "unexpected RUN lowering: {cmd:?}"
        );

        let digest = "08135c1b6349b0e4f894c36221952f0de00e6b4d82f80895abf359755e77103c";
        let steps = parse_script(&format!("ASSERT_EQ --hash={digest} $body\n"), lower_command)
            .expect("parses");
        let StepKind::AssertEq { hash, .. } = &steps[0].kind else {
            panic!("expected AssertEq, got {:?}", steps[0].kind);
        };
        assert_eq!(hash.as_deref(), Some(digest));

        // EXPAND still treats `--k=v` positionals as overrides.
        let steps = parse_script("EXPAND --k=v\n", lower_command).expect("parses");
        let StepKind::Expand { path, overrides } = &steps[0].kind else {
            panic!("expected Expand, got {:?}", steps[0].kind);
        };
        assert!(path.is_none());
        assert_eq!(overrides.len(), 1);
        assert_eq!(overrides[0].0.as_str(), "--k");
    }

    #[test]
    fn space_before_paren_is_not_a_call() {
        // The call head and `(` must be contiguous: `ECHO (1 + 2)` is an
        // instruction, never a function invocation.
        let steps = parse_script("ECHO (1 + 2)\n", lower_command).expect("parses");
        assert_eq!(steps.len(), 1);
        assert!(
            !matches!(steps[0].kind, crate::ast::StepKind::Call { .. }),
            "space before paren must not route to a call: {:?}",
            steps[0].kind
        );
    }

    #[test]
    fn unknown_type_tag_parses_as_custom() {
        // Open type tags: declarations carry plain names and resolve
        // against the descriptor table at runtime, so host types need no
        // grammar change (see the custom_types core tests).
        let steps = parse_script("LET $x: FOO = 1\n", lower_command).expect("custom tag parses");
        let StepKind::Assign { decl_type, .. } = &steps[0].kind else {
            panic!("expected Assign, got {:?}", steps[0].kind);
        };
        assert_eq!(decl_type, "FOO");
    }

    #[test]
    fn bare_for_without_types_is_rejected() {
        let err = parse_err("FOR $i IN [1] { ECHO hi }\n");
        assert!(err.contains("FOR requires explicit types"), "{err}");
    }

    #[test]
    fn mutate_statement_parses_without_keyword() {
        let steps = parse_script("$y = 2\n", lower_command).expect("mutation parses");
        assert!(matches!(steps[0].kind, StepKind::Set { .. }));
    }

    #[test]
    fn set_keyword_is_rejected_with_mutation_hint() {
        let err = parse_err("SET $y = 2\n");
        assert!(err.contains("not a keyword"), "{err}");
        assert!(err.contains("$var = <expr>"), "{err}");
    }

    #[test]
    fn structural_fallthrough_commits_per_keyword() {
        for (script, cmd) in [
            ("CANCEL foo\n", "CANCEL"),
            ("TIMEOUT foo\n", "TIMEOUT"),
            ("FOR foo\n", "FOR"),
            ("IF foo\n", "IF"),
            ("LET foo\n", "LET"),
            // NOTE: INHERIT_ENV is dual-registered as a leaf command
            // (`INHERIT_ENV <key>...`), so `INHERIT_ENV foo` lowers
            // successfully instead of erroring : excluded here.
            ("ASYNC\n", "ASYNC"),
            ("ELSE foo\n", "ELSE"),
        ] {
            let err = parse_err(script);
            assert!(
                err.contains(&format!("invalid syntax for command {cmd}")),
                "{cmd}: {err}"
            );
            assert!(!err.contains("unknown command"), "{cmd}: {err}");
        }
    }

    #[test]
    fn leaf_arity_errors_carry_invalid_syntax_prefix() {
        let err = parse_err("SLEEP 1s 2s\n");
        assert!(err.contains("invalid syntax for command SLEEP"), "{err}");
        assert!(!err.contains("unknown command"), "{err}");
    }

    #[test]
    fn list_append_lowers_variable_and_item() {
        let steps = parse_script("LIST_APPEND $items \"hi\"\n", lower_command).expect("parses");
        let StepKind::ListAppend { list, item } = &steps[0].kind else {
            panic!("expected ListAppend, got {:?}", steps[0].kind);
        };
        assert_eq!(list, "items");
        assert!(matches!(item, Arg::String(s, _) if s == "hi"));
        assert_eq!(steps[0].kind.to_string(), "LIST_APPEND $items \"hi\"");
    }

    #[test]
    fn list_append_rejects_wrong_arity() {
        for script in [
            "LIST_APPEND\n",
            "LIST_APPEND $items\n",
            "LIST_APPEND $items \"a\" \"b\"\n",
        ] {
            let err = parse_err(script);
            assert!(
                err.contains("invalid syntax for command LIST_APPEND"),
                "{script}: {err}"
            );
            assert!(!err.contains("unknown command"), "{script}: {err}");
        }
    }

    #[test]
    fn list_append_rejects_non_variable_target() {
        let err = parse_err("LIST_APPEND items \"a\"\n");
        assert!(
            err.contains("invalid syntax for command LIST_APPEND"),
            "{err}"
        );
        assert!(!err.contains("unknown command"), "{err}");
    }

    #[test]
    fn read_line_rejects_bare_word_target() {
        // The `$var` shape lives in the lower function now that the
        // central vocabulary holds value types only: a missing sigil
        // fails lowering with the command-specific error.
        let err = parse_err("READ_LINE reply\n");
        assert!(err.contains("READ_LINE requires a $variable"), "{err}");
        assert!(!err.contains("unknown command"), "{err}");
    }

    #[test]
    fn genuinely_unknown_command_keeps_bare_message() {
        let err = parse_err("FROBNICATE hi\n");
        assert!(err.contains("unknown command: FROBNICATE"), "{err}");
        assert!(!err.contains("did you mean"), "{err}");
    }

    #[test]
    fn lowercase_command_suggests_uppercase() {
        // Lowercase never reaches lowering through `parse_script` (the
        // grammar rejects it with its own uppercase hint), so exercise the
        // public `lower_command` dispatcher directly.
        let err = lower_command("echo", vec![Arg::String("hi".to_string(), false)])
            .expect_err("must fail")
            .to_string();
        assert!(err.contains("unknown command: echo"), "{err}");
        assert!(err.contains("did you mean `ECHO`"), "{err}");
    }

    #[test]
    fn func_def_requires_typed_uppercase_name() {
        let steps = parse_script(
            "FUNC GREET($name: STRING) {\n  RETURN $name\n}\n",
            lower_command,
        )
        .expect("func def parses");
        let StepKind::FuncDef { name, params, body } = &steps[0].kind else {
            panic!("expected FuncDef, got {:?}", steps[0].kind);
        };
        assert_eq!(name, "GREET");
        assert_eq!(
            params,
            &vec![("name".to_string(), "STRING".to_string())],
            "{params:?}"
        );
        assert!(matches!(body[0].kind, StepKind::Return { .. }));
    }

    #[test]
    fn lowercase_func_name_is_rejected() {
        let err = parse_err("FUNC greet($x: STRING) {\n  RETURN $x\n}\n");
        assert!(err.contains("FUNC"), "{err}");
    }

    #[test]
    fn call_and_while_lower_correctly() {
        let steps = parse_script(
            "FUNC GREET($name: STRING) {\n  RETURN $name\n}\nGREET(\"ada\")\n",
            lower_command,
        )
        .expect("call parses");
        assert!(
            matches!(&steps[1].kind, StepKind::Call { name, .. } if name == "SCRIPT::GREET"),
            "{:?}",
            steps[1].kind
        );
        let steps = parse_script(
            "FUNC GREET($a: STRING, $b: STRING) {\n  RETURN $a\n}\nGREET(\"ada\", \"bex\")\n",
            lower_command,
        )
        .expect("spaced call parses");
        assert!(
            matches!(&steps[1].kind, StepKind::Call { name, args } if name == "SCRIPT::GREET" && args.len() == 2),
            "{:?}",
            steps[1].kind
        );
        let steps = parse_script(
            indoc! {r#"
                WHILE !$done {
                  BREAK
                }
            "#},
            lower_command,
        )
        .expect("while parses");
        let StepKind::While { body, .. } = &steps[0].kind else {
            panic!("expected While, got {:?}", steps[0].kind);
        };
        assert!(matches!(body[0].kind, StepKind::Break));
    }

    #[test]
    fn let_capture_call_and_async_call_lower() {
        // A bare `NAME(...)` on the LET RHS stays an expression assignment;
        // only ASYNC/TIMEOUT/command captures produce AssignCapture.
        let steps = parse_script(
            indoc! {r#"
                FUNC GREET($name: STRING) {
                  RETURN $name
                }
                LET $r: STRING = GREET("ada")
            "#},
            lower_command,
        )
        .expect("capture call parses");
        let StepKind::Assign { var, expr, .. } = &steps[1].kind else {
            panic!("expected Assign, got {:?}", steps[1].kind);
        };
        assert_eq!(var, "r");
        assert!(
            matches!(expr, Expr::Call { name, .. } if name == "SCRIPT::GREET"),
            "{expr:?}"
        );
        let steps = parse_script(
            "FUNC GREET($name: STRING) {\n  RETURN $name\n}\nLET $t: HANDLE = ASYNC GREET(\"a\")\n",
            lower_command,
        )
        .expect("async call parses");
        assert!(
            matches!(&steps[1].kind, StepKind::AssignAsync { .. }),
            "{:?}",
            steps[1].kind
        );
    }

    #[test]
    fn multiline_call_args_span_lines() {
        // Regression: long invocations (e.g. 4-arg SSH_SERVE with an
        // options map) may put one argument per line. Bracket interiors
        // tolerate linebreaks while statement structure stays single-line.
        let steps = parse_script(
            "FUNC SERVE($b: STRING, $u: STRING, $p: STRING, $o: MAP) {\n  RETURN $b\n}\nLET $m: MAP = SERVE(\n  \"127.0.0.1:2241\",\n  \"test\",\n  \"test123\", {\n    key_path: \"test_key\"\n  }\n)\n",
            lower_command,
        )
        .expect("multiline call parses");
        let StepKind::Assign { expr, .. } = &steps[1].kind else {
            panic!("expected Assign, got {:?}", steps[1].kind);
        };
        let Expr::Call { name, args } = expr else {
            panic!("expected Call expr, got {expr:?}");
        };
        assert_eq!(name, "SCRIPT::SERVE");
        assert_eq!(args.len(), 4);
        assert!(matches!(&args[3], Expr::Map(entries) if entries.len() == 1));
        // Display stays single-line; reparsing the same text is identical.
        let rendered = steps[1].to_string();
        assert!(!rendered.contains('\n'), "{rendered}");
        let script = "FUNC SERVE($b: STRING, $u: STRING, $p: STRING, $o: MAP) {\n  RETURN $b\n}\nLET $m: MAP = SERVE(\n  \"127.0.0.1:2241\",\n  \"test\",\n  \"test123\", {\n    key_path: \"test_key\"\n  }\n)\n";
        let again = parse_script(script, lower_command).expect("reparse ok");
        assert_eq!(again, steps);
    }

    #[test]
    fn multiline_bare_call_and_list_span_lines() {
        let steps = parse_script(
            indoc! {r#"
                FUNC GREET($a: STRING) {
                  RETURN $a
                }
                GREET(
                  "ada"
                )
            "#},
            lower_command,
        )
        .expect("multiline bare call parses");
        let StepKind::Call { name, args } = &steps[1].kind else {
            panic!("expected Call, got {:?}", steps[1].kind);
        };
        assert_eq!(name, "SCRIPT::GREET");
        assert_eq!(args.len(), 1);
        let steps = parse_script(
            indoc! {r#"
                LET $l: LIST = [
                  "a",
                  "b"
                ]
            "#},
            lower_command,
        )
        .expect("multiline list parses");
        let StepKind::Assign { expr, .. } = &steps[0].kind else {
            panic!("expected Assign, got {:?}", steps[0].kind);
        };
        assert!(
            matches!(expr, Expr::List(items) if items.len() == 2),
            "{expr:?}"
        );
    }

    #[test]
    fn parse_duration_units() {
        use std::time::Duration;
        assert_eq!(parse_duration("500ms").unwrap(), Duration::from_millis(500));
        assert_eq!(parse_duration("10s").unwrap(), Duration::from_secs(10));
        assert_eq!(parse_duration("2m").unwrap(), Duration::from_secs(120));
        assert_eq!(parse_duration("1h").unwrap(), Duration::from_secs(3600));
        assert_eq!(parse_duration("30").unwrap(), Duration::from_secs(30));
    }

    #[test]
    fn parse_duration_rejects_garbage() {
        assert!(parse_duration("").is_err());
        assert!(parse_duration("banana").is_err());
        assert!(parse_duration("10x").is_err());
        assert!(parse_duration("0s").is_err());
        assert!(parse_duration("0").is_err());
        assert!(parse_duration("-5s").is_err());
    }

    #[test]
    fn format_duration_round_trips() {
        for text in ["500ms", "10s", "2m", "1h", "90s", "1500ms"] {
            let parsed = parse_duration(text).unwrap();
            let rendered = format_duration(&parsed);
            assert_eq!(
                parse_duration(&rendered).unwrap(),
                parsed,
                "round-trip failed for {text}"
            );
        }
        assert_eq!(format_duration(&parse_duration("90s").unwrap()), "90s");
        assert_eq!(format_duration(&parse_duration("2m").unwrap()), "2m");
    }

    #[test]
    fn structural_metadata_covers_all_structural_kinds() {
        use crate::ast::Value;

        // Tripwire: adding a structural StepKind variant without registering
        // documentation fails to compile here (non-exhaustive match). Leaf
        // commands map to None; they are covered by declare_commands!.
        fn metadata_name(kind: &StepKind) -> Option<&'static str> {
            match kind {
                StepKind::WithIo { .. } | StepKind::WithIoBlock { .. } => Some("WITH_IO"),
                StepKind::For { .. } => Some("FOR"),
                StepKind::If { .. } => Some("IF"),
                StepKind::Assign { .. } => Some("LET"),
                StepKind::Set { .. } => Some("MUTATION"),
                StepKind::AssignCapture { .. } => Some("LET"),
                StepKind::AwaitCapture { .. } => Some("AWAIT"),
                StepKind::AsyncBlock { .. } | StepKind::AssignAsync { .. } => Some("ASYNC"),
                StepKind::Await { .. } => Some("AWAIT"),
                StepKind::Cancel { .. } => Some("CANCEL"),
                StepKind::Timeout { .. } => Some("TIMEOUT"),
                StepKind::RemoteBlock { .. } => Some("REMOTE"),
                StepKind::FuncDef { .. } => Some("FUNC"),
                // Bare `NAME(...)` calls share the `FUNC` reference page;
                // there is no call keyword to document on its own.
                StepKind::Call { .. } => None,
                StepKind::Return { .. } => Some("RETURN"),
                StepKind::While { .. } => Some("WHILE"),
                StepKind::Break => Some("BREAK"),
                StepKind::Continue => Some("CONTINUE"),
                // Aliases are declared by `TYPE` but documented with the
                // type-word prose on the `LET` page (define-once,
                // use-everywhere); no dedicated reference page.
                StepKind::TypeAlias { .. } => Some("LET"),
                StepKind::RunExec { .. } => None,
                StepKind::Workdir(_)
                | StepKind::Workspace(_)
                | StepKind::Env { .. }
                | StepKind::InheritEnv { .. }
                | StepKind::Run(_)
                | StepKind::Echo(_)
                | StepKind::Copy { .. }
                | StepKind::Symlink { .. }
                | StepKind::Mkdir(_)
                | StepKind::Ls(_)
                | StepKind::Cwd
                | StepKind::Read(_)
                | StepKind::ReadLine { .. }
                | StepKind::Write { .. }
                | StepKind::Append { .. }
                | StepKind::Expand { .. }
                | StepKind::AssertEq { .. }
                | StepKind::AssertContains { .. }
                | StepKind::CopyGit { .. }
                | StepKind::HashSha256 { .. }
                | StepKind::Exit(_)
                | StepKind::Sleep { .. }
                | StepKind::ListAppend { .. } => None,
            }
        }

        // Exercise the matcher once per structural variant so the arms cannot
        // rot (a new variant breaks compilation above first).
        let dummies: Vec<StepKind> = vec![
            StepKind::WithIo {
                bindings: Vec::new(),
                cmd: Box::new(StepKind::Echo(crate::ast::Arg::String(
                    "x".to_string(),
                    false,
                ))),
            },
            StepKind::For {
                key_var: None,
                key_type: None,
                var: "i".to_string(),
                var_type: "STRING".to_string(),
                in_expr: Expr::Literal(Value::bool(true)),
                body: Vec::new(),
            },
            StepKind::If {
                cond: Box::new(Expr::Literal(Value::bool(true))),
                then_body: Vec::new(),
                else_ifs: Vec::new(),
                else_body: None,
            },
            StepKind::Assign {
                var: "v".to_string(),
                decl_type: "BOOL".to_string(),
                expr: Expr::Literal(Value::bool(true)),
            },
            StepKind::Set {
                var: "v".to_string(),
                expr: Expr::Literal(Value::bool(true)),
            },
            StepKind::AssignCapture {
                var: "v".to_string(),
                decl_type: "STRING".to_string(),
                cmd: Box::new(StepKind::Echo(crate::ast::Arg::String(
                    "x".to_string(),
                    false,
                ))),
            },
            StepKind::AwaitCapture {
                out_var: "o".to_string(),
                out_type: "STRING".to_string(),
                task_var: "t".to_string(),
            },
            StepKind::AsyncBlock { body: Vec::new() },
            StepKind::AssignAsync {
                var: "t".to_string(),
                decl_type: "HANDLE".to_string(),
                body: Vec::new(),
            },
            StepKind::Await {
                var: "t".to_string(),
            },
            StepKind::Cancel {
                var: "t".to_string(),
            },
            StepKind::Timeout {
                duration: Arg::String("1s".to_string(), false),
                body: Vec::new(),
            },
            StepKind::RemoteBlock {
                target: "prod".to_string(),
                vars: Vec::new(),
                env: Vec::new(),
                body: Vec::new(),
            },
            StepKind::FuncDef {
                name: "F".to_string(),
                params: Vec::new(),
                body: Vec::new(),
            },
            StepKind::Call {
                name: "F".to_string(),
                args: Vec::new(),
            },
            StepKind::Return {
                expr: Box::new(Expr::Literal(Value::bool(true))),
            },
            StepKind::While {
                cond: Box::new(Expr::Literal(Value::bool(true))),
                body: Vec::new(),
            },
            StepKind::Break,
            StepKind::Continue,
            StepKind::TypeAlias {
                name: "PERSON".to_string(),
                target: "MAP<name: STRING>".to_string(),
            },
        ];
        let registry = all_structural_metadata();
        for kind in &dummies {
            // Bare calls share the FUNC reference page and map to None.
            let Some(name) = metadata_name(kind) else {
                continue;
            };
            assert!(
                registry.iter().any(|meta| meta.name == name),
                "no structural metadata entry for {}",
                name
            );
        }
    }
}