tau_cli_term/

lib.rs

//! Higher-level terminal prompt with command completion.
//!
//! This crate is now a thin shell around [`tau_cli_term_raw`]: the
//! raw layer owns the input state machine (history navigation,
//! completion menu lifecycle, key dispatch). This crate plugs in the
//! *content* (which candidates exist for a given buffer) and the
//! *presentation* (how the menu is rendered as a styled block under
//! the prompt). It also handles `$EDITOR` integration, which doesn't
//! belong in the raw layer.
//! The ownership split and subprocess/editor contracts are summarized in
//! `ARCH-tau-cli-term`.

use std::{ffi as path_std_ffi, process as path_std_process, time as path_std_time};

mod bounded_command;
pub mod completion;
pub mod resolve;
#[cfg(test)]
mod tests;

use std::io;
use std::sync::{Arc, Mutex};

pub use bounded_command::ForegroundRestorationDiagnostic;
use bounded_command::{
    BoundedCommandError, ProcessOwnership, run_with_bounded_stdout,
    run_with_bounded_stdout_after_spawn, run_with_inherited_stdio,
};
pub use completion::{
    ArgCompleter, CommandCompletion, CommandName, CompletionData, CompletionItem, CompletionRule,
    CompletionRules,
};
#[cfg(test)]
pub(crate) use tau_cli_term_raw::RawEvent as TestRawEvent;
pub use tau_cli_term_raw::{
    Align, BlockId, Cell, Color, CursorShape, OpaquePresentationFact, OutputSnapshot,
    PresentationInvalidation, PresentationObservationKey, PriorityLine, PriorityLineAlignment,
    PriorityLinePriority, PriorityLineTruncation, RedrawSuppressionGuard, RendererDeliveryId, Span,
    Style, StyledBlock, StyledText, TermHandle, TerminalOptions, TwoLineElision, is_output_failure,
    sanitize_hyperlink_target,
};
use tau_cli_term_raw::{Candidate, Event as RawEvent};
use tau_term_screen::{display_width, truncate_to_width};
use tau_themes::Theme;

const PROMPT_TRAILER_MARKER: &str =
    "<!-- TAU trailer: everything after this line will be ignored -->";
// Keep user-facing command limit docs in FEATURES.md and
// docs/cli-keybindings.md in sync with these values.
const PROMPT_COMMAND_OUTPUT_LIMIT_BYTES: usize = 1024 * 1024;
const COMPLETION_COMMAND_OUTPUT_LIMIT_BYTES: usize = 256 * 1024;
const COMPLETION_COMMAND_TIMEOUT: std::time::Duration = path_std_time::Duration::from_secs(10);
const PROMPT_COMMAND_TIMEOUT: std::time::Duration = path_std_time::Duration::from_secs(60 * 60);
const AGENT_PICKER_OUTPUT_LIMIT_BYTES: usize = 64 * 1024;
const AGENT_PICKER_TIMEOUT: std::time::Duration = path_std_time::Duration::from_secs(5 * 60);
// Keep the first eleven fields synchronized with docs/list-agents.md#output.
// The final four source fields are picker-only cost/work status/activity; the
// presentation field is removed after fzf returns the complete input row.
const AGENT_PICKER_FZF_ARGS: &[&str] = &[
    "--height=100%",
    "--delimiter=\t",
    "--with-nth=16",
    "--no-multi",
    "--no-hscroll",
    "--prompt=agent> ",
];
const AGENT_PICKER_SOURCE_FIELDS: usize = 15;

const AGENT_PICKER_COLUMNS: [AgentPickerColumn; 4] = [
    AgentPickerColumn {
        source_field: usize::MAX,
        minimum_width: 8,
        preferred_width: 24,
        max_width: 40,
    },
    AgentPickerColumn {
        source_field: 10,
        minimum_width: 9,
        preferred_width: 9,
        max_width: 9,
    },
    AgentPickerColumn {
        source_field: 11,
        minimum_width: 9,
        preferred_width: 9,
        max_width: 9,
    },
    AgentPickerColumn {
        source_field: 13,
        minimum_width: 12,
        preferred_width: 24,
        max_width: 40,
    },
];
const AGENT_PICKER_COLUMN_GAP: &str = "  ";
// fzf reserves screen columns for its pointer, marker, and gutter.
const AGENT_PICKER_FZF_DECORATION_WIDTH: usize = 4;
const PROMPT_HISTORY_SEARCH_MAX_ROWS: usize = 200;
const PROMPT_HISTORY_SUMMARY_MAX_CHARS: usize = 240;
const PROMPT_HISTORY_PREVIEW_MAX_BYTES: usize = 64 * 1024;
const PROMPT_HISTORY_PREVIEW_TOTAL_BYTES: usize = 1024 * 1024;
/// Maximum number of nonempty search entries retained for one attachment.
const PROMPT_HISTORY_MAX_ENTRIES: usize = 1000;
/// Maximum primary UTF-8 text retained for one attachment's search entries.
const PROMPT_HISTORY_MAX_BYTES: usize = 16 * 1024 * 1024;
/// Independent HighTerm search-history retention limits for one attachment.
#[derive(Clone, Copy)]
struct PromptHistoryLimits {
    /// Maximum retained nonempty search entries.
    max_entries: usize,
    /// Maximum retained primary UTF-8 bytes.
    max_bytes: usize,
}
const COMPLETION_MENU_BLOCK_ID: BlockId = BlockId(u64::MAX);

/// One width-aware picker presentation column.
struct AgentPickerColumn {
    /// Zero-based index in the source TSV row.
    source_field: usize,
    /// Smallest useful width before lower-priority columns should be omitted.
    minimum_width: usize,
    /// Width targeted before distributing spare space toward natural width.
    preferred_width: usize,
    /// Maximum display width even when the terminal has spare space.
    max_width: usize,
}

/// Re-acquires raw terminal state on every external-command exit path.
struct ExternalResumeGuard<F: FnMut() -> io::Result<()>> {
    /// Resume operation for the terminal whose external pause remains armed.
    resume: F,
    /// Cleared only after an explicit successful-or-reported resume attempt.
    armed: bool,
}

/// Injectable terminal lifecycle and child-readiness operations for one picker.
struct AgentPickerHooks<P, R, A> {
    /// Releases terminal ownership before spawning the picker.
    pause: P,
    /// Restores terminal ownership after the picker finishes.
    resume: R,
    /// Observes child readiness before the command deadline begins.
    after_spawn: A,
}

impl<F: FnMut() -> io::Result<()>> ExternalResumeGuard<F> {
    fn new(resume: F) -> Self {
        Self {
            resume,
            armed: true,
        }
    }

    fn finish(mut self) -> io::Result<()> {
        self.armed = false;
        (self.resume)()
    }

    /// Prevents terminal resume when foreground ownership is unconfirmed.
    fn disarm(mut self) {
        self.armed = false;
    }
}

impl<F: FnMut() -> io::Result<()>> Drop for ExternalResumeGuard<F> {
    fn drop(&mut self) {
        if self.armed
            && let Err(error) = (self.resume)()
        {
            tracing::warn!(
                target: "tau_cli::input",
                %error,
                "failed to resume terminal after external command"
            );
        }
    }
}

/// Applies the fail-stop cut before an armed terminal-resume guard is dropped.
fn preserve_pause_on_unconfirmed_foreground<T, F: FnMut() -> io::Result<()>>(
    guard: ExternalResumeGuard<F>,
    result: Result<T, BoundedCommandError>,
) -> Result<T, BoundedCommandError> {
    if result
        .as_ref()
        .is_err_and(BoundedCommandError::is_foreground_ownership_unconfirmed)
    {
        guard.disarm();
    }
    result
}

/// High-level events surfaced to the caller.
pub enum Event {
    /// Upload one normalized paste while its original draft stays frozen.
    PasteUpload {
        /// Exact local attempt identity.
        id: u64,
        /// Normalized source kept outside editable prompt and histories.
        text: std::sync::Arc<str>,
    },
    /// Cancel one paste upload without canceling an agent prompt.
    PasteCancelled {
        /// Attempt canceled without changing the original draft.
        id: u64,
    },
    /// The user submitted a line (pressed Enter by default, Ctrl-Enter,
    /// or ran `submit-prompt` with no completion preview).
    Line(String),
    /// The user signalled EOF (Ctrl-D on empty line).
    Eof,
    /// The user requested prompt cancellation with a second consecutive Ctrl-C.
    CancelPrompt,
    /// The terminal was resized.
    Resize { width: u16, height: u16 },
    /// The terminal reported focus gained or lost.
    FocusChanged { focused: bool },
    /// The input buffer changed (or the completion menu cycled,
    /// opened, or closed). Caller should redraw any prompt-derived
    /// UI.
    BufferChanged,
    /// Shift+Tab pressed outside an open completion menu.
    BackTab,
    /// Escape pressed outside an open completion menu.
    Escape,
    /// A binding requested an application-defined action without touching the
    /// prompt draft.
    Action(String),
}

/// Failure while an interactive external program temporarily owns the terminal.
#[derive(Debug)]
pub enum ExternalProgramError {
    /// Ordinary command failure after terminal ownership was restored.
    Command(String),
    /// Fatal failure because Tau could not confirm foreground ownership.
    ForegroundOwnershipUnconfirmed {
        /// Complete user-facing failure message retained for top-level
        /// reporting.
        message: String,
        /// Bounded private diagnostic for the failed restoration syscall.
        diagnostic: ForegroundRestorationDiagnostic,
    },
}

impl ExternalProgramError {
    /// Returns whether the interactive attachment must exit without resuming.
    #[must_use]
    pub fn is_foreground_ownership_unconfirmed(&self) -> bool {
        matches!(self, Self::ForegroundOwnershipUnconfirmed { .. })
    }

    /// Returns the bounded restoration diagnostic for an ownership fail-stop.
    #[must_use]
    pub fn foreground_restoration_diagnostic(&self) -> Option<ForegroundRestorationDiagnostic> {
        match self {
            Self::ForegroundOwnershipUnconfirmed { diagnostic, .. } => Some(*diagnostic),
            Self::Command(_) => None,
        }
    }
}

impl std::fmt::Display for ExternalProgramError {
    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Command(error) => formatter.write_str(error),
            Self::ForegroundOwnershipUnconfirmed { message, .. } => formatter.write_str(message),
        }
    }
}

impl std::error::Error for ExternalProgramError {}

impl From<BoundedCommandError> for ExternalProgramError {
    fn from(error: BoundedCommandError) -> Self {
        match error.foreground_restoration_diagnostic() {
            Some(diagnostic) => Self::ForegroundOwnershipUnconfirmed {
                message: error.to_string(),
                diagnostic,
            },
            None => Self::Command(error.to_string()),
        }
    }
}

impl From<String> for ExternalProgramError {
    fn from(error: String) -> Self {
        Self::Command(error)
    }
}

/// Returns whether an input I/O error requires attachment-only terminal
/// fail-stop.
#[must_use]
pub fn is_foreground_ownership_unconfirmed(error: &io::Error) -> bool {
    foreground_restoration_diagnostic(error).is_some()
}

/// Returns the bounded restoration diagnostic from an input I/O fail-stop.
#[must_use]
pub fn foreground_restoration_diagnostic(
    error: &io::Error,
) -> Option<ForegroundRestorationDiagnostic> {
    error
        .get_ref()
        .and_then(|source| source.downcast_ref::<BoundedCommandError>())
        .and_then(BoundedCommandError::foreground_restoration_diagnostic)
}

/// Higher-level terminal prompt with completion support.
pub struct HighTerm {
    term: tau_cli_term_raw::Term,
    handle: TermHandle,
    theme: Theme,
    editor_context: Arc<Mutex<EditorContext>>,
    /// Editor command resolved once at startup: `$EDITOR`, else
    /// `$VISUAL`, else the first of `hx`/`vim`/`vi`/`nano` found on
    /// `$PATH`. Passed to shell actions as `$TAU_EDITOR`.
    external_editor: Option<String>,
    /// Block id for the completion menu, allocated lazily on first
    /// open. Reused across opens; content swapped to empty when the
    /// menu is hidden.
    menu_block_id: Option<BlockId>,
    /// Submitted prompt history used by prompt-history search. Seeded
    /// from persistent history at startup and extended with submitted
    /// prompts from this process.
    prompt_history: Vec<String>,
    /// Whether the most recently submitted prompt survived search-history
    /// retention and can be replaced by its final redacted form.
    last_submitted_prompt_retained: bool,
    /// Optional small limits used by focused test-only history state machines.
    #[cfg(test)]
    prompt_history_limit_override: Option<PromptHistoryLimits>,
    completion_command_rules: completion::CompletionCommandRules,
    last_command_completion_token: Option<String>,
}

impl HighTerm {
    /// Runs an in-process interactive callback while it exclusively owns the
    /// terminal.
    ///
    /// Tau pauses redraws and releases raw terminal features before invoking
    /// `callback`, then restores terminal state and requests a fresh repaint.
    /// A release failure prevents the callback from running. The resume guard
    /// also restores terminal state if the callback unwinds.
    pub fn run_with_external_terminal<T>(&self, callback: impl FnOnce() -> T) -> io::Result<T> {
        run_with_external_terminal_hooks(
            callback,
            || self.term.pause_for_external(),
            || self.term.resume_after_external(),
        )
    }

    /// Creates a new terminal with the given prompt and commands.
    ///
    /// Returns the terminal, a thread-safe handle for rendering, and a
    /// [`CompletionData`] handle for pushing dynamic argument completions
    /// from background threads.
    pub fn new(
        left_prompt: impl Into<StyledText>,
        commands: Vec<CommandCompletion>,
        theme: Theme,
        bindings: impl IntoIterator<Item = (String, String)>,
        terminal_options: TerminalOptions,
    ) -> io::Result<(Self, TermHandle, CompletionData)> {
        Self::new_with_completion_rules(
            left_prompt,
            commands,
            theme,
            bindings,
            std::iter::empty(),
            CompletionRules::default(),
            terminal_options,
        )
    }

    /// Creates a new terminal and seeds prompt input history.
    pub fn new_with_input_history(
        left_prompt: impl Into<StyledText>,
        commands: Vec<CommandCompletion>,
        theme: Theme,
        bindings: impl IntoIterator<Item = (String, String)>,
        input_history: impl IntoIterator<Item = String>,
        terminal_options: TerminalOptions,
    ) -> io::Result<(Self, TermHandle, CompletionData)> {
        Self::new_with_completion_rules(
            left_prompt,
            commands,
            theme,
            bindings,
            input_history,
            CompletionRules::default(),
            terminal_options,
        )
    }

    /// Creates a new terminal with explicit prompt completion rules.
    pub fn new_with_completion_rules(
        left_prompt: impl Into<StyledText>,
        commands: Vec<CommandCompletion>,
        theme: Theme,
        bindings: impl IntoIterator<Item = (String, String)>,
        input_history: impl IntoIterator<Item = String>,
        completion_rules: CompletionRules,
        terminal_options: TerminalOptions,
    ) -> io::Result<(Self, TermHandle, CompletionData)> {
        let data = CompletionData::new();
        let (term, handle) = Self::new_with_completion_rules_and_data(
            left_prompt,
            commands,
            theme,
            bindings,
            input_history,
            completion_rules,
            terminal_options,
            data.clone(),
        )?;
        Ok((term, handle, data))
    }

    /// Creates a terminal with caller-owned mutable completion state.
    ///
    /// Callers may populate this state before the input loop starts, then
    /// update it later from background owners through its shared handle.
    #[allow(clippy::too_many_arguments)]
    pub fn new_with_completion_rules_and_data(
        left_prompt: impl Into<StyledText>,
        commands: Vec<CommandCompletion>,
        theme: Theme,
        bindings: impl IntoIterator<Item = (String, String)>,
        input_history: impl IntoIterator<Item = String>,
        completion_rules: CompletionRules,
        terminal_options: TerminalOptions,
        data: CompletionData,
    ) -> io::Result<(Self, TermHandle)> {
        let input_history: Vec<String> = input_history.into_iter().collect();
        let (mut term, handle) = tau_cli_term_raw::Term::new(left_prompt, terminal_options)?;
        term.defer_submitted_input_history_limit();
        term.seed_input_history(input_history.clone());
        term.set_bindings(bindings);
        let handle_clone = handle.clone();
        let completion_command_rules = completion_rules.command_rules().clone();
        term.set_completion_source(Some(make_completion_source(
            commands,
            data,
            completion_rules.clone(),
        )));
        let external_editor = resolve_external_editor();
        Ok((
            Self {
                term,
                handle,
                theme,
                editor_context: Arc::new(Mutex::new(EditorContext::default())),
                external_editor,
                menu_block_id: None,
                prompt_history: bounded_seeded_prompt_history(
                    input_history
                        .into_iter()
                        .filter(|entry| !entry.is_empty())
                        .collect(),
                ),
                last_submitted_prompt_retained: false,
                #[cfg(test)]
                prompt_history_limit_override: None,
                completion_command_rules,
                last_command_completion_token: None,
            },
            handle_clone,
        ))
    }

    #[cfg(test)]
    pub(crate) fn new_for_test(
        term: tau_cli_term_raw::Term,
        handle: TermHandle,
        commands: Vec<CommandCompletion>,
        theme: Theme,
        bindings: impl IntoIterator<Item = (String, String)>,
    ) -> (Self, CompletionData) {
        Self::new_for_test_with_completion_rules(
            term,
            handle,
            commands,
            theme,
            bindings,
            CompletionRules::default(),
        )
    }

    /// Creates a virtual terminal with explicit completion rules for focused
    /// runtime tests.
    #[cfg(test)]
    pub(crate) fn new_for_test_with_completion_rules(
        mut term: tau_cli_term_raw::Term,
        handle: TermHandle,
        commands: Vec<CommandCompletion>,
        theme: Theme,
        bindings: impl IntoIterator<Item = (String, String)>,
        completion_rules: CompletionRules,
    ) -> (Self, CompletionData) {
        term.defer_submitted_input_history_limit();
        let data = CompletionData::new();
        let data_clone = data.clone();
        let completion_command_rules = completion_rules.command_rules().clone();
        term.set_completion_source(Some(make_completion_source(
            commands,
            data,
            completion_rules.clone(),
        )));
        term.set_bindings(bindings);
        (
            Self {
                term,
                handle,
                theme,
                editor_context: Arc::new(Mutex::new(EditorContext::default())),
                external_editor: None,
                menu_block_id: None,
                prompt_history: Vec::new(),
                last_submitted_prompt_retained: false,
                prompt_history_limit_override: None,
                completion_command_rules,
                last_command_completion_token: None,
            },
            data_clone,
        )
    }

    /// Returns a reference to the [`TermHandle`].
    pub fn handle(&self) -> &TermHandle {
        &self.handle
    }

    /// Replaces the editor-context storage with a shared handle.
    ///
    /// Use this when another component (e.g. the event renderer) owns
    /// the authoritative context and needs the prompt's external-editor
    /// integration to read conversation context and write prompt-trailer
    /// recovery state through the same `Arc`. The previously-owned
    /// `EditorContext` is dropped.
    /// `EditorContext` is dropped.
    pub fn set_editor_context_handle(&mut self, editor_context: Arc<Mutex<EditorContext>>) {
        self.editor_context = editor_context;
    }

    /// Opens the ordinary prompt editor with one explicitly selected response.
    ///
    /// The selected response temporarily replaces the renderer-published
    /// response context. Existing editor recovery survives the invocation,
    /// and any newly edited trailer text becomes the next recovery value.
    pub fn edit_prompt_with_response(&mut self, response: String) -> io::Result<()> {
        let mut context = self
            .editor_context
            .lock()
            .expect("editor context mutex poisoned")
            .clone();
        context.current_response = None;
        context.last_response = Some(response);
        context.previous_prompt = None;
        context.chat_markdown = None;
        self.edit_prompt_with_context(context, None)
    }

    /// Opens the ordinary prompt editor with an exact Markdown chat export
    /// below the established trailer marker.
    ///
    /// Terminal pause/resume, command failure, prompt replacement, and edited
    /// trailer recovery use the same path as `shell-prompt-edit`.
    pub fn edit_prompt_with_chat(&mut self, chat_markdown: String) -> io::Result<()> {
        let recovery = self
            .editor_context
            .lock()
            .expect("editor context mutex poisoned")
            .edited_trailer_recovery
            .clone();
        self.edit_prompt_with_context(
            EditorContext {
                chat_markdown: Some(chat_markdown),
                edited_trailer_recovery: recovery.clone(),
                ..EditorContext::default()
            },
            recovery,
        )
    }

    /// Runs one editor invocation with an isolated trailer projection.
    fn edit_prompt_with_context(
        &mut self,
        context: EditorContext,
        hidden_recovery: Option<String>,
    ) -> io::Result<()> {
        let temporary = Arc::new(Mutex::new(context));
        let outcome = self.run_prompt_action_with_context(
            PromptShellAction::Edit(PromptShellCommand {
                command: "$TAU_EDITOR \"$TAU_PROMPT_PATH\"".to_owned(),
                trim: false,
            }),
            temporary.clone(),
        );
        let edited_recovery = temporary
            .lock()
            .expect("editor context mutex poisoned")
            .edited_trailer_recovery
            .clone()
            .or(hidden_recovery);
        self.editor_context
            .lock()
            .expect("editor context mutex poisoned")
            .edited_trailer_recovery = edited_recovery;
        if !matches!(outcome, PromptActionOutcome::Fatal(_)) {
            self.handle.redraw_sync();
        }
        match outcome {
            PromptActionOutcome::Fatal(error) => Err(error),
            PromptActionOutcome::BufferChanged
            | PromptActionOutcome::Continue
            | PromptActionOutcome::Return(_) => Ok(()),
        }
    }

    /// Replaces the prompt UI theme for future local rendering.
    pub fn set_theme(&mut self, theme: Theme) {
        self.theme = theme;
        self.sync_menu_block();
        self.handle.redraw();
    }

    /// Triggers a redraw.
    pub fn redraw(&self) {
        self.handle.redraw();
    }

    /// Replaces the most recently submitted prompt in navigation and search
    /// history.
    ///
    /// Higher layers use this when a submitted line has a safer presentation
    /// form than the raw value that must remain available to the immediate
    /// routing stack.
    pub fn replace_last_submitted_prompt(&mut self, text: String) {
        if self.last_submitted_prompt_retained {
            if let Some(last) = self.prompt_history.last_mut() {
                *last = text.clone();
            }
        } else if !text.is_empty() {
            self.prompt_history.push(text.clone());
        }
        self.term.replace_last_submitted_input(text);
    }

    /// Runs the optional `fzf` agent-row picker while safely releasing raw
    /// mode.
    ///
    /// `rows` must be headerless TSV with the stable agent id in field one.
    /// Cancellation returns `Ok(None)`. The external `fzf` interaction uses a
    /// five-minute command timeout, including for an empty input roster. Tau
    /// checks foreground restoration after settling `fzf` and preserves the
    /// primary outcome or failure alongside any restoration failure. If
    /// ownership remains unconfirmed, the caller must exit the affected
    /// attachment without resuming terminal input or output.
    pub fn pick_agent_row_with_fzf(
        &self,
        rows: &str,
    ) -> Result<Option<String>, ExternalProgramError> {
        self.pick_agent_row_with_command(
            path_std_ffi::OsStr::new("fzf"),
            rows,
            AGENT_PICKER_TIMEOUT,
            ProcessOwnership::ForegroundProcessGroup,
        )
    }

    /// Runs an injected picker with explicit execution bounds and ownership.
    fn pick_agent_row_with_command(
        &self,
        program: &std::ffi::OsStr,
        rows: &str,
        timeout: std::time::Duration,
        ownership: ProcessOwnership,
    ) -> Result<Option<String>, ExternalProgramError> {
        self.pick_agent_row_with_command_and_terminal(
            program,
            rows,
            timeout,
            ownership,
            AgentPickerHooks {
                pause: || self.term.pause_for_external(),
                resume: || self.term.resume_after_external(),
                after_spawn: || Ok(()),
            },
        )
    }

    /// Runs an injected picker between explicit terminal pause/resume
    /// operations.
    fn pick_agent_row_with_command_and_terminal(
        &self,
        program: &std::ffi::OsStr,
        rows: &str,
        timeout: std::time::Duration,
        ownership: ProcessOwnership,
        hooks: AgentPickerHooks<
            impl FnOnce() -> io::Result<()>,
            impl FnMut() -> io::Result<()>,
            impl FnOnce() -> Result<(), String>,
        >,
    ) -> Result<Option<String>, ExternalProgramError> {
        let picker_rows = format_agent_picker_rows(rows, self.handle.size().0)
            .map_err(ExternalProgramError::Command)?;
        (hooks.pause)().map_err(|error| format!("could not release terminal: {error}"))?;
        let guard = ExternalResumeGuard::new(hooks.resume);
        let selection = run_agent_fzf_command_with_ownership(
            program,
            &picker_rows,
            timeout,
            ownership,
            hooks.after_spawn,
        );
        if selection
            .as_ref()
            .is_err_and(BoundedCommandError::is_foreground_ownership_unconfirmed)
        {
            guard.disarm();
            return selection.map_err(ExternalProgramError::from);
        }
        guard.finish().map_err(|error| {
            ExternalProgramError::Command(format!(
                "could not resume terminal after agent picker: {error}"
            ))
        })?;
        selection.map_err(ExternalProgramError::from)
    }

    /// Closes the active completion menu, if any, and updates its rendered
    /// block.
    ///
    /// Returns `true` when a menu was open. Call this from application-level
    /// state transitions that make the active completion context stale but do
    /// not otherwise change the prompt buffer. Dismissing a previewed candidate
    /// may restore the previous buffer and cursor; this method updates the
    /// rendered menu but does not emit a [`Event::BufferChanged`] event.
    pub fn dismiss_completion_menu(&mut self) -> bool {
        let dismissed = self.term.dismiss_completion_menu();
        if dismissed {
            self.sync_menu_block();
            self.handle.redraw();
        }
        dismissed
    }

    /// Appends persistent output to history.
    pub fn print_output(
        &self,
        debug_id: impl Into<String>,
        block: impl Into<StyledBlock>,
    ) -> BlockId {
        self.handle.print_output(debug_id, block)
    }

    /// Blocks until the next high-level event, syncing the
    /// completion menu block to the raw term's current state.
    pub fn get_next_event(&mut self) -> io::Result<Event> {
        loop {
            let raw = self.term.get_next_event()?;
            match self.handle_next_raw_event(raw) {
                NextEventStep::Return(event) => return Ok(event),
                NextEventStep::Continue => continue,
                NextEventStep::Fatal(error) => return Err(error),
            }
        }
    }

    fn handle_next_raw_event(&mut self, raw: RawEvent) -> NextEventStep {
        match raw {
            RawEvent::PasteUpload { id, text } => {
                NextEventStep::Return(Event::PasteUpload { id, text })
            }
            RawEvent::PasteCancelled { id } => NextEventStep::Return(Event::PasteCancelled { id }),
            RawEvent::BufferChanged => self.handle_buffer_changed_event(),
            RawEvent::CompletionRefresh => {
                self.sync_menu_block();
                self.handle.redraw();
                NextEventStep::Continue
            }
            RawEvent::CompletionAccept => self.handle_completion_accept_event(),
            RawEvent::BackTab => NextEventStep::Return(Event::BackTab),
            RawEvent::Escape => NextEventStep::Return(Event::Escape),
            RawEvent::Line(line) => self.handle_line_event(line),
            RawEvent::Eof => {
                self.sync_menu_block();
                NextEventStep::Return(Event::Eof)
            }
            RawEvent::CancelPrompt => {
                self.sync_menu_block();
                self.handle.redraw_sync();
                NextEventStep::Return(Event::CancelPrompt)
            }
            RawEvent::Resize { width, height } => {
                self.sync_menu_block();
                self.handle.redraw();
                NextEventStep::Return(Event::Resize { width, height })
            }
            RawEvent::FocusChanged { focused } => {
                NextEventStep::Return(Event::FocusChanged { focused })
            }
            RawEvent::Notice(message) => self.handle_notice_event(&message),
            RawEvent::ExternalEditor => self.handle_external_editor_event(),
            RawEvent::Binding(action) => self.handle_binding_event(&action),
        }
    }

    fn handle_buffer_changed_event(&mut self) -> NextEventStep {
        let completion_changed = match self.maybe_run_command_completion() {
            Ok(changed) => changed,
            Err(error) => return NextEventStep::Fatal(error),
        };
        if completion_changed {
            self.sync_menu_block();
            self.handle.redraw_sync();
            return NextEventStep::Return(Event::BufferChanged);
        }
        self.sync_menu_block();
        self.handle.redraw();
        NextEventStep::Return(Event::BufferChanged)
    }

    fn handle_completion_accept_event(&mut self) -> NextEventStep {
        // Accept-without-submit: the buffer already reflects the chosen
        // candidate. Sync the menu (now closed) and loop so the user has to
        // press Enter again to actually submit.
        self.sync_menu_block();
        self.handle.redraw();
        NextEventStep::Continue
    }

    fn handle_line_event(&mut self, line: String) -> NextEventStep {
        let started = path_std_time::Instant::now();
        self.record_submitted_prompt(&line);
        let history_finished = path_std_time::Instant::now();
        let redraw_requested = self.sync_menu_block();
        let menu_finished = path_std_time::Instant::now();
        if redraw_requested {
            self.handle.redraw();
        }
        tracing::trace!(
            target: "tau_cli::prompt_submission",
            stage = "highterm_history_menu",
            prompt_bytes = line.len(),
            history_us = history_finished.duration_since(started).as_micros(),
            menu_sync_us = menu_finished.duration_since(history_finished).as_micros(),
            redraw_request_us = menu_finished.elapsed().as_micros(),
            redraw_requested,
            stage_us = started.elapsed().as_micros(),
            "content-free prompt submission stage"
        );
        NextEventStep::Return(Event::Line(line))
    }

    fn handle_notice_event(&mut self, message: &str) -> NextEventStep {
        self.sync_menu_block();
        self.print_local(message);
        self.handle.redraw_sync();
        NextEventStep::Return(Event::BufferChanged)
    }

    fn handle_external_editor_event(&mut self) -> NextEventStep {
        self.sync_menu_block();
        let outcome = self.run_prompt_action(PromptShellAction::Edit(PromptShellCommand {
            command: "$TAU_EDITOR \"$TAU_PROMPT_PATH\"".to_owned(),
            trim: false,
        }));
        if !matches!(outcome, PromptActionOutcome::Fatal(_)) {
            self.handle.redraw_sync();
        }
        outcome.into_next_event_step()
    }

    fn handle_binding_event(&mut self, action: &str) -> NextEventStep {
        self.sync_menu_block();
        let outcome = self.run_binding(action);
        if !matches!(outcome, PromptActionOutcome::Fatal(_)) {
            self.handle.redraw_sync();
        }
        outcome.into_next_event_step()
    }

    /// Updates the suggestion block to match the raw term's completion state.
    ///
    /// Returns whether synchronization wrote visible menu state.
    /// Callers that already have a redraw request for another prompt mutation
    /// can skip a second request when this returns `false`.
    fn sync_menu_block(&mut self) -> bool {
        match self.term.completion_state() {
            Some(view) => {
                let (width, height) = self.handle.size();
                let block = completion::render_menu_block(&view, &self.theme, width, height);
                let id = match self.menu_block_id {
                    Some(id) => id,
                    None => {
                        let id = COMPLETION_MENU_BLOCK_ID;
                        self.handle.set_block(id, "");
                        self.handle.push_suggestions(id);
                        self.menu_block_id = Some(id);
                        id
                    }
                };
                self.handle.set_block(id, block);
                true
            }
            None => {
                if let Some(id) = self.menu_block_id.take() {
                    self.handle.remove_suggestions(id);
                    self.handle.remove_block(id);
                    true
                } else {
                    false
                }
            }
        }
    }

    fn run_binding(&mut self, action: &str) -> PromptActionOutcome {
        tracing::trace!(target: "tau_cli::input", action, "running prompt binding");
        if tau_cli_term_raw::Term::is_named_action(action) {
            return self
                .term
                .trigger_named_action(action)
                .map_or(PromptActionOutcome::Continue, |raw| {
                    self.apply_raw_prompt_event(raw)
                });
        }
        let Some(action) = PromptShellAction::parse(action) else {
            self.print_local(&format!("binding: unknown action `{action}`"));
            return PromptActionOutcome::BufferChanged;
        };
        self.run_prompt_action(action)
    }

    /// Runs a [`PromptShellAction`] and applies its result to the
    /// input buffer. Errors (spawn failure, bad utf-8, no editor)
    /// surface as a themed info line above the prompt.
    fn run_prompt_action(&mut self, action: PromptShellAction) -> PromptActionOutcome {
        self.run_prompt_action_with_context(action, self.editor_context.clone())
    }

    /// Runs a prompt action against one explicit editor-context projection.
    fn run_prompt_action_with_context(
        &mut self,
        action: PromptShellAction,
        editor_context: Arc<Mutex<EditorContext>>,
    ) -> PromptActionOutcome {
        match run_prompt_shell_action(
            &self.term,
            &self.handle,
            editor_context,
            self.external_editor.as_deref(),
            &self.prompt_history,
            action,
        ) {
            Ok(Some(PromptShellResult::Replace(new_text))) => {
                let cursor = new_text.len();
                self.handle.set_buffer(new_text, cursor);
                self.sync_menu_block();
            }
            Ok(Some(PromptShellResult::ReplacePreservingUndo(new_text))) => {
                let cursor = new_text.len();
                self.handle.set_buffer_preserving_undo(new_text, cursor);
                self.sync_menu_block();
            }
            Ok(Some(PromptShellResult::Insert(text))) => {
                let mut buffer = self.handle.get_buffer();
                let cursor = self.handle.get_cursor();
                buffer.insert_str(cursor, &text);
                self.handle.set_buffer(buffer, cursor + text.len());
                self.sync_menu_block();
            }
            Ok(Some(PromptShellResult::Action(action))) => {
                return PromptActionOutcome::Return(Event::Action(action));
            }
            Ok(Some(PromptShellResult::History(delta))) => {
                self.term.trigger_history_step(delta);
                self.sync_menu_block();
            }
            Ok(Some(PromptShellResult::Undo)) => {
                self.term.trigger_undo();
                self.sync_menu_block();
            }
            Ok(Some(PromptShellResult::Redo)) => {
                self.term.trigger_redo();
                self.sync_menu_block();
            }
            Ok(Some(PromptShellResult::RawEvent(raw))) => {
                return self.apply_raw_prompt_event(raw);
            }
            Ok(None) => {} // shell exited non-zero or no output applies.
            Err(error) if error.is_foreground_ownership_unconfirmed() => {
                return PromptActionOutcome::Fatal(fatal_terminal_ownership(error));
            }
            Err(error) => self.print_local(&format!("prompt action: {error}")),
        }
        PromptActionOutcome::BufferChanged
    }

    fn apply_raw_prompt_event(&mut self, raw: RawEvent) -> PromptActionOutcome {
        match raw {
            RawEvent::PasteUpload { id, text } => {
                PromptActionOutcome::Return(Event::PasteUpload { id, text })
            }
            RawEvent::PasteCancelled { id } => {
                PromptActionOutcome::Return(Event::PasteCancelled { id })
            }
            RawEvent::BufferChanged => {
                self.sync_menu_block();
                PromptActionOutcome::BufferChanged
            }
            RawEvent::CompletionRefresh => {
                self.sync_menu_block();
                self.handle.redraw();
                PromptActionOutcome::Continue
            }
            RawEvent::CompletionAccept => {
                self.sync_menu_block();
                PromptActionOutcome::Continue
            }
            RawEvent::Line(line) => {
                let started = path_std_time::Instant::now();
                self.record_submitted_prompt(&line);
                let history_finished = path_std_time::Instant::now();
                self.sync_menu_block();
                tracing::trace!(
                    target: "tau_cli::prompt_submission",
                    stage = "highterm_history_menu",
                    prompt_bytes = line.len(),
                    history_us = history_finished.duration_since(started).as_micros(),
                    menu_sync_us = history_finished.elapsed().as_micros(),
                    redraw_request_us = 0_u64,
                    redraw_requested = false,
                    stage_us = started.elapsed().as_micros(),
                    "content-free prompt submission stage"
                );
                PromptActionOutcome::Return(Event::Line(line))
            }
            RawEvent::Eof => PromptActionOutcome::Return(Event::Eof),
            RawEvent::CancelPrompt => PromptActionOutcome::Return(Event::CancelPrompt),
            RawEvent::Resize { width, height } => {
                PromptActionOutcome::Return(Event::Resize { width, height })
            }
            RawEvent::FocusChanged { focused } => {
                PromptActionOutcome::Return(Event::FocusChanged { focused })
            }
            RawEvent::BackTab => PromptActionOutcome::Return(Event::BackTab),
            RawEvent::Escape => PromptActionOutcome::Return(Event::Escape),
            RawEvent::Notice(message) => {
                self.print_local(&message);
                PromptActionOutcome::BufferChanged
            }
            RawEvent::Binding(_) | RawEvent::ExternalEditor => {
                unreachable!("unsupported prompt action event")
            }
        }
    }

    /// Records one submitted prompt using the canonical literal-escape
    /// spelling.
    fn record_submitted_prompt(&mut self, line: &str) {
        if line.is_empty() {
            return;
        }
        let history_line = canonical_literal_colon_prompt(line).unwrap_or_else(|| line.to_owned());
        self.prompt_history.push(history_line.clone());
        self.last_submitted_prompt_retained = true;
        self.term.replace_last_submitted_input(history_line);
    }

    /// Bounds both histories after the input loop has finalized this
    /// submission's canonical or redacted presentation.
    pub fn finalize_last_submitted_prompt_history(&mut self) {
        let history = std::mem::take(&mut self.prompt_history);
        self.prompt_history = self.bounded_prompt_history_for_attachment(history);
        self.last_submitted_prompt_retained = self.prompt_history.last().is_some();
        self.term.finalize_submitted_input_history();
    }

    /// Returns an attachment-local bounded suffix using this instance's limits.
    fn bounded_prompt_history_for_attachment(&self, prompt_history: Vec<String>) -> Vec<String> {
        Self::bounded_prompt_history_with_limits(
            prompt_history,
            self.effective_prompt_history_limits(),
        )
    }

    /// Returns the newest permitted suffix under explicit entry and byte
    /// limits.
    fn bounded_prompt_history_with_limits(
        mut prompt_history: Vec<String>,
        limits: PromptHistoryLimits,
    ) -> Vec<String> {
        prompt_history.retain(|entry| !entry.is_empty() && entry.len() <= limits.max_bytes);

        let mut retained_bytes = 0;
        let mut retained_start = prompt_history.len();
        for (retained_entries, (index, entry)) in
            prompt_history.iter().enumerate().rev().enumerate()
        {
            if retained_entries == limits.max_entries
                || entry.len() > limits.max_bytes - retained_bytes
            {
                break;
            }
            retained_bytes += entry.len();
            retained_start = index;
        }
        prompt_history.drain(..retained_start);
        prompt_history
    }

    /// Returns the production HighTerm history limits.
    fn prompt_history_limits() -> PromptHistoryLimits {
        PromptHistoryLimits {
            max_entries: PROMPT_HISTORY_MAX_ENTRIES,
            max_bytes: PROMPT_HISTORY_MAX_BYTES,
        }
    }

    /// Returns production limits or a focused test's explicit local limits.
    fn effective_prompt_history_limits(&self) -> PromptHistoryLimits {
        #[cfg(test)]
        if let Some(limits) = self.prompt_history_limit_override {
            return limits;
        }
        Self::prompt_history_limits()
    }

    fn maybe_run_command_completion(&mut self) -> io::Result<bool> {
        let buffer = self.handle.get_buffer();
        let cursor = self.handle.get_cursor();
        let Some((command, before, after)) = self
            .completion_command_rules
            .command_for_exact_token(&buffer, cursor)
        else {
            self.last_command_completion_token = None;
            return Ok(false);
        };
        let token_key = format!("{before}\0{cursor}");
        if self.last_command_completion_token.as_deref() == Some(token_key.as_str()) {
            return Ok(false);
        }
        self.last_command_completion_token = Some(token_key);
        let completion_result = match command {
            completion::CommandCompletionMatch::Command(command) => {
                run_completion_command(&self.term, command)
            }
            completion::CommandCompletionMatch::EmptyCommand => {
                Err(empty_completion_command_error())
            }
        };
        match completion_result {
            Ok(Some(text)) => {
                let new_text = format!("{before}{text}{after}");
                let new_cursor = before.len() + text.len();
                self.handle.set_buffer(new_text, new_cursor);
                self.last_command_completion_token = None;
                Ok(true)
            }
            Ok(None) => Ok(false),
            Err(error) if error.is_foreground_ownership_unconfirmed() => {
                Err(fatal_terminal_ownership(error))
            }
            Err(error) => {
                self.print_local(&format!("completion command: {error}"));
                Ok(true)
            }
        }
    }

    fn print_local(&self, message: &str) {
        let block = resolve::themed_block(
            &self.theme,
            tau_themes::names::SYSTEM_INFO,
            message.to_owned(),
        );
        self.handle.print_output("prompt-action-error", block);
    }
}

/// Runs one callback between injectable terminal release and resume operations.
fn run_with_external_terminal_hooks<T>(
    callback: impl FnOnce() -> T,
    pause: impl FnOnce() -> io::Result<()>,
    resume: impl FnMut() -> io::Result<()>,
) -> io::Result<T> {
    pause()?;
    let guard = ExternalResumeGuard::new(resume);
    let result = callback();
    guard.finish()?;
    Ok(result)
}

/// Bounds startup-seeded history with production limits before an attachment
/// owns it. Attachment mutations use
/// [`HighTerm::bounded_prompt_history_for_attachment`].
fn bounded_seeded_prompt_history(prompt_history: Vec<String>) -> Vec<String> {
    HighTerm::bounded_prompt_history_with_limits(prompt_history, HighTerm::prompt_history_limits())
}

/// Returns canonical literal-colon prompt text for a line beginning with `::`.
///
/// Leading whitespace is preserved while exactly one colon is removed from the
/// first non-whitespace token. Lines that do not use the escape return `None`.
#[must_use]
pub fn canonical_literal_colon_prompt(line: &str) -> Option<String> {
    let leading_len = line.len() - line.trim_start().len();
    line.get(leading_len..)?
        .strip_prefix("::")
        .map(|suffix| format!("{}:{suffix}", &line[..leading_len]))
}

fn run_agent_fzf_command_with_ownership(
    program: &std::ffi::OsStr,
    rows: &str,
    timeout: std::time::Duration,
    ownership: ProcessOwnership,
    after_spawn: impl FnOnce() -> Result<(), String>,
) -> Result<Option<String>, BoundedCommandError> {
    let mut command = path_std_process::Command::new(program);
    command
        .args(AGENT_PICKER_FZF_ARGS)
        .stdout(path_std_process::Stdio::piped())
        .stderr(path_std_process::Stdio::null());
    let output = run_with_bounded_stdout_after_spawn(
        &mut command,
        Some(rows.as_bytes()),
        AGENT_PICKER_OUTPUT_LIMIT_BYTES,
        timeout,
        ownership,
        after_spawn,
    )
    .map_err(|error| match error {
        BoundedCommandError::Command(error) => {
            BoundedCommandError::Command(format!("fzf failed: {error}"))
        }
        error @ BoundedCommandError::ForegroundOwnershipUnconfirmed { .. } => error,
    })?;
    match output.status.code() {
        Some(1 | 130) => Ok(None),
        _ if !output.status.success() => Err(format!(
            "fzf exited with status {}",
            output.status.code().map_or_else(
                || "terminated by signal".to_owned(),
                |code| code.to_string()
            )
        )
        .into()),
        _ => parse_agent_fzf_output(output.stdout).map_err(BoundedCommandError::Command),
    }
}

fn format_agent_picker_rows(rows: &str, terminal_width: usize) -> Result<String, String> {
    let fields = rows
        .lines()
        .map(|row| {
            let fields = row.split('\t').collect::<Vec<_>>();
            if fields.len() != AGENT_PICKER_SOURCE_FIELDS {
                return Err(format!(
                    "agent row has {} fields instead of {AGENT_PICKER_SOURCE_FIELDS}",
                    fields.len()
                ));
            }
            Ok(fields)
        })
        .collect::<Result<Vec<_>, _>>()?;
    let content_width = terminal_width.saturating_sub(AGENT_PICKER_FZF_DECORATION_WIDTH);
    let column_widths = agent_picker_column_widths(&fields, content_width);
    let mut output = String::new();
    for fields in fields {
        let display = AGENT_PICKER_COLUMNS
            .iter()
            .zip(&column_widths)
            .map(|(column, &width)| {
                let source = agent_picker_column_value(&fields, column.source_field);
                let value = truncate_to_width(&source, width);
                let padding = width.saturating_sub(display_width(&value));
                format!("{value}{}", " ".repeat(padding))
            })
            .collect::<Vec<_>>()
            .join(AGENT_PICKER_COLUMN_GAP)
            .trim_end()
            .to_owned();
        output.push_str(&fields.join("\t"));
        output.push('\t');
        output.push_str(&display);
        output.push('\n');
    }
    Ok(output)
}

fn agent_picker_column_widths(rows: &[Vec<&str>], content_width: usize) -> Vec<usize> {
    let mut column_count = AGENT_PICKER_COLUMNS.len();
    while 1 < column_count
        && content_width
            < AGENT_PICKER_COLUMNS[..column_count]
                .iter()
                .map(|column| column.minimum_width)
                .sum::<usize>()
                + AGENT_PICKER_COLUMN_GAP.len() * column_count.saturating_sub(1)
    {
        column_count -= 1;
    }
    if content_width == 0 {
        return Vec::new();
    }
    let gap_width = AGENT_PICKER_COLUMN_GAP.len() * column_count.saturating_sub(1);
    let available = content_width.saturating_sub(gap_width);
    let natural = AGENT_PICKER_COLUMNS[..column_count]
        .iter()
        .map(|column| {
            rows.iter()
                .map(|row| display_width(&agent_picker_column_value(row, column.source_field)))
                .max()
                .unwrap_or(1)
                .max(1)
                .min(column.max_width)
        })
        .collect::<Vec<_>>();
    let mut widths = vec![1; column_count];
    let mut remaining = available.saturating_sub(column_count);
    for targets in [
        &AGENT_PICKER_COLUMNS[..column_count]
            .iter()
            .map(|column| column.minimum_width)
            .collect::<Vec<_>>(),
        &AGENT_PICKER_COLUMNS[..column_count]
            .iter()
            .map(|column| column.preferred_width)
            .collect::<Vec<_>>(),
        natural.as_slice(),
    ] {
        while 0 < remaining {
            let mut grew = false;
            for (column, width) in widths.iter_mut().enumerate() {
                let target = targets[column].min(natural[column]);
                if *width < target && 0 < remaining {
                    *width += 1;
                    remaining -= 1;
                    grew = true;
                }
            }
            if !grew {
                break;
            }
        }
    }
    widths
}

/// Return one source column, synthesizing the mandatory visual prefix.
fn agent_picker_column_value(fields: &[&str], source_field: usize) -> String {
    if source_field == usize::MAX {
        format!("{}{} @{}", fields[12], fields[14], fields[0])
    } else {
        fields[source_field].to_owned()
    }
}

fn parse_agent_fzf_output(output: Vec<u8>) -> Result<Option<String>, String> {
    let output =
        String::from_utf8(output).map_err(|error| format!("fzf output was not UTF-8: {error}"))?;
    let output = output.strip_suffix('\n').unwrap_or(&output);
    let output = output.strip_suffix('\r').unwrap_or(output);
    if output.is_empty() {
        return Ok(None);
    }
    if output.contains(['\n', '\r']) {
        return Err("fzf returned more than one row".to_owned());
    }
    let (row, _) = output
        .rsplit_once('\t')
        .ok_or_else(|| "fzf returned a malformed agent row".to_owned())?;
    if row.split('\t').count() != AGENT_PICKER_SOURCE_FIELDS {
        return Err("fzf returned a malformed agent row".to_owned());
    }
    Ok(Some(row.to_owned()))
}

fn make_completion_source(
    commands: Vec<CommandCompletion>,
    data: CompletionData,
    rules: CompletionRules,
) -> Box<dyn tau_cli_term_raw::CompletionSource> {
    let commands = Arc::new(commands);
    let rules = Arc::new(rules);
    Box::new(move |buffer: &str, cursor: usize| -> Vec<Candidate> {
        completion::build_candidates_with_rules(&commands, &data, &rules, buffer, cursor)
    })
}

fn run_completion_command(
    term: &tau_cli_term_raw::Term,
    command: &completion::CompletionCommand,
) -> Result<Option<String>, BoundedCommandError> {
    term.pause_for_external()
        .map_err(|e| format!("could not release terminal: {e}"))?;
    let guard = ExternalResumeGuard::new(|| term.resume_after_external());
    let mut command_builder = path_std_process::Command::new(command.program());
    command_builder
        .args(command.args())
        .stdin(path_std_process::Stdio::null())
        .stdout(path_std_process::Stdio::piped())
        .stderr(path_std_process::Stdio::null());
    let output = preserve_pause_on_unconfirmed_foreground(
        guard,
        run_with_bounded_stdout(
            &mut command_builder,
            None,
            COMPLETION_COMMAND_OUTPUT_LIMIT_BYTES,
            COMPLETION_COMMAND_TIMEOUT,
            ProcessOwnership::ForegroundProcessGroup,
        ),
    )?;
    if !output.status.success() {
        return Ok(None);
    }
    let text = String::from_utf8(output.stdout)
        .map_err(|e| format!("command output was not utf-8: {e}"))?;
    let text = text.trim().to_owned();
    if text.is_empty() {
        Ok(None)
    } else {
        Ok(Some(text))
    }
}

/// Returns the established diagnostic for a public completion command with no
/// executable argv element.
fn empty_completion_command_error() -> BoundedCommandError {
    "empty command".to_owned().into()
}

struct PromptShellCommand {
    command: String,
    trim: bool,
}

enum PromptShellAction {
    Insert(PromptShellCommand),
    Edit(PromptShellCommand),
    HistorySearch(PromptShellCommand),
    Action(String),
    PromptNext,
    PromptPrevious,
    PromptUndo,
    PromptRedo,
    SubmitPrompt,
    InsertNewline,
}

/// Conversation context and prompt-editor recovery state appended below the
/// prompt trailer when the user edits the prompt in an external editor.
#[derive(Clone, Default)]
pub struct EditorContext {
    /// Response text currently streaming or otherwise in progress, included as
    /// read-only context when editing the next prompt.
    pub current_response: Option<String>,
    /// Most recent completed response text, included as read-only context when
    /// editing the next prompt.
    pub last_response: Option<String>,
    /// Previous submitted prompt text, included as read-only context when
    /// editing the next prompt.
    pub previous_prompt: Option<String>,
    /// Exact Markdown conversation rendered directly below the trailer marker
    /// for a history-aware editor invocation.
    pub chat_markdown: Option<String>,
    /// Text recovered from a previous edit where the normally ignored trailer
    /// section was modified before the editor exited. This is set only when the
    /// edited trailer differs from the generated trailer, cleared when the
    /// trailer is unchanged or the marker is deleted, rendered below the marker
    /// on the next edit, and never promoted into the prompt unless the user
    /// manually moves it above the marker.
    pub edited_trailer_recovery: Option<String>,
}

impl EditorContext {
    fn update_edited_trailer_recovery(&mut self, original_text: &str, edited_text: &str) {
        let Some((_, original_trailer)) = split_at_prompt_trailer_marker(original_text) else {
            return;
        };
        self.edited_trailer_recovery = edited_trailer_recovery(original_trailer, edited_text);
    }
}

enum PromptShellResult {
    Insert(String),
    Replace(String),
    ReplacePreservingUndo(String),
    Action(String),
    History(isize),
    Undo,
    Redo,
    RawEvent(RawEvent),
}

#[derive(Clone, Copy)]
enum PromptShellExternalKind {
    Insert,
    Edit,
    HistorySearch,
}

struct PromptShellExternalAction {
    kind: PromptShellExternalKind,
    shell: PromptShellCommand,
}

enum PromptShellDispatch {
    Immediate(PromptShellResult),
    External(PromptShellExternalAction),
}

struct PromptHistoryPicker {
    rows: String,
    prompt_dir: tempfile::TempDir,
}

enum PromptShellCommandOutput {
    Edited,
    Captured(Vec<u8>),
}

enum PromptActionOutcome {
    BufferChanged,
    Continue,
    Return(Event),
    Fatal(io::Error),
}

enum NextEventStep {
    Return(Event),
    Continue,
    Fatal(io::Error),
}

impl PromptActionOutcome {
    fn into_next_event_step(self) -> NextEventStep {
        match self {
            Self::BufferChanged => NextEventStep::Return(Event::BufferChanged),
            Self::Continue => NextEventStep::Continue,
            Self::Return(event) => NextEventStep::Return(event),
            Self::Fatal(error) => NextEventStep::Fatal(error),
        }
    }
}

/// Converts terminal-ownership loss into a fatal interactive-input error.
fn fatal_terminal_ownership(error: BoundedCommandError) -> io::Error {
    io::Error::other(error)
}

impl PromptShellAction {
    // Keep prompt-local action names, Term::trigger_named_action,
    // crates/tau-cli/src/chat.rs::encode_binding_action,
    // built-in.cli-bindings.yaml, and docs/cli-keybindings.md in sync.
    fn parse(action: &str) -> Option<Self> {
        match action {
            "prompt-next" => return Some(Self::PromptNext),
            "prompt-previous" => return Some(Self::PromptPrevious),
            "prompt-undo" => return Some(Self::PromptUndo),
            "prompt-redo" => return Some(Self::PromptRedo),
            "submit-prompt" => return Some(Self::SubmitPrompt),
            "insert-newline" => return Some(Self::InsertNewline),
            _ => {}
        }
        let mut parts = action.splitn(3, ':');
        let name = parts.next()?;
        let (Some(mode), Some(command)) = (parts.next(), parts.next()) else {
            return (!action.is_empty() && !action.contains(':'))
                .then(|| Self::Action(action.to_owned()));
        };
        let trim = match mode {
            "trim" => true,
            "raw" => false,
            _ => return None,
        };
        let command = PromptShellCommand {
            command: command.to_owned(),
            trim,
        };
        match name {
            "shell-prompt-insert" => Some(Self::Insert(command)),
            "shell-prompt-edit" => Some(Self::Edit(command)),
            "prompt-history-search" => Some(Self::HistorySearch(command)),
            _ => None,
        }
    }
}

fn prompt_shell_dispatch(
    action: PromptShellAction,
    term: &tau_cli_term_raw::Term,
) -> PromptShellDispatch {
    match action {
        PromptShellAction::PromptNext => {
            PromptShellDispatch::Immediate(PromptShellResult::History(1))
        }
        PromptShellAction::PromptPrevious => {
            PromptShellDispatch::Immediate(PromptShellResult::History(-1))
        }
        PromptShellAction::PromptUndo => PromptShellDispatch::Immediate(PromptShellResult::Undo),
        PromptShellAction::PromptRedo => PromptShellDispatch::Immediate(PromptShellResult::Redo),
        PromptShellAction::Action(action) => {
            PromptShellDispatch::Immediate(PromptShellResult::Action(action))
        }
        PromptShellAction::SubmitPrompt => PromptShellDispatch::Immediate(
            PromptShellResult::RawEvent(term.trigger_submit_or_accept_completion()),
        ),
        PromptShellAction::InsertNewline => PromptShellDispatch::Immediate(
            PromptShellResult::RawEvent(term.trigger_insert_newline()),
        ),
        PromptShellAction::Insert(shell) => {
            PromptShellDispatch::External(PromptShellExternalAction {
                kind: PromptShellExternalKind::Insert,
                shell,
            })
        }
        PromptShellAction::Edit(shell) => {
            PromptShellDispatch::External(PromptShellExternalAction {
                kind: PromptShellExternalKind::Edit,
                shell,
            })
        }
        PromptShellAction::HistorySearch(shell) => {
            PromptShellDispatch::External(PromptShellExternalAction {
                kind: PromptShellExternalKind::HistorySearch,
                shell,
            })
        }
    }
}

fn run_prompt_shell_action(
    term: &tau_cli_term_raw::Term,
    handle: &TermHandle,
    editor_context: Arc<Mutex<EditorContext>>,
    external_editor: Option<&str>,
    prompt_history: &[String],
    action: PromptShellAction,
) -> Result<Option<PromptShellResult>, BoundedCommandError> {
    let external_action = match prompt_shell_dispatch(action, term) {
        PromptShellDispatch::Immediate(result) => return Ok(Some(result)),
        PromptShellDispatch::External(external_action) => external_action,
    };
    let current = trim_prompt_newlines(&handle.get_buffer()).to_owned();
    let cursor = handle.get_cursor();
    let tmp = tempfile::Builder::new()
        .prefix("tau-prompt-")
        .suffix(".tau.md")
        .tempfile()
        .map_err(|e| format!("could not create tempfile: {e}"))?;
    let file_text = prompt_shell_file_text(external_action.kind, &current, &editor_context);
    std::fs::write(tmp.path(), file_text.as_bytes())
        .map_err(|e| format!("could not write tempfile: {e}"))?;

    let history_picker = match external_action.kind {
        PromptShellExternalKind::HistorySearch => {
            match prepare_history_picker(term, prompt_history)? {
                Some(history_picker) => Some(history_picker),
                None => return Ok(None),
            }
        }
        PromptShellExternalKind::Insert | PromptShellExternalKind::Edit => None,
    };

    let command = external_action.shell.command.as_str();
    tracing::trace!(
        target: "tau_cli::input",
        command,
        prompt_path = %tmp.path().display(),
        cursor,
        "spawning prompt shell action"
    );
    if command.trim().is_empty() {
        return Err("empty shell command".to_owned().into());
    }

    term.pause_for_external()
        .map_err(|e| format!("could not release terminal: {e}"))?;
    // RAII so a spawn error / panic still restores raw mode.
    let guard = ExternalResumeGuard::new(|| term.resume_after_external());

    let mut command_builder = prompt_shell_command_builder(
        command,
        tmp.path(),
        cursor,
        external_editor,
        history_picker.as_ref(),
    );
    let command_result = preserve_pause_on_unconfirmed_foreground(
        guard,
        run_external_prompt_shell_command(
            &mut command_builder,
            external_action.kind,
            history_picker.as_ref(),
        ),
    );
    let Some(output) = command_result? else {
        return Ok(None);
    };
    match output {
        PromptShellCommandOutput::Captured(stdout) => {
            return Ok(prompt_shell_captured_result(
                external_action.kind,
                external_action.shell.trim,
                stdout,
                prompt_history,
            )?);
        }
        PromptShellCommandOutput::Edited => {}
    }

    let new_text =
        std::fs::read_to_string(tmp.path()).map_err(|e| format!("could not read tempfile: {e}"))?;
    editor_context
        .lock()
        .expect("editor context mutex poisoned")
        .update_edited_trailer_recovery(&file_text, &new_text);
    let new_text = strip_prompt_trailer(&new_text);
    let new_text = trim_prompt_newlines(new_text).to_owned();
    Ok(Some(PromptShellResult::Replace(new_text)))
}

fn prompt_shell_file_text(
    kind: PromptShellExternalKind,
    current: &str,
    editor_context: &Arc<Mutex<EditorContext>>,
) -> String {
    match kind {
        PromptShellExternalKind::Edit => append_prompt_trailer(current, editor_context),
        PromptShellExternalKind::Insert | PromptShellExternalKind::HistorySearch => {
            current.to_owned()
        }
    }
}

fn prepare_history_picker(
    term: &tau_cli_term_raw::Term,
    prompt_history: &[String],
) -> Result<Option<PromptHistoryPicker>, String> {
    let rows = prompt_history_search_rows(prompt_history);
    if rows.is_empty() {
        return Ok(None);
    }
    let prompt_dir = prompt_history_preview_dir(prompt_history)?;
    term.record_prompt_undo();
    Ok(Some(PromptHistoryPicker { rows, prompt_dir }))
}

fn prompt_shell_command_builder(
    command: &str,
    prompt_path: &std::path::Path,
    cursor: usize,
    external_editor: Option<&str>,
    history_picker: Option<&PromptHistoryPicker>,
) -> std::process::Command {
    let mut command_builder = path_std_process::Command::new("sh");
    command_builder
        .arg("-c")
        .arg(command)
        .env("TAU_PROMPT_PATH", prompt_path)
        .env("TAU_PROMPT_COLUMN", (cursor + 1).to_string())
        .env("TAU_PROMPT_ROW", "1")
        .env("TAU_EDITOR", external_editor.unwrap_or(""));
    if let Some(history_picker) = history_picker {
        command_builder.env("TAU_PROMPT_HISTORY_DIR", history_picker.prompt_dir.path());
    }
    command_builder
}

fn run_external_prompt_shell_command(
    command_builder: &mut std::process::Command,
    kind: PromptShellExternalKind,
    history_picker: Option<&PromptHistoryPicker>,
) -> Result<Option<PromptShellCommandOutput>, BoundedCommandError> {
    match kind {
        PromptShellExternalKind::Edit => run_prompt_edit_command(command_builder),
        PromptShellExternalKind::Insert | PromptShellExternalKind::HistorySearch => {
            run_prompt_capture_command(command_builder, history_picker)
        }
    }
}

fn run_prompt_edit_command(
    command_builder: &mut std::process::Command,
) -> Result<Option<PromptShellCommandOutput>, BoundedCommandError> {
    command_builder
        .stdin(path_std_process::Stdio::inherit())
        .stdout(path_std_process::Stdio::inherit())
        .stderr(path_std_process::Stdio::inherit());
    let status = run_with_inherited_stdio(
        command_builder,
        PROMPT_COMMAND_TIMEOUT,
        ProcessOwnership::ForegroundProcessGroup,
    )?
    .status;
    Ok(status.success().then_some(PromptShellCommandOutput::Edited))
}

fn run_prompt_capture_command(
    command_builder: &mut std::process::Command,
    history_picker: Option<&PromptHistoryPicker>,
) -> Result<Option<PromptShellCommandOutput>, BoundedCommandError> {
    command_builder.stdin(if history_picker.is_some() {
        path_std_process::Stdio::piped()
    } else {
        path_std_process::Stdio::null()
    });
    command_builder
        .stdout(path_std_process::Stdio::piped())
        .stderr(path_std_process::Stdio::null());
    let stdin_input = history_picker.map(|history_picker| history_picker.rows.as_bytes());
    let output = run_with_bounded_stdout(
        command_builder,
        stdin_input,
        PROMPT_COMMAND_OUTPUT_LIMIT_BYTES,
        PROMPT_COMMAND_TIMEOUT,
        ProcessOwnership::ForegroundProcessGroup,
    )?;
    Ok(output
        .status
        .success()
        .then_some(PromptShellCommandOutput::Captured(output.stdout)))
}

fn prompt_shell_captured_result(
    kind: PromptShellExternalKind,
    trim: bool,
    stdout: Vec<u8>,
    prompt_history: &[String],
) -> Result<Option<PromptShellResult>, String> {
    let text =
        String::from_utf8(stdout).map_err(|e| format!("command output was not utf-8: {e}"))?;
    let text = trim_prompt_shell_output(text, trim);
    match kind {
        PromptShellExternalKind::Insert => Ok(Some(PromptShellResult::Insert(text))),
        PromptShellExternalKind::HistorySearch => {
            selected_prompt_history_result(text, prompt_history)
        }
        PromptShellExternalKind::Edit => unreachable!(),
    }
}

fn trim_prompt_shell_output(text: String, trim: bool) -> String {
    if trim { text.trim().to_owned() } else { text }
}

fn selected_prompt_history_result(
    selected: String,
    prompt_history: &[String],
) -> Result<Option<PromptShellResult>, String> {
    let selected_index = selected.split('\t').next().unwrap_or("").trim();
    if selected_index.is_empty() {
        return Ok(None);
    }
    let index = selected_index
        .parse::<usize>()
        .map_err(|e| format!("history selection was not an index: {e}"))?;
    let text = prompt_history
        .get(index)
        .ok_or_else(|| format!("history selection index {index} is out of range"))?
        .clone();
    Ok(Some(PromptShellResult::ReplacePreservingUndo(text)))
}

fn prompt_history_search_rows(prompt_history: &[String]) -> String {
    let mut rows = String::new();
    for (index, prompt) in bounded_prompt_history_entries(prompt_history) {
        rows.push_str(&index.to_string());
        rows.push('\t');
        rows.push_str(&prompt_history_summary(prompt));
        rows.push('\n');
    }
    rows
}

fn prompt_history_preview_dir(prompt_history: &[String]) -> Result<tempfile::TempDir, String> {
    let dir = tempfile::Builder::new()
        .prefix("tau-prompt-history-")
        .tempdir()
        .map_err(|e| format!("could not create prompt history tempdir: {e}"))?;
    let mut remaining_total = PROMPT_HISTORY_PREVIEW_TOTAL_BYTES;
    for (index, prompt) in bounded_prompt_history_entries(prompt_history) {
        let preview = bounded_prompt_history_preview(prompt, &mut remaining_total);
        std::fs::write(dir.path().join(index.to_string()), preview.as_bytes())
            .map_err(|e| format!("could not write prompt history preview {index}: {e}"))?;
    }
    Ok(dir)
}

fn bounded_prompt_history_entries(
    prompt_history: &[String],
) -> impl Iterator<Item = (usize, &str)> {
    prompt_history
        .iter()
        .enumerate()
        .rev()
        .filter(|(_, prompt)| !prompt.is_empty())
        .take(PROMPT_HISTORY_SEARCH_MAX_ROWS)
        .map(|(index, prompt)| (index, prompt.as_str()))
}

fn prompt_history_summary(prompt: &str) -> String {
    let mut summary = String::new();
    let mut summary_chars = 0usize;
    let mut pending_space = false;

    for ch in prompt.chars() {
        if ch.is_whitespace() {
            pending_space = !summary.is_empty();
            continue;
        }

        if pending_space {
            if summary_chars + 1 >= PROMPT_HISTORY_SUMMARY_MAX_CHARS {
                append_prompt_history_summary_ellipsis(&mut summary, &mut summary_chars);
                return summary;
            }
            summary.push(' ');
            summary_chars += 1;
            pending_space = false;
        }

        if summary_chars + 1 >= PROMPT_HISTORY_SUMMARY_MAX_CHARS {
            append_prompt_history_summary_ellipsis(&mut summary, &mut summary_chars);
            return summary;
        }
        summary.push(ch);
        summary_chars += 1;
    }

    summary
}

fn append_prompt_history_summary_ellipsis(summary: &mut String, summary_chars: &mut usize) {
    if *summary_chars == PROMPT_HISTORY_SUMMARY_MAX_CHARS {
        summary.pop();
        *summary_chars -= 1;
    }
    summary.push('…');
    *summary_chars += 1;
}

fn bounded_prompt_history_preview(prompt: &str, remaining_total: &mut usize) -> String {
    const TRUNCATED: &str = "\n[history preview truncated]\n";
    if *remaining_total == 0 {
        return String::new();
    }

    let budget = PROMPT_HISTORY_PREVIEW_MAX_BYTES.min(*remaining_total);
    if prompt.len() <= budget {
        *remaining_total = remaining_total.saturating_sub(prompt.len());
        return prompt.to_owned();
    }

    let content_budget = if budget > TRUNCATED.len() {
        budget - TRUNCATED.len()
    } else {
        budget
    };
    let end = previous_char_boundary(prompt, content_budget);
    let mut preview = prompt[..end].to_owned();
    if preview.len() + TRUNCATED.len() <= budget {
        preview.push_str(TRUNCATED);
    }
    *remaining_total = remaining_total.saturating_sub(preview.len());
    preview
}

fn previous_char_boundary(text: &str, index: usize) -> usize {
    let mut index = index.min(text.len());
    while !text.is_char_boundary(index) {
        index -= 1;
    }
    index
}

fn append_prompt_trailer(current: &str, editor_context: &Arc<Mutex<EditorContext>>) -> String {
    let context = editor_context
        .lock()
        .expect("editor context mutex poisoned")
        .clone();
    if context.current_response.is_none()
        && context.last_response.is_none()
        && context.previous_prompt.is_none()
        && context.chat_markdown.is_none()
        && context.edited_trailer_recovery.is_none()
    {
        return current.to_owned();
    }

    let mut out = trim_prompt_newlines(current).to_owned();
    out.push_str("\n\n");
    out.push_str(PROMPT_TRAILER_MARKER);
    out.push('\n');
    if let Some(chat_markdown) = context.chat_markdown {
        out.push_str(&chat_markdown);
        return out;
    }
    if let Some(text) = context
        .current_response
        .as_deref()
        .filter(|t| !t.is_empty())
    {
        out.push_str("\n## Current response in progress\n\n");
        push_markdown_quote(&mut out, text);
    }
    if let Some(text) = context.last_response.as_deref().filter(|t| !t.is_empty()) {
        out.push_str("\n## Last response\n\n");
        push_markdown_quote(&mut out, text);
    }
    if let Some(text) = context.previous_prompt.as_deref().filter(|t| !t.is_empty()) {
        out.push_str("\n## Previous prompt\n\n");
        push_markdown_quote(&mut out, text);
    }
    if let Some(text) = context
        .edited_trailer_recovery
        .as_deref()
        .filter(|t| !t.is_empty())
    {
        out.push_str("\n## Previously edited text below TAU trailer\n\n");
        out.push_str(
            "Move anything you want to keep above the TAU trailer marker; \
             leaving this section unchanged will discard it after this editor session.\n\n",
        );
        out.push_str(text);
        if !out.ends_with('\n') {
            out.push('\n');
        }
    }
    out
}

fn trim_prompt_newlines(text: &str) -> &str {
    text.trim_matches(['\n', '\r'])
}

fn strip_prompt_trailer(text: &str) -> &str {
    let Some((before, _)) = split_at_prompt_trailer_marker(text) else {
        return text;
    };
    trim_prompt_before_trailer_marker(before)
}

fn edited_trailer_recovery(original_trailer: &str, edited_text: &str) -> Option<String> {
    let (_, edited_trailer) = split_at_prompt_trailer_marker(edited_text)?;
    if edited_trailer == original_trailer {
        return None;
    }
    let trimmed = trim_prompt_newlines(edited_trailer);
    (!trimmed.is_empty()).then(|| trimmed.to_owned())
}

fn trim_prompt_before_trailer_marker(before: &str) -> &str {
    before
        .strip_suffix("\n\n")
        .or_else(|| before.strip_suffix("\r\n\r\n"))
        .or_else(|| before.strip_suffix('\n'))
        .or_else(|| before.strip_suffix("\r\n"))
        .unwrap_or(before)
}

fn split_at_prompt_trailer_marker(text: &str) -> Option<(&str, &str)> {
    let mut line_start = 0;
    for line in text.split_inclusive('\n') {
        let line_without_newline = line.strip_suffix('\n').unwrap_or(line);
        let line_without_ending = line_without_newline
            .strip_suffix('\r')
            .unwrap_or(line_without_newline);
        if line_without_ending == PROMPT_TRAILER_MARKER {
            let trailer_start = line_start + line.len();
            return Some((&text[..line_start], &text[trailer_start..]));
        }
        line_start += line.len();
    }
    None
}

fn push_markdown_quote(out: &mut String, text: &str) {
    for line in text.lines() {
        out.push_str("> ");
        out.push_str(line);
        out.push('\n');
    }
}

/// Resolves the external editor once at startup: `$EDITOR`, then `$VISUAL`,
/// then the first of `hx`/`vim`/`vi`/`nano` found on `$PATH`. The resolved
/// command is exposed to prompt shell actions as `$TAU_EDITOR`.
fn resolve_external_editor() -> Option<String> {
    for var in ["EDITOR", "VISUAL"] {
        if let Some(val) = std::env::var_os(var) {
            let s = val.to_string_lossy();
            let trimmed = s.trim();
            if !trimmed.is_empty() {
                return Some(trimmed.to_owned());
            }
        }
    }
    ["hx", "vim", "vi", "nano"]
        .into_iter()
        .find(|cand| which::which(cand).is_ok())
        .map(str::to_owned)
}