harn_vm/stdlib/

http_response.rs

//! HTTP response codec primitives for `.harn` HTTP handlers.
//!
//! Handlers hosted on `harn-serve` build their replies with these
//! builtins instead of bare JSON dicts. The codec on the server side
//! (`harn_serve::http_codec`) recognises the tagged record, lifts
//! status/headers/body out of it, and renders it as a proper HTTP
//! response — JSON, no-content, error envelope, buffered stream, or
//! Server-Sent Events.
//!
//! The tag is the literal string `"v1"` under the
//! `__http_response__` key. Plain dicts that happen to define
//! `status`/`headers`/`body` are not picked up by the codec; only the
//! tagged record is.
//!
//! Channel-bearing bodies (`http_stream(channel)`, `http_sse(channel)`)
//! are drained inside the builtin before the handler returns. This
//! keeps the v1 codec wire-format JSON-only — the channel is
//! materialised into a list of chunks/events. Authors write the same
//! code that true streaming will accept; only the on-the-wire timing
//! differs.

use crate::value::VmDictExt;
use std::collections::BTreeMap;

use sha2::{Digest, Sha256};

use crate::stdlib::macros::{harn_builtin, VmBuiltinDef};
use crate::value::{VmError, VmValue};
use crate::vm::Vm;

/// Tag key + version that the harn-serve codec keys off of.
pub const HTTP_RESPONSE_TAG_KEY: &str = "__http_response__";
pub const HTTP_RESPONSE_TAG_VERSION: &str = "v1";

const BODY_KIND_JSON: &str = "json";
const BODY_KIND_NONE: &str = "none";
const BODY_KIND_BYTES: &str = "bytes";
const BODY_KIND_STREAM: &str = "stream";
const BODY_KIND_SSE: &str = "sse";

pub(crate) fn register_http_response_builtins(vm: &mut Vm) {
    for def in MODULE_BUILTINS {
        vm.register_builtin_def(def);
    }
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_ok(body: any?) -> dict", category = "http_response"
)]
fn http_ok_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let body = args.first().cloned().unwrap_or(VmValue::Nil);
    Ok(envelope(
        200,
        body,
        BODY_KIND_JSON,
        crate::value::DictMap::new(),
    ))
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_created(body: any?, location?: string?) -> dict",
    category = "http_response"
)]
fn http_created_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let body = args.first().cloned().unwrap_or(VmValue::Nil);
    let mut headers = crate::value::DictMap::new();
    if let Some(location) = args.get(1).and_then(string_or_nil) {
        headers.put_str("Location", location);
    }
    Ok(envelope(201, body, BODY_KIND_JSON, headers))
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_no_content() -> dict", category = "http_response"
)]
fn http_no_content_impl(_args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    Ok(envelope(
        204,
        VmValue::Nil,
        BODY_KIND_NONE,
        crate::value::DictMap::new(),
    ))
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_error(status: int, code: string, message: string, details?: any) -> dict",
    category = "http_response"
)]
fn http_error_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let status = require_status(args.first(), "http_error")?;
    if !(400..=599).contains(&status) {
        return Err(thrown_err(format!(
            "http_error: status must be 4xx or 5xx (got {status})"
        )));
    }
    let code = require_nonempty_string(args.get(1), "http_error", "code")?;
    let message = require_nonempty_string(args.get(2), "http_error", "message")?;
    let details = args.get(3).cloned().unwrap_or(VmValue::Nil);

    let mut body = crate::value::DictMap::new();
    body.put_str("code", code);
    body.put_str("message", message);
    if !matches!(details, VmValue::Nil) {
        body.insert(crate::value::intern_key("details"), details);
    }
    let mut env = envelope_map(
        status,
        VmValue::dict(body),
        BODY_KIND_JSON,
        crate::value::DictMap::new(),
    );
    env.insert(crate::value::intern_key("is_error"), VmValue::Bool(true));
    Ok(VmValue::dict(env))
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_reply(status: int, body?: any, headers?: dict) -> dict",
    category = "http_response"
)]
fn http_reply_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let status = require_status(args.first(), "http_reply")?;
    let body = args.get(1).cloned().unwrap_or(VmValue::Nil);
    let headers = parse_headers(args.get(2), "http_reply")?;
    let body_kind = if status == 204 || status == 304 || matches!(body, VmValue::Nil) {
        BODY_KIND_NONE
    } else if matches!(body, VmValue::Bytes(_)) {
        BODY_KIND_BYTES
    } else {
        BODY_KIND_JSON
    };
    let body_for_envelope = if body_kind == BODY_KIND_NONE {
        VmValue::Nil
    } else {
        body
    };
    Ok(envelope(status, body_for_envelope, body_kind, headers))
}

/// Convert a host/adapter response record with `status`, `body`, `headers`,
/// and optional `body_kind` into a tagged HTTP response envelope.
#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_reply_from(result: dict) -> dict",
    category = "http_response"
)]
fn http_reply_from_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let result = args
        .first()
        .and_then(VmValue::as_dict)
        .ok_or_else(|| thrown_err("http_reply_from: result must be a dict"))?;

    if is_http_response_envelope(result) {
        return Ok(VmValue::Dict(result.clone().into()));
    }

    let status = require_status(result.get("status"), "http_reply_from")?;
    let headers = parse_headers(result.get("headers"), "http_reply_from")?;
    let body_kind = match result.get("body_kind") {
        None | Some(VmValue::Nil) => None,
        Some(VmValue::String(kind)) => Some(kind.as_str()),
        Some(other) => {
            return Err(thrown_err(format!(
                "http_reply_from: body_kind must be a string (got {})",
                other.type_name()
            )));
        }
    };

    match body_kind {
        Some(BODY_KIND_NONE) => Ok(envelope(status, VmValue::Nil, BODY_KIND_NONE, headers)),
        Some(BODY_KIND_BYTES) => {
            let body = result
                .get("raw_body")
                .filter(|value| matches!(value, VmValue::Bytes(_)))
                .or_else(|| result.get("body"))
                .cloned()
                .unwrap_or(VmValue::Nil);
            match body {
                VmValue::Bytes(_) | VmValue::Nil => {
                    Ok(envelope(status, body, BODY_KIND_BYTES, headers))
                }
                other => Err(thrown_err(format!(
                    "http_reply_from: body_kind bytes requires bytes `raw_body` or `body` (got {})",
                    other.type_name()
                ))),
            }
        }
        Some(BODY_KIND_STREAM) => {
            let body = result
                .get("body")
                .cloned()
                .map(stream_body_chunks)
                .unwrap_or_else(empty_list);
            Ok(envelope(status, body, BODY_KIND_STREAM, headers))
        }
        Some(BODY_KIND_SSE) => {
            let body = result
                .get("body")
                .cloned()
                .map(stream_body_chunks)
                .unwrap_or_else(empty_list);
            Ok(envelope(status, body, BODY_KIND_SSE, headers))
        }
        Some(BODY_KIND_JSON) => {
            let body = result.get("body").cloned().unwrap_or(VmValue::Nil);
            Ok(envelope(status, body, BODY_KIND_JSON, headers))
        }
        _ => {
            let body = result.get("body").cloned().unwrap_or(VmValue::Nil);
            let args = [VmValue::Int(status), body, VmValue::dict(headers)];
            http_reply_impl(&args, &mut String::new())
        }
    }
}

fn empty_list() -> VmValue {
    VmValue::List(std::sync::Arc::new(Vec::new()))
}

fn stream_body_chunks(body: VmValue) -> VmValue {
    match body {
        VmValue::List(_) => body,
        VmValue::Nil => empty_list(),
        other => VmValue::List(std::sync::Arc::new(vec![other])),
    }
}

#[harn_builtin(
    exposure = "capability_arg:0",
    effects = ["state.mutate@arg0"],
    sig = "http_stream(source: any, content_type?: string?) -> dict",
    kind = "async",
    category = "http_response"
)]
async fn http_stream_impl(
    _ctx: crate::vm::AsyncBuiltinCtx,
    args: Vec<VmValue>,
) -> Result<VmValue, VmError> {
    let source = args
        .first()
        .cloned()
        .ok_or_else(|| thrown_err("http_stream: source is required"))?;
    let content_type = args
        .get(1)
        .and_then(string_or_nil)
        .unwrap_or_else(|| "application/octet-stream".to_string());

    let chunks = drain_to_list(source, "http_stream").await?;
    let mut headers = crate::value::DictMap::new();
    headers.put_str("Content-Type", content_type);
    Ok(envelope(
        200,
        VmValue::List(std::sync::Arc::new(chunks)),
        BODY_KIND_STREAM,
        headers,
    ))
}

#[harn_builtin(
    exposure = "capability_arg:0",
    effects = ["state.mutate@arg0"],
    sig = "http_sse(source: any, retry_ms?: int?) -> dict",
    kind = "async",
    category = "http_response"
)]
async fn http_sse_impl(
    _ctx: crate::vm::AsyncBuiltinCtx,
    args: Vec<VmValue>,
) -> Result<VmValue, VmError> {
    let source = args
        .first()
        .cloned()
        .ok_or_else(|| thrown_err("http_sse: source is required"))?;
    let retry_ms = match args.get(1) {
        None | Some(VmValue::Nil) => None,
        Some(VmValue::Int(value)) => {
            if *value < 0 {
                return Err(thrown_err(format!(
                    "http_sse: retry_ms must be non-negative (got {value})"
                )));
            }
            Some(*value)
        }
        Some(other) => {
            return Err(thrown_err(format!(
                "http_sse: retry_ms must be an integer (got {})",
                other.type_name()
            )));
        }
    };

    let events = drain_to_list(source, "http_sse").await?;
    let mut headers = crate::value::DictMap::new();
    headers.put_str("Content-Type", "text/event-stream");
    headers.put_str("Cache-Control", "no-cache");
    let mut env = envelope_map(
        200,
        VmValue::List(std::sync::Arc::new(events)),
        BODY_KIND_SSE,
        headers,
    );
    if let Some(retry_ms) = retry_ms {
        env.insert(crate::value::intern_key("retry_ms"), VmValue::Int(retry_ms));
    }
    Ok(VmValue::dict(env))
}

fn envelope(
    status: i64,
    body: VmValue,
    body_kind: &str,
    headers: crate::value::DictMap,
) -> VmValue {
    VmValue::dict(envelope_map(status, body, body_kind, headers))
}

fn envelope_map(
    status: i64,
    body: VmValue,
    body_kind: &str,
    headers: crate::value::DictMap,
) -> crate::value::DictMap {
    let mut map = crate::value::DictMap::new();
    map.insert(
        crate::value::intern_key(HTTP_RESPONSE_TAG_KEY),
        VmValue::String(arcstr::ArcStr::from(HTTP_RESPONSE_TAG_VERSION)),
    );
    map.insert(crate::value::intern_key("status"), VmValue::Int(status));
    map.put_str("body_kind", body_kind);
    map.insert(crate::value::intern_key("headers"), VmValue::dict(headers));
    if !matches!(body, VmValue::Nil) {
        map.insert(crate::value::intern_key("body"), body);
    }
    map
}

fn require_status(value: Option<&VmValue>, fn_name: &str) -> Result<i64, VmError> {
    let status = match value {
        Some(VmValue::Int(value)) => *value,
        Some(other) => {
            return Err(thrown_err(format!(
                "{fn_name}: status must be an integer (got {})",
                other.type_name()
            )));
        }
        None => {
            return Err(thrown_err(format!("{fn_name}: status is required")));
        }
    };
    if !(100..=599).contains(&status) {
        return Err(thrown_err(format!(
            "{fn_name}: status {status} is out of range (100-599)"
        )));
    }
    Ok(status)
}

fn require_nonempty_string(
    value: Option<&VmValue>,
    fn_name: &str,
    arg_name: &str,
) -> Result<String, VmError> {
    let text = match value {
        Some(VmValue::String(text)) => text.to_string(),
        Some(other) => {
            return Err(thrown_err(format!(
                "{fn_name}: {arg_name} must be a string (got {})",
                other.type_name()
            )));
        }
        None => {
            return Err(thrown_err(format!("{fn_name}: {arg_name} is required")));
        }
    };
    if text.is_empty() {
        return Err(thrown_err(format!(
            "{fn_name}: {arg_name} must be non-empty"
        )));
    }
    Ok(text)
}

fn string_or_nil(value: &VmValue) -> Option<String> {
    match value {
        VmValue::String(text) if !text.is_empty() => Some(text.to_string()),
        _ => None,
    }
}

fn parse_headers(value: Option<&VmValue>, fn_name: &str) -> Result<crate::value::DictMap, VmError> {
    match value {
        None | Some(VmValue::Nil) => Ok(crate::value::DictMap::new()),
        Some(VmValue::Dict(dict)) => Ok((**dict).clone()),
        Some(other) => Err(thrown_err(format!(
            "{fn_name}: headers must be a dict (got {})",
            other.type_name()
        ))),
    }
}

/// Drain a channel into a `Vec<VmValue>`, or pass a list through verbatim.
///
/// The drain stops when the channel is closed and all queued values have been
/// consumed. Channel handles can close through a flag/signal pair without
/// dropping every sender clone, so the drain polls with `try_recv` and observes
/// the closed state; when both report empty, the drain terminates.
async fn drain_to_list(value: VmValue, fn_name: &str) -> Result<Vec<VmValue>, VmError> {
    use tokio::sync::mpsc::error::TryRecvError;

    match value {
        VmValue::List(items) => Ok(items.iter().cloned().collect()),
        VmValue::Channel(handle) => {
            let mut items = Vec::new();
            let mut rx = handle.receiver.lock().await;
            loop {
                match rx.try_recv() {
                    Ok(value) => items.push(value),
                    Err(TryRecvError::Empty) => {
                        if handle.is_closed() {
                            break;
                        }
                        // Yield so the producer can deliver the next
                        // value; if the channel is still empty after
                        // the producer has done its work, the next
                        // pass will see it closed.
                        tokio::task::yield_now().await;
                    }
                    Err(TryRecvError::Disconnected) => break,
                }
            }
            Ok(items)
        }
        other => Err(thrown_err(format!(
            "{fn_name}: source must be a list or channel (got {})",
            other.type_name()
        ))),
    }
}

fn thrown_err(message: impl Into<String>) -> VmError {
    VmError::Thrown(VmValue::String(arcstr::ArcStr::from(message.into())))
}

/// Return `Some(envelope)` if the JSON value is a tagged HTTP response.
///
/// Used by the harn-serve HTTP codec to detect that a `.harn` handler
/// opted into structured HTTP semantics rather than the default `200
/// {result}` shape.
pub fn parse_envelope(value: &serde_json::Value) -> Option<HttpEnvelope> {
    let obj = value.as_object()?;
    let tag = obj.get(HTTP_RESPONSE_TAG_KEY)?.as_str()?;
    if tag != HTTP_RESPONSE_TAG_VERSION {
        return None;
    }
    let status = obj.get("status")?.as_u64()? as u16;
    let body_kind = obj
        .get("body_kind")
        .and_then(|v| v.as_str())
        .unwrap_or(BODY_KIND_JSON)
        .to_string();
    let headers = obj
        .get("headers")
        .and_then(|v| v.as_object())
        .map(|map| {
            map.iter()
                .map(|(key, value)| {
                    let header = match value {
                        serde_json::Value::String(s) => HttpHeaderValue::Single(s.clone()),
                        serde_json::Value::Array(values) => HttpHeaderValue::Multi(
                            values
                                .iter()
                                .filter_map(|v| v.as_str().map(str::to_string))
                                .collect(),
                        ),
                        other => HttpHeaderValue::Single(other.to_string()),
                    };
                    (key.clone(), header)
                })
                .collect::<BTreeMap<_, _>>()
        })
        .unwrap_or_default();
    let body = obj.get("body").cloned();
    let retry_ms = obj.get("retry_ms").and_then(|v| v.as_u64());
    let is_error = obj
        .get("is_error")
        .and_then(|v| v.as_bool())
        .unwrap_or(false);
    let ws_upgrade = obj
        .get("ws_upgrade")
        .and_then(|v| v.as_object())
        .map(|map| {
            let subprotocol = map
                .get("subprotocol")
                .and_then(|v| v.as_str())
                .map(str::to_string);
            let offered = map
                .get("offered")
                .and_then(|v| v.as_array())
                .map(|values| {
                    values
                        .iter()
                        .filter_map(|v| v.as_str().map(str::to_string))
                        .collect()
                })
                .unwrap_or_default();
            let idle_ping_ms = map.get("idle_ping_ms").and_then(|v| v.as_u64());
            let max_message_bytes = map.get("max_message_bytes").and_then(|v| v.as_u64());
            let on_message = map
                .get("on_message")
                .and_then(|v| v.as_str())
                .map(str::to_string);
            WsUpgradeSpec {
                subprotocol,
                offered,
                idle_ping_ms,
                max_message_bytes,
                on_message,
            }
        });
    Some(HttpEnvelope {
        status,
        body_kind,
        headers,
        body,
        retry_ms,
        is_error,
        ws_upgrade,
    })
}

#[derive(Debug, Clone)]
pub struct HttpEnvelope {
    pub status: u16,
    pub body_kind: String,
    pub headers: BTreeMap<String, HttpHeaderValue>,
    pub body: Option<serde_json::Value>,
    pub retry_ms: Option<u64>,
    pub is_error: bool,
    /// Populated when the handler returned an `http_upgrade_ws(...)`
    /// envelope. The hosting adapter detects this and routes the
    /// upgrade through `harn_serve::ws_route` instead of rendering the
    /// 101 response as plain HTTP.
    pub ws_upgrade: Option<WsUpgradeSpec>,
}

#[derive(Debug, Clone, Default)]
pub struct WsUpgradeSpec {
    pub subprotocol: Option<String>,
    pub offered: Vec<String>,
    pub idle_ping_ms: Option<u64>,
    pub max_message_bytes: Option<u64>,
    /// Exported `.harn` function the hosting adapter dispatches once per
    /// inbound WebSocket frame. The function receives a `{type, data}`
    /// message dict and its return value is rendered back to the client
    /// (a string is sent verbatim; any other value is JSON-encoded; `nil`
    /// sends nothing). `None` leaves the socket inbound-only — the adapter
    /// drains and discards frames, which suits server-push handlers that
    /// never read from the client.
    pub on_message: Option<String>,
}

#[derive(Debug, Clone)]
pub enum HttpHeaderValue {
    Single(String),
    Multi(Vec<String>),
}

impl HttpHeaderValue {
    pub fn values(&self) -> Box<dyn Iterator<Item = &str> + '_> {
        match self {
            Self::Single(value) => Box::new(std::iter::once(value.as_str())),
            Self::Multi(values) => Box::new(values.iter().map(String::as_str)),
        }
    }
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_etag(body: any) -> string", category = "http_response"
)]
fn http_etag_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let body = args
        .first()
        .ok_or_else(|| thrown_err("http_etag: body is required"))?;
    let bytes = value_as_bytes(body);
    let mut hasher = Sha256::new();
    hasher.update(&bytes);
    let digest = hasher.finalize();
    Ok(VmValue::String(arcstr::ArcStr::from(format!(
        "\"{}\"",
        hex::encode(digest)
    ))))
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_choose(accept: string?, offers: list, default?: string?) -> string",
    category = "http_response"
)]
fn http_choose_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let accept = optional_string_arg(args.first(), "http_choose", "accept")?;
    let offers_value = args
        .get(1)
        .ok_or_else(|| thrown_err("http_choose: offers is required"))?;
    let offers = expect_string_list(offers_value, "http_choose", "offers")?;
    if offers.is_empty() {
        return Err(thrown_err("http_choose: offers must be non-empty"));
    }
    let default = optional_string_arg(args.get(2), "http_choose", "default")?
        .unwrap_or_else(|| offers[0].clone());

    let chosen = match accept.as_deref() {
        None | Some("") | Some("*/*") => default,
        Some(header) => negotiate_accept(header, &offers).unwrap_or(default),
    };
    Ok(VmValue::String(arcstr::ArcStr::from(chosen)))
}

fn optional_string_arg(
    value: Option<&VmValue>,
    builtin: &str,
    arg_name: &str,
) -> Result<Option<String>, VmError> {
    match value {
        None | Some(VmValue::Nil) => Ok(None),
        Some(VmValue::String(text)) => Ok(Some(text.to_string())),
        Some(other) => Err(thrown_err(format!(
            "{builtin}: {arg_name} must be a string or nil (got {})",
            other.type_name()
        ))),
    }
}

fn expect_string_list(
    value: &VmValue,
    builtin: &str,
    arg_name: &str,
) -> Result<Vec<String>, VmError> {
    let items = match value {
        VmValue::List(items) => items,
        other => {
            return Err(thrown_err(format!(
                "{builtin}: {arg_name} must be a list (got {})",
                other.type_name()
            )));
        }
    };
    items
        .iter()
        .map(|value| match value {
            VmValue::String(text) => Ok(text.to_string()),
            other => Err(thrown_err(format!(
                "{builtin}: {arg_name} must contain strings (got {})",
                other.type_name()
            ))),
        })
        .collect()
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_not_modified(etag?: string?, headers?: dict) -> dict",
    category = "http_response"
)]
fn http_not_modified_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let mut headers = parse_headers(args.get(1), "http_not_modified")?;
    if let Some(etag) = args.first().and_then(string_or_nil) {
        headers.put_str("ETag", etag);
    }
    Ok(envelope(304, VmValue::Nil, BODY_KIND_NONE, headers))
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_push_hints(envelope: dict, paths: list) -> dict",
    category = "http_response"
)]
fn http_push_hints_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    // Pull the inbound envelope. We require it to be a tagged
    // `http_response` envelope so the codec on the other side actually
    // applies the Link headers — wrapping a plain dict would silently
    // no-op once it reached the wire.
    let envelope = args
        .first()
        .and_then(VmValue::as_dict)
        .ok_or_else(|| thrown_err("http_push_hints: envelope must be a dict"))?;
    if !is_http_response_envelope(envelope) {
        return Err(thrown_err(
            "http_push_hints: envelope must be an http_response envelope \
             (use http_ok, http_reply, etc. before calling this)",
        ));
    }

    let paths = match args.get(1) {
        Some(VmValue::List(items)) => items.clone(),
        Some(other) => {
            return Err(thrown_err(format!(
                "http_push_hints: paths must be a list (got {})",
                other.type_name()
            )));
        }
        None => {
            return Err(thrown_err("http_push_hints: paths is required"));
        }
    };

    let mut new_links: Vec<String> = Vec::with_capacity(paths.len());
    for item in paths.iter() {
        match item {
            VmValue::String(text) => {
                let path = text.as_str();
                if path.is_empty() {
                    return Err(thrown_err(
                        "http_push_hints: paths must not contain empty strings",
                    ));
                }
                new_links.push(format_link_header(path));
            }
            other => {
                return Err(thrown_err(format!(
                    "http_push_hints: paths must contain strings (got {})",
                    other.type_name()
                )));
            }
        }
    }

    if new_links.is_empty() {
        return Ok(VmValue::Dict(envelope.clone().into()));
    }

    let mut envelope_map = (*envelope).clone();
    let mut headers = envelope_map
        .get("headers")
        .and_then(VmValue::as_dict)
        .cloned()
        .unwrap_or_default();

    // Preserve any pre-existing Link header(s) the handler already set.
    let mut combined: Vec<VmValue> = match headers.get("Link") {
        Some(VmValue::String(existing)) => vec![VmValue::String(existing.clone())],
        Some(VmValue::List(items)) => items.iter().cloned().collect(),
        _ => Vec::new(),
    };
    combined.extend(
        new_links
            .into_iter()
            .map(|link| VmValue::String(arcstr::ArcStr::from(link))),
    );

    headers.insert(
        crate::value::intern_key("Link"),
        VmValue::List(std::sync::Arc::new(combined)),
    );
    envelope_map.insert(crate::value::intern_key("headers"), VmValue::dict(headers));
    Ok(VmValue::dict(envelope_map))
}

fn is_http_response_envelope(map: &crate::value::DictMap) -> bool {
    matches!(
        map.get(HTTP_RESPONSE_TAG_KEY),
        Some(VmValue::String(tag)) if tag.as_str() == HTTP_RESPONSE_TAG_VERSION,
    )
}

fn format_link_header(path: &str) -> String {
    match infer_preload_as(path) {
        Some(kind) => format!("<{path}>; rel=preload; as={kind}"),
        None => format!("<{path}>; rel=preload"),
    }
}

/// Map the asset extension to its `as=` attribute per the HTML living
/// standard's [destination table]. Unknown extensions emit a bare
/// `rel=preload` and let the browser decline the hint rather than
/// guessing — `as=` mismatch silently invalidates the preload.
///
/// [destination table]: https://html.spec.whatwg.org/multipage/links.html#link-type-preload
fn infer_preload_as(path: &str) -> Option<&'static str> {
    // Strip any query string / fragment before extension lookup. Paths
    // like `/main.js?v=42` should still infer `script`.
    let pre_query = path.split(['?', '#']).next().unwrap_or(path);
    let dot = pre_query.rfind('.')?;
    let ext = &pre_query[dot + 1..];
    Some(match ext.to_ascii_lowercase().as_str() {
        "css" => "style",
        "js" | "mjs" => "script",
        "json" => "fetch",
        "png" | "jpg" | "jpeg" | "gif" | "webp" | "svg" | "avif" | "ico" => "image",
        "woff" | "woff2" | "ttf" | "otf" => "font",
        _ => return None,
    })
}

#[harn_builtin(
    exposure = "pure",
    effects = [],
    sig = "http_upgrade_ws(req: dict, options?: dict) -> dict",
    category = "http_response"
)]
fn http_upgrade_ws_impl(args: &[VmValue], _out: &mut String) -> Result<VmValue, VmError> {
    let req = args
        .first()
        .and_then(VmValue::as_dict)
        .ok_or_else(|| thrown_err("http_upgrade_ws: req must be a dict"))?;
    let options = args.get(1).and_then(VmValue::as_dict);

    let request_subprotocols = req
        .get("headers")
        .and_then(VmValue::as_dict)
        .and_then(|headers| header_lookup(headers, "sec-websocket-protocol"))
        .map(|raw| {
            raw.split(',')
                .map(|s| s.trim().to_string())
                .filter(|s| !s.is_empty())
                .collect::<Vec<_>>()
        })
        .unwrap_or_default();
    let offered_subprotocols = options
        .and_then(|opts| opts.get("subprotocols"))
        .and_then(|value| match value {
            VmValue::List(items) => Some(
                items
                    .iter()
                    .filter_map(|v| match v {
                        VmValue::String(s) => Some(s.to_string()),
                        _ => None,
                    })
                    .collect::<Vec<_>>(),
            ),
            _ => None,
        })
        .unwrap_or_default();

    // Pick the first *client-preferred* subprotocol the server can
    // serve. This must match the convention in
    // `harn_serve::ws::negotiate_subprotocol` — if the two
    // disagreed, the builtin's envelope would carry one subprotocol
    // while the actual upgrade handshake echoed back another.
    let negotiated = request_subprotocols
        .iter()
        .find(|client| offered_subprotocols.iter().any(|name| name == *client))
        .cloned();

    let mut headers = crate::value::DictMap::new();
    headers.put_str("Upgrade", "websocket");
    headers.put_str("Connection", "Upgrade");
    if let Some(name) = &negotiated {
        headers.put_str("Sec-WebSocket-Protocol", name.clone());
    }

    let idle_ping_ms = options
        .and_then(|opts| opts.get("idle_ping_ms"))
        .and_then(|v| v.as_int());
    let max_message_bytes = options
        .and_then(|opts| opts.get("max_message_bytes"))
        .and_then(|v| v.as_int());
    let on_message = options
        .and_then(|opts| opts.get("on_message"))
        .and_then(|v| match v {
            VmValue::String(name) => Some(name.to_string()),
            _ => None,
        });

    let mut env_map = envelope_map(101, VmValue::Nil, BODY_KIND_NONE, headers);
    env_map.insert(
        crate::value::intern_key("ws_upgrade"),
        VmValue::dict({
            let mut map = crate::value::DictMap::new();
            map.insert(
                crate::value::intern_key("subprotocol"),
                match &negotiated {
                    Some(name) => VmValue::String(arcstr::ArcStr::from(name.clone())),
                    None => VmValue::Nil,
                },
            );
            map.insert(
                crate::value::intern_key("offered"),
                VmValue::List(std::sync::Arc::new(
                    offered_subprotocols
                        .iter()
                        .map(|s| VmValue::String(arcstr::ArcStr::from(s.clone())))
                        .collect(),
                )),
            );
            if let Some(ms) = idle_ping_ms {
                map.insert(crate::value::intern_key("idle_ping_ms"), VmValue::Int(ms));
            }
            if let Some(bytes) = max_message_bytes {
                map.insert(
                    crate::value::intern_key("max_message_bytes"),
                    VmValue::Int(bytes),
                );
            }
            if let Some(handler) = &on_message {
                map.put_str("on_message", handler.clone());
            }
            map
        }),
    );
    Ok(VmValue::dict(env_map))
}

fn header_lookup(headers: &crate::value::DictMap, name: &str) -> Option<String> {
    let needle = name.to_ascii_lowercase();
    headers
        .iter()
        .find(|(key, _)| key.to_ascii_lowercase() == needle)
        .and_then(|(_, value)| match value {
            VmValue::String(text) => Some(text.to_string()),
            _ => None,
        })
}

fn value_as_bytes(value: &VmValue) -> Vec<u8> {
    match value {
        VmValue::Bytes(bytes) => bytes.as_ref().clone(),
        VmValue::String(text) => text.as_bytes().to_vec(),
        VmValue::Nil => Vec::new(),
        // For dicts / lists / structs, fall through to the stdlib JSON
        // encoder so the ETag derives from a stable canonical form
        // (dict keys sorted, bytes base64-tagged) rather than the
        // less-stable `display()` representation. We reuse
        // `stdlib::json` directly instead of reaching for the
        // llm-helpers encoder so the abstraction boundary stays
        // sibling-module, not cross-subsystem.
        other => crate::stdlib::json::vm_value_to_json(other).into_bytes(),
    }
}

/// Parse an HTTP `Accept` header and return the best matching offer.
///
/// Standard Q-value scoring per RFC 9110 §12.5.1: each media-range
/// gets a `q` (1.0 by default); each offer is scored by its
/// best-matching range, with ties broken by offer order. Wildcard
/// matches (`type/*`, `*/*`) score below exact-type matches.
fn negotiate_accept(header: &str, offers: &[String]) -> Option<String> {
    let ranges: Vec<MediaRange> = header
        .split(',')
        .filter_map(MediaRange::parse)
        .filter(|range| range.q > 0.0)
        .collect();
    if ranges.is_empty() {
        return None;
    }

    let mut best: Option<(usize, f32, u8)> = None;
    for (index, offer) in offers.iter().enumerate() {
        let (offer_type, offer_subtype) = split_media(offer)?;
        for range in &ranges {
            let score = range.match_score(offer_type, offer_subtype);
            let Some(score) = score else { continue };
            let q = range.q;
            let candidate = (index, q, score);
            best = Some(match best {
                None => candidate,
                Some(current) => {
                    // Prefer higher q first; if equal, higher specificity;
                    // if equal, earlier offer wins.
                    if q > current.1
                        || (q == current.1 && score > current.2)
                        || (q == current.1 && score == current.2 && index < current.0)
                    {
                        candidate
                    } else {
                        current
                    }
                }
            });
        }
    }
    best.map(|(index, _, _)| offers[index].clone())
}

struct MediaRange<'a> {
    type_: &'a str,
    subtype: &'a str,
    q: f32,
}

impl<'a> MediaRange<'a> {
    fn parse(raw: &'a str) -> Option<Self> {
        let trimmed = raw.trim();
        let mut parts = trimmed.split(';');
        let media = parts.next()?.trim();
        let (type_, subtype) = split_media(media)?;
        let mut q = 1.0;
        for param in parts {
            let param = param.trim();
            if let Some(value) = param
                .strip_prefix("q=")
                .or_else(|| param.strip_prefix("Q="))
            {
                if let Ok(parsed) = value.trim().parse::<f32>() {
                    if (0.0..=1.0).contains(&parsed) {
                        q = parsed;
                    }
                }
            }
        }
        Some(Self { type_, subtype, q })
    }

    fn match_score(&self, offer_type: &str, offer_subtype: &str) -> Option<u8> {
        let type_match = self.type_ == "*" || self.type_.eq_ignore_ascii_case(offer_type);
        let subtype_match = self.subtype == "*" || self.subtype.eq_ignore_ascii_case(offer_subtype);
        if !type_match || !subtype_match {
            return None;
        }
        Some(match (self.type_, self.subtype) {
            ("*", _) => 1,
            (_, "*") => 2,
            _ => 3,
        })
    }
}

fn split_media(value: &str) -> Option<(&str, &str)> {
    let mut iter = value.splitn(2, '/');
    let type_ = iter.next()?.trim();
    let subtype = iter.next()?.trim();
    if type_.is_empty() || subtype.is_empty() {
        return None;
    }
    Some((type_, subtype))
}

pub(crate) const MODULE_BUILTINS: &[&VmBuiltinDef] = &[
    &HTTP_OK_IMPL_DEF,
    &HTTP_CREATED_IMPL_DEF,
    &HTTP_NO_CONTENT_IMPL_DEF,
    &HTTP_ERROR_IMPL_DEF,
    &HTTP_REPLY_IMPL_DEF,
    &HTTP_REPLY_FROM_IMPL_DEF,
    &HTTP_STREAM_IMPL_DEF,
    &HTTP_SSE_IMPL_DEF,
    &HTTP_ETAG_IMPL_DEF,
    &HTTP_CHOOSE_IMPL_DEF,
    &HTTP_NOT_MODIFIED_IMPL_DEF,
    &HTTP_PUSH_HINTS_IMPL_DEF,
    &HTTP_UPGRADE_WS_IMPL_DEF,
];

#[cfg(test)]
mod tests {
    use super::*;
    use crate::llm::helpers::vm_value_to_json;

    fn dict(value: &VmValue) -> &crate::value::DictMap {
        value.as_dict().expect("envelope is a dict")
    }

    fn run_sync<F, Fut>(future: F) -> Fut::Output
    where
        F: FnOnce() -> Fut,
        Fut: std::future::Future,
    {
        tokio::runtime::Builder::new_current_thread()
            .enable_all()
            .build()
            .expect("rt")
            .block_on(future())
    }

    #[test]
    fn http_ok_produces_tagged_envelope() {
        let body = VmValue::String(arcstr::ArcStr::from("hello"));
        let response = http_ok_impl(&[body], &mut String::new()).expect("ok");
        let map = dict(&response);
        assert_eq!(
            map.get(HTTP_RESPONSE_TAG_KEY).and_then(|v| match v {
                VmValue::String(s) => Some(s.as_str()),
                _ => None,
            }),
            Some(HTTP_RESPONSE_TAG_VERSION)
        );
        assert!(matches!(map.get("status"), Some(VmValue::Int(200))));
        assert_eq!(
            map.get("body").map(|v| v.display()).as_deref(),
            Some("hello")
        );
    }

    #[test]
    fn http_created_sets_location_header() {
        let body = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("id"),
            VmValue::String(arcstr::ArcStr::from("sess_1")),
        )]));
        let location = VmValue::String(arcstr::ArcStr::from("/v1/sessions/sess_1"));
        let response = http_created_impl(&[body, location], &mut String::new()).expect("created");
        let map = dict(&response);
        assert!(matches!(map.get("status"), Some(VmValue::Int(201))));
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        assert_eq!(
            headers.get("Location").map(|v| v.display()).as_deref(),
            Some("/v1/sessions/sess_1")
        );
    }

    #[test]
    fn http_no_content_omits_body_marker() {
        let response = http_no_content_impl(&[], &mut String::new()).expect("no_content");
        let map = dict(&response);
        assert!(matches!(map.get("status"), Some(VmValue::Int(204))));
        assert!(map.get("body").is_none());
        assert_eq!(
            map.get("body_kind").and_then(|v| match v {
                VmValue::String(s) => Some(s.as_str()),
                _ => None,
            }),
            Some(BODY_KIND_NONE)
        );
    }

    #[test]
    fn http_error_carries_code_message_and_marker() {
        let response = http_error_impl(
            &[
                VmValue::Int(422),
                VmValue::String(arcstr::ArcStr::from("invalid_input")),
                VmValue::String(arcstr::ArcStr::from("bad payload")),
                VmValue::Nil,
            ],
            &mut String::new(),
        )
        .expect("error");
        let map = dict(&response);
        assert!(matches!(map.get("status"), Some(VmValue::Int(422))));
        assert!(matches!(map.get("is_error"), Some(VmValue::Bool(true))));
        let body = map
            .get("body")
            .and_then(VmValue::as_dict)
            .expect("body dict");
        assert_eq!(
            body.get("code").map(|v| v.display()).as_deref(),
            Some("invalid_input")
        );
        assert_eq!(
            body.get("message").map(|v| v.display()).as_deref(),
            Some("bad payload")
        );
    }

    #[test]
    fn http_error_rejects_2xx_status() {
        let err = http_error_impl(
            &[
                VmValue::Int(200),
                VmValue::String(arcstr::ArcStr::from("x")),
                VmValue::String(arcstr::ArcStr::from("y")),
            ],
            &mut String::new(),
        )
        .expect_err("expected reject");
        match err {
            VmError::Thrown(VmValue::String(text)) => {
                assert!(text.contains("4xx or 5xx"), "got: {text}");
            }
            other => panic!("unexpected error: {other:?}"),
        }
    }

    #[test]
    fn http_reply_rejects_out_of_range_status() {
        let err =
            http_reply_impl(&[VmValue::Int(999)], &mut String::new()).expect_err("out of range");
        match err {
            VmError::Thrown(VmValue::String(text)) => {
                assert!(text.contains("100-599"), "got: {text}");
            }
            other => panic!("unexpected error: {other:?}"),
        }
    }

    #[test]
    fn http_reply_bytes_uses_bytes_body_kind() {
        let bytes = VmValue::Bytes(std::sync::Arc::new(vec![0x00, 0xff, 0xfe, 0x80]));
        let headers = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("Content-Type"),
            VmValue::String(arcstr::ArcStr::from("application/octet-stream")),
        )]));
        let response =
            http_reply_impl(&[VmValue::Int(200), bytes, headers], &mut String::new()).unwrap();
        let map = dict(&response);
        assert_eq!(
            map.get("body_kind").and_then(|v| match v {
                VmValue::String(s) => Some(s.as_str()),
                _ => None,
            }),
            Some(BODY_KIND_BYTES)
        );
        assert!(matches!(map.get("body"), Some(VmValue::Bytes(_))));
    }

    #[test]
    fn http_reply_from_wraps_stream_body_as_chunk_list() {
        let result = VmValue::dict(crate::value::DictMap::from_iter([
            (crate::value::intern_key("status"), VmValue::Int(202)),
            (
                crate::value::intern_key("body_kind"),
                VmValue::string("stream"),
            ),
            (
                crate::value::intern_key("headers"),
                VmValue::dict(crate::value::DictMap::from_iter([(
                    crate::value::intern_key("Content-Type"),
                    VmValue::string("text/plain"),
                )])),
            ),
            (crate::value::intern_key("body"), VmValue::string("queued")),
        ]));

        let response = http_reply_from_impl(&[result], &mut String::new()).unwrap();
        let map = dict(&response);
        assert!(matches!(map.get("status"), Some(VmValue::Int(202))));
        assert_eq!(
            map.get("body_kind").and_then(|v| match v {
                VmValue::String(s) => Some(s.as_str()),
                _ => None,
            }),
            Some(BODY_KIND_STREAM)
        );
        let body = match map.get("body") {
            Some(VmValue::List(items)) => items,
            other => panic!("expected stream body chunk list, got {other:?}"),
        };
        assert_eq!(body.len(), 1);
        assert_eq!(body[0].display(), "queued");
    }

    #[test]
    fn http_reply_from_preserves_existing_stream_chunks() {
        let chunks = VmValue::List(std::sync::Arc::new(vec![
            VmValue::string("alpha"),
            VmValue::string("bravo"),
        ]));
        let result = VmValue::dict(crate::value::DictMap::from_iter([
            (crate::value::intern_key("status"), VmValue::Int(200)),
            (
                crate::value::intern_key("body_kind"),
                VmValue::string("stream"),
            ),
            (crate::value::intern_key("body"), chunks),
        ]));

        let response = http_reply_from_impl(&[result], &mut String::new()).unwrap();
        let map = dict(&response);
        let body = match map.get("body") {
            Some(VmValue::List(items)) => items,
            other => panic!("expected stream body chunk list, got {other:?}"),
        };
        assert_eq!(body.len(), 2);
        assert_eq!(body[0].display(), "alpha");
        assert_eq!(body[1].display(), "bravo");
    }

    #[test]
    fn http_reply_from_preserves_raw_body_for_bytes_kind() {
        let raw = VmValue::Bytes(std::sync::Arc::new(vec![0x00, 0xff, 0xfe, 0x80]));
        let result = VmValue::dict(crate::value::DictMap::from_iter([
            (crate::value::intern_key("status"), VmValue::Int(200)),
            (
                crate::value::intern_key("body_kind"),
                VmValue::string("bytes"),
            ),
            (crate::value::intern_key("body"), VmValue::string("<lossy>")),
            (crate::value::intern_key("raw_body"), raw),
        ]));

        let response = http_reply_from_impl(&[result], &mut String::new()).unwrap();
        let map = dict(&response);
        assert_eq!(
            map.get("body_kind").and_then(|v| match v {
                VmValue::String(s) => Some(s.as_str()),
                _ => None,
            }),
            Some(BODY_KIND_BYTES)
        );
        match map.get("body") {
            Some(VmValue::Bytes(bytes)) => assert_eq!(bytes.as_ref(), &[0x00, 0xff, 0xfe, 0x80]),
            other => panic!("expected bytes body, got {other:?}"),
        }
    }

    #[test]
    fn http_reply_from_rejects_non_bytes_for_bytes_kind() {
        let result = VmValue::dict(crate::value::DictMap::from_iter([
            (crate::value::intern_key("status"), VmValue::Int(200)),
            (
                crate::value::intern_key("body_kind"),
                VmValue::string("bytes"),
            ),
            (
                crate::value::intern_key("body"),
                VmValue::string("not bytes"),
            ),
        ]));

        let err =
            http_reply_from_impl(&[result], &mut String::new()).expect_err("expected bytes error");
        match err {
            VmError::Thrown(VmValue::String(text)) => {
                assert!(text.contains("requires bytes"), "unexpected error: {text}");
            }
            other => panic!("unexpected error: {other:?}"),
        }
    }

    #[test]
    fn http_reply_from_falls_back_to_http_reply_for_text_kind() {
        let result = VmValue::dict(crate::value::DictMap::from_iter([
            (crate::value::intern_key("status"), VmValue::Int(200)),
            (
                crate::value::intern_key("body_kind"),
                VmValue::string("text"),
            ),
            (crate::value::intern_key("body"), VmValue::string("hello")),
        ]));

        let response = http_reply_from_impl(&[result], &mut String::new()).unwrap();
        let map = dict(&response);
        assert_eq!(
            map.get("body_kind").and_then(|v| match v {
                VmValue::String(s) => Some(s.as_str()),
                _ => None,
            }),
            Some(BODY_KIND_JSON)
        );
        assert_eq!(
            map.get("body").map(VmValue::display).as_deref(),
            Some("hello")
        );
    }

    #[test]
    fn http_reply_from_rejects_non_dict_result() {
        let err = http_reply_from_impl(&[VmValue::string("nope")], &mut String::new())
            .expect_err("expected result type error");
        match err {
            VmError::Thrown(VmValue::String(text)) => {
                assert!(
                    text.contains("result must be a dict"),
                    "unexpected error: {text}"
                );
            }
            other => panic!("unexpected error: {other:?}"),
        }
    }

    #[test]
    fn http_stream_buffers_list_source() {
        let items = vec![
            VmValue::String(arcstr::ArcStr::from("a")),
            VmValue::String(arcstr::ArcStr::from("b")),
        ];
        let response = run_sync(|| {
            http_stream_impl(
                crate::vm::AsyncBuiltinCtx::for_test(Vm::new()),
                vec![
                    VmValue::List(std::sync::Arc::new(items.clone())),
                    VmValue::String(arcstr::ArcStr::from("text/plain")),
                ],
            )
        })
        .expect("stream");
        let map = dict(&response);
        assert_eq!(
            map.get("body_kind").and_then(|v| match v {
                VmValue::String(s) => Some(s.as_str()),
                _ => None,
            }),
            Some(BODY_KIND_STREAM)
        );
        let body = map.get("body").expect("body");
        match body {
            VmValue::List(values) => {
                assert_eq!(values.len(), 2);
            }
            other => panic!("expected list body, got {other:?}"),
        }
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        assert_eq!(
            headers.get("Content-Type").map(|v| v.display()).as_deref(),
            Some("text/plain")
        );
    }

    #[test]
    fn http_sse_sets_event_stream_headers_and_optional_retry() {
        let events = vec![VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("data"),
            VmValue::String(arcstr::ArcStr::from("ping")),
        )]))];
        let response = run_sync(|| {
            http_sse_impl(
                crate::vm::AsyncBuiltinCtx::for_test(Vm::new()),
                vec![
                    VmValue::List(std::sync::Arc::new(events.clone())),
                    VmValue::Int(2500),
                ],
            )
        })
        .expect("sse");
        let map = dict(&response);
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        assert_eq!(
            headers.get("Content-Type").map(|v| v.display()).as_deref(),
            Some("text/event-stream")
        );
        assert_eq!(
            headers.get("Cache-Control").map(|v| v.display()).as_deref(),
            Some("no-cache")
        );
        assert!(matches!(map.get("retry_ms"), Some(VmValue::Int(2500))));
    }

    #[test]
    fn parse_envelope_round_trip_through_json() {
        let response = http_error_impl(
            &[
                VmValue::Int(404),
                VmValue::String(arcstr::ArcStr::from("not_found")),
                VmValue::String(arcstr::ArcStr::from("missing")),
                VmValue::dict(crate::value::DictMap::from_iter([(
                    crate::value::intern_key("id"),
                    VmValue::String(arcstr::ArcStr::from("sess_404")),
                )])),
            ],
            &mut String::new(),
        )
        .expect("error");
        let json = vm_value_to_json(&response);
        let envelope = parse_envelope(&json).expect("envelope parses");
        assert_eq!(envelope.status, 404);
        assert!(envelope.is_error);
        let body = envelope.body.expect("body");
        assert_eq!(body["code"], "not_found");
        assert_eq!(body["details"]["id"], "sess_404");
    }

    #[test]
    fn parse_envelope_ignores_untagged_dicts() {
        let plain = serde_json::json!({"status": 200, "body": {}});
        assert!(parse_envelope(&plain).is_none());
    }

    #[test]
    fn http_etag_is_quoted_hex_sha256_of_payload() {
        let value = VmValue::String(arcstr::ArcStr::from("hello"));
        let etag = http_etag_impl(&[value], &mut String::new()).expect("etag");
        match etag {
            VmValue::String(text) => {
                assert_eq!(
                    text.as_str(),
                    "\"2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824\""
                );
            }
            other => panic!("expected string, got {other:?}"),
        }
    }

    #[test]
    fn http_etag_stable_across_string_and_bytes_for_same_payload() {
        let from_string = http_etag_impl(
            &[VmValue::String(arcstr::ArcStr::from("hello"))],
            &mut String::new(),
        )
        .unwrap();
        let from_bytes = http_etag_impl(
            &[VmValue::Bytes(std::sync::Arc::new(b"hello".to_vec()))],
            &mut String::new(),
        )
        .unwrap();
        assert_eq!(from_string.display(), from_bytes.display());
    }

    #[test]
    fn http_choose_returns_best_q_match() {
        let accept = VmValue::String(arcstr::ArcStr::from(
            "application/xml;q=0.5, application/json;q=0.9",
        ));
        let offers = VmValue::List(std::sync::Arc::new(vec![
            VmValue::String(arcstr::ArcStr::from("application/xml")),
            VmValue::String(arcstr::ArcStr::from("application/json")),
        ]));
        let chosen = http_choose_impl(&[accept, offers], &mut String::new()).unwrap();
        assert_eq!(chosen.display(), "application/json");
    }

    #[test]
    fn http_choose_prefers_specific_over_wildcard() {
        let accept = VmValue::String(arcstr::ArcStr::from("text/*;q=0.5, application/json"));
        let offers = VmValue::List(std::sync::Arc::new(vec![
            VmValue::String(arcstr::ArcStr::from("text/plain")),
            VmValue::String(arcstr::ArcStr::from("application/json")),
        ]));
        let chosen = http_choose_impl(&[accept, offers], &mut String::new()).unwrap();
        assert_eq!(chosen.display(), "application/json");
    }

    #[test]
    fn http_choose_returns_default_for_no_accept() {
        let offers = VmValue::List(std::sync::Arc::new(vec![
            VmValue::String(arcstr::ArcStr::from("text/plain")),
            VmValue::String(arcstr::ArcStr::from("application/json")),
        ]));
        let chosen = http_choose_impl(&[VmValue::Nil, offers], &mut String::new()).unwrap();
        assert_eq!(chosen.display(), "text/plain");
    }

    #[test]
    fn http_choose_overrides_default_with_explicit() {
        let offers = VmValue::List(std::sync::Arc::new(vec![
            VmValue::String(arcstr::ArcStr::from("text/plain")),
            VmValue::String(arcstr::ArcStr::from("application/json")),
        ]));
        let chosen = http_choose_impl(
            &[
                VmValue::Nil,
                offers,
                VmValue::String(arcstr::ArcStr::from("application/json")),
            ],
            &mut String::new(),
        )
        .unwrap();
        assert_eq!(chosen.display(), "application/json");
    }

    #[test]
    fn http_choose_wildcard_accept_yields_default() {
        let offers = VmValue::List(std::sync::Arc::new(vec![VmValue::String(
            arcstr::ArcStr::from("application/json"),
        )]));
        let chosen = http_choose_impl(
            &[VmValue::String(arcstr::ArcStr::from("*/*")), offers],
            &mut String::new(),
        )
        .unwrap();
        assert_eq!(chosen.display(), "application/json");
    }

    #[test]
    fn http_not_modified_envelope_carries_etag() {
        let etag = VmValue::String(arcstr::ArcStr::from("\"abc\""));
        let response = http_not_modified_impl(&[etag, VmValue::Nil], &mut String::new()).unwrap();
        let map = dict(&response);
        assert!(matches!(map.get("status"), Some(VmValue::Int(304))));
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        assert_eq!(
            headers.get("ETag").map(|v| v.display()).as_deref(),
            Some("\"abc\"")
        );
    }

    #[test]
    fn http_push_hints_appends_link_headers_with_inferred_as() {
        let envelope = http_ok_impl(
            &[VmValue::dict(crate::value::DictMap::new())],
            &mut String::new(),
        )
        .unwrap();
        let paths = VmValue::List(std::sync::Arc::new(vec![
            VmValue::String(arcstr::ArcStr::from("/main.css")),
            VmValue::String(arcstr::ArcStr::from("/app.js")),
            VmValue::String(arcstr::ArcStr::from("/hero.webp")),
            VmValue::String(arcstr::ArcStr::from("/inter.woff2")),
            VmValue::String(arcstr::ArcStr::from("/manifest.json")),
            VmValue::String(arcstr::ArcStr::from("/unknown.xyz")),
        ]));
        let response =
            http_push_hints_impl(&[envelope, paths], &mut String::new()).expect("push_hints");
        let map = dict(&response);
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        let links = match headers.get("Link") {
            Some(VmValue::List(items)) => items.clone(),
            other => panic!("Link should be a list, got {other:?}"),
        };
        let rendered: Vec<String> = links
            .iter()
            .map(|v| match v {
                VmValue::String(s) => s.to_string(),
                other => panic!("Link entry is not a string: {other:?}"),
            })
            .collect();
        assert_eq!(
            rendered,
            vec![
                "</main.css>; rel=preload; as=style",
                "</app.js>; rel=preload; as=script",
                "</hero.webp>; rel=preload; as=image",
                "</inter.woff2>; rel=preload; as=font",
                "</manifest.json>; rel=preload; as=fetch",
                "</unknown.xyz>; rel=preload",
            ]
        );
    }

    #[test]
    fn http_push_hints_handles_querystring_in_path() {
        let envelope = http_ok_impl(&[VmValue::Nil], &mut String::new()).unwrap();
        let paths = VmValue::List(std::sync::Arc::new(vec![VmValue::String(
            arcstr::ArcStr::from("/static/app.js?v=42"),
        )]));
        let response =
            http_push_hints_impl(&[envelope, paths], &mut String::new()).expect("push_hints");
        let map = dict(&response);
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        let links = match headers.get("Link") {
            Some(VmValue::List(items)) => items.clone(),
            other => panic!("Link should be a list, got {other:?}"),
        };
        assert_eq!(
            links[0].display(),
            "</static/app.js?v=42>; rel=preload; as=script"
        );
    }

    #[test]
    fn http_push_hints_rejects_untagged_envelope() {
        let plain = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("status"),
            VmValue::Int(200),
        )]));
        let paths = VmValue::List(std::sync::Arc::new(vec![VmValue::String(
            arcstr::ArcStr::from("/main.css"),
        )]));
        let result = http_push_hints_impl(&[plain, paths], &mut String::new());
        assert!(
            matches!(result, Err(VmError::Thrown(_))),
            "untagged dict should be rejected, got {result:?}"
        );
    }

    #[test]
    fn http_push_hints_preserves_existing_link_header() {
        let envelope = http_reply_impl(
            &[
                VmValue::Int(200),
                VmValue::dict(crate::value::DictMap::new()),
                VmValue::dict(crate::value::DictMap::from_iter([(
                    crate::value::intern_key("Link"),
                    VmValue::String(arcstr::ArcStr::from("</legacy.css>; rel=preload; as=style")),
                )])),
            ],
            &mut String::new(),
        )
        .unwrap();
        let paths = VmValue::List(std::sync::Arc::new(vec![VmValue::String(
            arcstr::ArcStr::from("/app.js"),
        )]));
        let response = http_push_hints_impl(&[envelope, paths], &mut String::new()).unwrap();
        let map = dict(&response);
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        let links = match headers.get("Link") {
            Some(VmValue::List(items)) => items.clone(),
            other => panic!("Link should be a list once preloads are added, got {other:?}"),
        };
        assert_eq!(links.len(), 2);
        assert_eq!(links[0].display(), "</legacy.css>; rel=preload; as=style");
        assert_eq!(links[1].display(), "</app.js>; rel=preload; as=script");
    }

    #[test]
    fn http_upgrade_ws_envelope_negotiates_subprotocol() {
        let req = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("headers"),
            VmValue::dict(crate::value::DictMap::from_iter([(
                crate::value::intern_key("Sec-WebSocket-Protocol"),
                VmValue::String(arcstr::ArcStr::from("v0.harn, v1.harn")),
            )])),
        )]));
        let options = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("subprotocols"),
            VmValue::List(std::sync::Arc::new(vec![
                VmValue::String(arcstr::ArcStr::from("v1.harn")),
                VmValue::String(arcstr::ArcStr::from("v2.harn")),
            ])),
        )]));
        let response = http_upgrade_ws_impl(&[req, options], &mut String::new()).unwrap();
        let map = dict(&response);
        assert!(matches!(map.get("status"), Some(VmValue::Int(101))));
        let upgrade = map
            .get("ws_upgrade")
            .and_then(VmValue::as_dict)
            .expect("ws_upgrade");
        assert_eq!(
            upgrade.get("subprotocol").map(|v| v.display()).as_deref(),
            Some("v1.harn")
        );
        let headers = map
            .get("headers")
            .and_then(VmValue::as_dict)
            .expect("headers");
        assert_eq!(
            headers.get("Upgrade").map(|v| v.display()).as_deref(),
            Some("websocket")
        );
        assert_eq!(
            headers
                .get("Sec-WebSocket-Protocol")
                .map(|v| v.display())
                .as_deref(),
            Some("v1.harn")
        );
    }

    #[test]
    fn http_upgrade_ws_picks_client_preferred_when_both_overlap() {
        // Regression for the divergence between
        // `http_upgrade_ws_impl`'s envelope-side negotiation and
        // `harn_serve::ws::negotiate_subprotocol`'s wire-side
        // negotiation. With client "v2.harn, v1.harn" and server
        // ["v1.harn", "v2.harn"] the two implementations used to
        // disagree (server-order picked v1; client-order picks v2).
        // The envelope MUST match what the upgrade handshake echoes
        // back, so we honour client preference everywhere.
        let req = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("headers"),
            VmValue::dict(crate::value::DictMap::from_iter([(
                crate::value::intern_key("Sec-WebSocket-Protocol"),
                VmValue::String(arcstr::ArcStr::from("v2.harn, v1.harn")),
            )])),
        )]));
        let options = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("subprotocols"),
            VmValue::List(std::sync::Arc::new(vec![
                VmValue::String(arcstr::ArcStr::from("v1.harn")),
                VmValue::String(arcstr::ArcStr::from("v2.harn")),
            ])),
        )]));
        let response = http_upgrade_ws_impl(&[req, options], &mut String::new()).unwrap();
        let upgrade = dict(&response)
            .get("ws_upgrade")
            .and_then(VmValue::as_dict)
            .expect("ws_upgrade");
        assert_eq!(
            upgrade.get("subprotocol").map(|v| v.display()).as_deref(),
            Some("v2.harn")
        );
    }

    #[test]
    fn parse_envelope_round_trips_ws_upgrade_marker() {
        let req = VmValue::dict(crate::value::DictMap::from_iter([(
            crate::value::intern_key("headers"),
            VmValue::dict(crate::value::DictMap::from_iter([(
                crate::value::intern_key("Sec-WebSocket-Protocol"),
                VmValue::String(arcstr::ArcStr::from("v1.harn")),
            )])),
        )]));
        let options = VmValue::dict(crate::value::DictMap::from_iter([
            (
                crate::value::intern_key("subprotocols"),
                VmValue::List(std::sync::Arc::new(vec![VmValue::String(
                    arcstr::ArcStr::from("v1.harn"),
                )])),
            ),
            (
                crate::value::intern_key("idle_ping_ms"),
                VmValue::Int(15_000),
            ),
        ]));
        let response = http_upgrade_ws_impl(&[req, options], &mut String::new()).unwrap();
        let json = vm_value_to_json(&response);
        let envelope = parse_envelope(&json).expect("envelope parses");
        let ws = envelope.ws_upgrade.expect("ws_upgrade present");
        assert_eq!(ws.subprotocol.as_deref(), Some("v1.harn"));
        assert_eq!(ws.offered, vec!["v1.harn"]);
        assert_eq!(ws.idle_ping_ms, Some(15_000));
        assert_eq!(envelope.status, 101);
    }

    #[test]
    fn http_upgrade_ws_falls_through_when_no_subprotocols_offered() {
        let req = VmValue::dict(crate::value::DictMap::new());
        let response = http_upgrade_ws_impl(&[req], &mut String::new()).unwrap();
        let map = dict(&response);
        let upgrade = map
            .get("ws_upgrade")
            .and_then(VmValue::as_dict)
            .expect("ws_upgrade");
        assert!(matches!(upgrade.get("subprotocol"), Some(VmValue::Nil)));
    }
}