devflow_core/

canary.rs

//! The delivery canary (D-13): the guard that notices when the undocumented
//! CLI behaviour this whole arc rests on has gone away.
//!
//! # Why a planted token rather than a version check
//!
//! Claude Code's `task-notification` delivery — the CLI waking a live session
//! back up after a background task finishes — is **undocumented behaviour**,
//! observed only on `claude_code_version 2.1.220`. A CLI update can withdraw it
//! without any announcement, and if it is withdrawn then every multi-plan wave
//! silently orphans its dispatched work: exactly the 999.64 shape this phase
//! exists to close. Reading the version string would guard a *proxy* for the
//! behaviour, not the behaviour, and would go on reporting healthy the moment
//! the same version number stopped meaning the same thing. So the guard plants
//! a value only DevFlow knows and confirms it comes back.
//!
//! # What a `Confirmed` outcome does and does not mean
//!
//! It means **the notification path is alive**. It NEVER means the dispatched
//! work happened. The agent can read the token out of its own prompt and emit
//! it without doing anything at all — that is 999.67's shape, accepted here
//! deliberately (threat T-31-11) rather than mitigated, because mitigating it
//! needs per-child tokens and D-14 defers those on size. Summaries and merges
//! remain the evidence of work (D-16/D-18). Nothing in this module may be
//! rephrased to imply otherwise.
//!
//! # Where the trust decision is made
//!
//! Not here. The CLI echoes the operator's prompt back into the same stdout as
//! a `user` event, so the planted token **will** appear in the capture whether
//! or not anything was delivered — that echo is what produced the checkpoint
//! false positive 30-05 had to fix. The question "did this token come back from
//! somewhere trustworthy?" is therefore answered by exactly one function in
//! this codebase, [`crate::agent_result::token_reported_in_capture`], which
//! confines the match to events that are both `type: "result"` and
//! orchestrator-authored. This module delegates to it and holds no notion of
//! its own about which lines are trustworthy — a second such notion would be
//! free to drift away from the first, and the drift would be invisible.

use crate::agent_result;
use crate::agents::{AgentAdapter, ClaudeAgent};
use crate::git::hermetic_command;
use crate::monitor::{self, CloseRule};
use crate::phase_id::PhaseId;
use serde::{Deserialize, Serialize};
use std::io::{BufRead, BufReader, Write};
use std::path::{Path, PathBuf};
use std::process::Stdio;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::mpsc;
use std::time::{Duration, Instant};
use tracing::warn;

/// The fixed, greppable prefix every declared canary token carries.
///
/// Exposed so the run's provenance can record WHICH guard ran without
/// recording the token itself (T-31-13).
pub const TOKEN_PREFIX: &str = "DEVFLOW_DELIVERY_CANARY_";

/// File name of the canary's own throwaway capture, inside the capture dir.
///
/// Deliberately NOT the phase capture (`.devflow/phase-NN-stdout.log`): that
/// file is the one artifact the entire Layer 1 cascade decides a stage on, and
/// a guard that clobbered it would break the thing it exists to protect.
const CAPTURE_FILE: &str = "delivery-canary.jsonl";

/// Monotonic within one process — the third input to [`declare_token`].
static TOKEN_SEQ: AtomicU64 = AtomicU64::new(0);

/// Declare a fresh success token for one canary run.
///
/// **This is a nonce, not a secret, and must not be "upgraded" into one.** The
/// only property required (RESEARCH § ASVS V6) is that an agent cannot produce
/// the value by chance inside its own generated text. A 64-bit hash of the
/// current wall-clock nanos, this process's pid and a per-process counter
/// clears that bar by a wide margin, and it costs no new dependency — which is
/// why `std::hash::DefaultHasher` is used here rather than a CSPRNG crate.
/// Nothing downstream authenticates anything with this value.
///
/// Two calls in one process differ because the counter feeds the hash. That
/// makes distinctness overwhelming (a 64-bit collision), not absolute; the
/// token is a nonce and nothing breaks on the ~2⁻⁶⁴ tie.
pub fn declare_token() -> String {
    use std::hash::{Hash, Hasher};

    let nanos = std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map(|d| d.as_nanos())
        .unwrap_or(0);
    let seq = TOKEN_SEQ.fetch_add(1, Ordering::Relaxed);

    let mut hasher = std::collections::hash_map::DefaultHasher::new();
    nanos.hash(&mut hasher);
    std::process::id().hash(&mut hasher);
    seq.hash(&mut hasher);

    format!("{TOKEN_PREFIX}{:016x}", hasher.finish())
}

/// What one canary run established.
///
/// `Absent` and `Unverified` are kept apart on purpose, and collapsing them
/// would be a real loss of information: "the CLI ran and the behaviour is gone"
/// and "the CLI could not be run at all" call for completely different operator
/// action, and a merged variant would report a missing binary as a broken
/// premise (threat T-31-12 — the risk this guard carries is a FALSE refusal).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum CanaryOutcome {
    /// The declared token came back from a trustworthy place. The notification
    /// path is alive. This says NOTHING about whether work happened.
    Confirmed,
    /// The CLI ran and the token did not come back. The premise this arc rests
    /// on is no longer backed by observed behaviour.
    Absent,
    /// The guard itself could not reach a conclusion — carries the reason. Not
    /// a statement about the CLI's behaviour.
    Unverified(String),
}

/// How the canary gets a child to talk to.
///
/// The seam exists so the matcher can be tested without spawning an agent: every
/// test in this module injects a launcher that writes a canned capture, and none
/// of them runs `claude`. `run` returns `Err` ONLY when the child could not be
/// run to the point of producing a capture — a child that ran and said nothing
/// useful is `Ok`, because that is a fact about the CLI's behaviour and belongs
/// in the `Absent`/`Confirmed` decision rather than in the `Unverified` one.
pub trait CanaryLauncher {
    /// Run one throwaway agent turn against `prompt`, teeing its stdout to
    /// `capture`.
    fn run(&self, prompt: &str, capture: &Path) -> Result<(), String>;
}

/// Where one canary run's throwaway capture lands.
pub fn canary_capture_path(capture_dir: &Path) -> PathBuf {
    capture_dir.join(CAPTURE_FILE)
}

/// The throwaway prompt: dispatch one trivial background task, wait for its
/// completion notification, and only then report.
///
/// The `DEVFLOW_RESULT:` line and the bare token line are separate on purpose.
/// The marker line is what a pipe-owning supervisor's close rule watches for
/// (it must parse as the existing marker grammar, so nothing may be added
/// inside its JSON body); the bare token line is what
/// [`agent_result::token_reported_in_capture`] matches. Folding the token into
/// the marker's JSON would couple this prompt to `AgentResult`'s schema for no
/// gain.
pub fn canary_prompt(token: &str) -> String {
    format!(
        "DevFlow startup check of Claude Code's background-task notification path. \
         Do exactly the following and nothing else — do not read, create or modify any file, \
         and do not run any command.\n\
         \n\
         1. Dispatch ONE background task whose entire job is to reply with the word `ok`.\n\
         2. Wait for that task's completion notification to arrive. Do not finish before it does.\n\
         3. In the turn that follows that notification, end your message with these two lines, \
         each on its own line and exactly as written:\n\
         \n\
         {token}\n\
         DEVFLOW_RESULT: {{\"status\":\"success\"}}\n\
         \n\
         The first line is a single-use token supplied by DevFlow. Reproduce it character for \
         character; do not shorten, summarise, quote or comment on it."
    )
}

/// Run one delivery canary and report what it established.
///
/// Declares a fresh token, plants it in a throwaway prompt, runs `launcher`
/// against a capture inside `capture_dir`, and hands the resulting capture text
/// to [`agent_result::token_reported_in_capture`] — the one function in this
/// codebase that decides whether a token came back from somewhere trustworthy.
/// See this module's header for why that decision is not made here.
/// Every failure mode below is `Unverified`, never `Absent`. `Absent` is a
/// claim about the CLI's behaviour and may only be made after the CLI actually
/// ran and produced a capture that could be read.
pub fn run_delivery_canary<L: CanaryLauncher>(launcher: &L, capture_dir: &Path) -> CanaryOutcome {
    let token = declare_token();
    let capture = canary_capture_path(capture_dir);

    // Through `ensure_devflow_dir` rather than a bare `create_dir_all`: it also
    // self-protects a `.devflow` in the path with a `*` .gitignore, and the
    // canary capture is agent output that must not be sweepable into a
    // downstream repo by a routine `git add .` (T-31-13, ROADMAP §999.69).
    if let Err(err) = crate::workflow::ensure_devflow_dir(capture_dir) {
        return CanaryOutcome::Unverified(format!(
            "could not prepare the canary capture directory {}: {err}",
            capture_dir.display()
        ));
    }

    if let Err(reason) = launcher.run(&canary_prompt(&token), &capture) {
        return CanaryOutcome::Unverified(reason);
    }

    // Lossy decode, matching the ONE capture-decode policy the rest of this
    // codebase reads through (`agent_result::read_capture`, CR-01): a single
    // invalid UTF-8 byte from a raw pipe must not silently disable the guard.
    // REPLACE rather than drop — dropping joins the tokens on either side.
    let text = match std::fs::read(&capture) {
        Ok(bytes) => String::from_utf8_lossy(&bytes).into_owned(),
        Err(err) => {
            return CanaryOutcome::Unverified(format!(
                "the canary ran but its capture {} could not be read: {err}",
                capture.display()
            ));
        }
    };

    if agent_result::token_reported_in_capture(&text, &token) {
        CanaryOutcome::Confirmed
    } else {
        CanaryOutcome::Absent
    }
}

/// How long the canary waits with NOTHING arriving on the child's stdout before
/// concluding nothing more is coming.
///
/// Deliberately its OWN constant rather than a reuse of the stage monitor's
/// idle timeout: the canary waits for one trivial background task, the monitor
/// waits for a whole stage, and coupling them would let a future change to the
/// stage timeout silently change how patient the guard is. That separation is
/// still right — but it is exactly why this constant has to be re-derived when
/// the evidence moves, rather than tracking the other one for free.
///
/// **Raised 30s -> 120s on 2026-08-03. The previous value's stated "~4x margin"
/// was refuted by measurement.** That figure came from Phase 30d's *backgrounded*
/// 10s/22s sleeps. Direct measurement (CLI 2.1.220, five workload-controlled
/// trials, two workload types, negative control) found the CLI emits
/// `tool_progress` keepalives on a **fixed 30.00s interval**, with the first gap
/// after `task_started` consistently ~26.4s. So 30s of stream silence is normal
/// healthy behaviour, and a 30s patience budget had roughly 1.1x margin, not 4x.
/// See `IDLE_TIMEOUT_FLOOR_SECS` in `monitor.rs` and the phase's
/// `31-IDLE-GAP-MEASUREMENTS.md`.
///
/// **Why a false `Absent` is the expensive direction here.** This guard *refuses
/// to run* on `Absent`/`Unverified` (D-15). A canary that gives up during a
/// normal keepalive gap does not degrade the run — it locks the operator out of
/// every `stream-json` launch until they diagnose it. Being slower to detect a
/// genuinely dead delivery path costs one wait, bounded anyway by
/// [`CANARY_DEADLINE_SECS`]; being wrong in the other direction costs the tool.
const CANARY_IDLE_SECS: u64 = 120;

/// Absolute wall-clock cap on one canary run.
///
/// The guard runs SYNCHRONOUSLY inside the operator's `devflow start`, so a
/// child that never speaks and never exits would wedge the launch outright.
/// The idle timeout above already covers a silent child; this covers a chatty
/// one that never converges.
const CANARY_DEADLINE_SECS: u64 = 300;

/// How long the child gets to exit on its own after its stdin is released,
/// before being killed.
const CANARY_REAP_GRACE_SECS: u64 = 10;

/// Poll interval while reaping.
const REAP_POLL: Duration = Duration::from_millis(100);

/// The real launcher: runs one throwaway `claude` turn over the same
/// bidirectional `stream-json` transport a production stage uses.
///
/// **Nothing in this plan's test suite executes this type.** Every test injects
/// a launcher that writes a canned capture, by design — a guard whose own tests
/// spend real agent invocations is a guard nobody runs. The consequence is that
/// this implementation is reasoned, not witnessed; plan 31-05's acceptance run
/// against the real CLI is what witnesses it.
pub struct ClaudeCanaryLauncher {
    /// Working directory for the throwaway child.
    ///
    /// Carried as a field because [`CanaryLauncher::run`] has nowhere to put a
    /// cwd and [`hermetic_command`] requires one. Deriving it from the capture
    /// path instead would silently couple the child's working directory to
    /// where DevFlow happens to keep its runtime files.
    pub workdir: PathBuf,
}

impl CanaryLauncher for ClaudeCanaryLauncher {
    fn run(&self, prompt: &str, capture: &Path) -> Result<(), String> {
        // Phase 0 — the codebase's "not attributable to a real phase" sentinel
        // (see `advance`'s `events::emit(project_root, 0, …)`). `exec_command`
        // ignores both the phase and the prompt: under `--input-format
        // stream-json` the prompt travels on stdin, not argv.
        let (program, args) = ClaudeAgent.exec_command(PhaseId::new(0), prompt, &[]);

        let mut capture_file = std::fs::File::create(capture).map_err(|err| {
            format!(
                "could not create the canary capture {}: {err}",
                capture.display()
            )
        })?;

        // No `.process_group(0)` here, deliberately — the opposite choice from
        // `run_pipe_owning_monitor`'s detached child. This one runs in the
        // FOREGROUND of the operator's own CLI, so it should stay in the
        // terminal's process group and die with a Ctrl-C like any other
        // foreground child. Group isolation would leave a canary running with
        // nothing left to reap it.
        let mut child = hermetic_command(program, &self.workdir)
            .args(&args)
            .stdin(Stdio::piped())
            .stdout(Stdio::piped())
            // stderr is discarded rather than teed: the capture must stay
            // parseable JSONL, and nothing reads a canary's diagnostics.
            .stderr(Stdio::null())
            .spawn()
            .map_err(|err| format!("could not run `{program}`: {err}"))?;

        let mut child_stdin = child
            .stdin
            .take()
            .ok_or_else(|| "the canary child exposed no stdin pipe".to_string())?;
        let child_stdout = child
            .stdout
            .take()
            .ok_or_else(|| "the canary child exposed no stdout pipe".to_string())?;

        // Same three-participant threading model as the production monitor, and
        // for the same reason (T-31-04): writing the turn synchronously before
        // reading stdout is the textbook two-pipe deadlock.
        let (close_tx, close_rx) = mpsc::channel::<()>();
        let turn = monitor::user_turn_line(prompt);
        let writer = std::thread::spawn(move || {
            let wrote = child_stdin
                .write_all(turn.as_bytes())
                .and_then(|()| child_stdin.write_all(b"\n"))
                .and_then(|()| child_stdin.flush());
            if let Err(err) = wrote {
                warn!("could not write the canary's user turn to the child's stdin: {err}");
                return;
            }
            // Held open past the first turn ON PURPOSE. Releasing it here would
            // end the session before any task-notification turn could be
            // delivered — which is the very behaviour being measured, so the
            // guard would report `Absent` against a perfectly healthy CLI.
            let _ = close_rx.recv();
            drop(child_stdin);
        });

        let (line_tx, line_rx) = mpsc::channel::<String>();
        let reader = std::thread::spawn(move || {
            for line in BufReader::new(child_stdout).lines() {
                let Ok(line) = line else {
                    break;
                };
                if let Err(err) = writeln!(capture_file, "{line}") {
                    warn!("could not append to the canary capture: {err}");
                }
                let _ = capture_file.flush();
                if line_tx.send(line).is_err() {
                    break;
                }
            }
        });

        // The SAME close rule the production monitor applies (constraint 4's
        // AND: a top-level marker plus a drained background-task list), reused
        // rather than reimplemented. This governs only when stdin is released —
        // it is a lifecycle decision, not the trust decision. The trust
        // decision is made once, afterwards, by `run_delivery_canary`.
        let mut rule = CloseRule::default();
        let mut close_signalled = false;
        let idle = Duration::from_secs(CANARY_IDLE_SECS);
        let deadline = Instant::now() + Duration::from_secs(CANARY_DEADLINE_SECS);

        loop {
            let remaining = deadline.saturating_duration_since(Instant::now());
            if remaining.is_zero() {
                break;
            }
            match line_rx.recv_timeout(idle.min(remaining)) {
                Ok(line) => {
                    if close_signalled {
                        continue;
                    }
                    rule.observe(&line);
                    if rule.should_close() {
                        let _ = close_tx.send(());
                        close_signalled = true;
                    }
                }
                // Idle expiry, deadline expiry and stdout EOF all mean the same
                // thing here: stop waiting and go read what was captured. A
                // timeout is NOT an error — a child that ran and said nothing
                // useful is a fact about the CLI, and belongs in the
                // `Absent` decision rather than in `Unverified`.
                Err(mpsc::RecvTimeoutError::Disconnected | mpsc::RecvTimeoutError::Timeout) => {
                    break;
                }
            }
        }

        // Release stdin before waiting: a child still holding an open stdin may
        // never exit on its own.
        drop(close_tx);
        reap(&mut child);
        let _ = writer.join();
        let _ = reader.join();
        Ok(())
    }
}

/// Wait a bounded time for the canary child to exit, then kill it.
///
/// `try_wait`/`kill`/`wait` rather than [`crate::agent::terminate_and_verify`]:
/// that helper polls `/proc` liveness, and this child is a DIRECT child of the
/// current process, so it becomes an unreaped zombie whose `/proc` entry
/// outlives it — the liveness poll would report a dead child as alive for the
/// full timeout. `wait()` is the correct liveness answer for a direct child.
///
/// Known limitation, recorded rather than solved: this signals the child only,
/// not a process group, so a descendant the canary child itself spawned can
/// outlive the kill. The canary child is short-lived, capped by
/// [`CANARY_DEADLINE_SECS`], and dispatches a task that touches nothing.
fn reap(child: &mut std::process::Child) {
    let deadline = Instant::now() + Duration::from_secs(CANARY_REAP_GRACE_SECS);
    loop {
        match child.try_wait() {
            Ok(Some(_)) => return,
            Ok(None) => {}
            Err(err) => {
                warn!("could not poll the canary child: {err}");
                return;
            }
        }
        if Instant::now() >= deadline {
            break;
        }
        std::thread::sleep(REAP_POLL);
    }
    let _ = child.kill();
    let _ = child.wait();
}

/// The `claude --version` string, for the run's provenance.
///
/// Recorded alongside a canary outcome so a later forensic read can tell WHICH
/// CLI the behaviour was (or was not) witnessed on — the whole premise is
/// version-fragile, and an outcome with no version attached cannot be compared
/// against a later one. Fail-soft: `None` when the binary is missing or says
/// nothing, because a guard's provenance must never be the reason a launch
/// fails.
///
/// This is NOT the guard. A version string is a proxy for the behaviour, which
/// is exactly what D-13 rejected; it is recorded as context beside the real
/// measurement, never in place of it.
pub fn claude_cli_version() -> Option<String> {
    let output = std::process::Command::new("claude")
        .arg("--version")
        .output()
        .ok()?;
    if !output.status.success() {
        return None;
    }
    let version = String::from_utf8_lossy(&output.stdout).trim().to_string();
    (!version.is_empty()).then_some(version)
}

#[cfg(test)]
mod tests {
    use super::*;

    // ---- fixtures --------------------------------------------------------
    //
    // Event shapes are taken from the real archived capture at
    // `.planning/phases/30-keep-the-session-alive-past-turn-end/30a-evidence/
    // raw_output_v3.jsonl` (lines 5, 19, 54) by way of 31-RESEARCH.md § "Code
    // Examples": a `system`/`init` line, a top-level `result` carrying the
    // agent's own final text, and the echoed `user` turn the CLI writes back
    // into the same stdout. Identifiers are generalized; shapes are not.

    const INIT_LINE: &str = r#"{"type":"system","subtype":"init","cwd":"/tmp/work","session_id":"s-1","claude_code_version":"2.1.220","uuid":"u-init"}"#;

    /// The CLI's echo of the operator's prompt, re-emitted as a `user` event.
    /// This is the shape that produced the checkpoint false positive 30-05
    /// fixed, and the reason the canary may never substring-scan the capture.
    fn echoed_prompt_line(prompt: &str) -> String {
        serde_json::json!({
            "type": "user",
            "message": { "role": "user", "content": prompt },
            "session_id": "s-1",
            "uuid": "u-echo",
        })
        .to_string()
    }

    /// A TOP-LEVEL `result` event — no `parent_tool_use_id`, so the
    /// orchestrator session authored it.
    fn top_level_result_line(text: &str) -> String {
        serde_json::json!({
            "type": "result",
            "subtype": "success",
            "is_error": false,
            "num_turns": 3,
            "stop_reason": "end_turn",
            "session_id": "s-1",
            "uuid": "u-res",
            "result": text,
        })
        .to_string()
    }

    /// A `result` event forwarded from a SUBAGENT — same type, non-null
    /// `parent_tool_use_id`, therefore not the orchestrator speaking.
    fn subagent_result_line(text: &str) -> String {
        serde_json::json!({
            "type": "result",
            "subtype": "success",
            "is_error": false,
            "session_id": "s-1",
            "uuid": "u-sub",
            "parent_tool_use_id": "toolu_01CanarySubagent",
            "result": text,
        })
        .to_string()
    }

    /// Recover the declared token from the prompt the canary handed the
    /// launcher — the same way the real agent gets it. Keeps the tests honest:
    /// the token is generated inside `run_delivery_canary`, so a test launcher
    /// that hard-coded one would be answering a question nobody asked.
    fn token_in(prompt: &str) -> String {
        let start = prompt
            .find(TOKEN_PREFIX)
            .expect("the canary prompt must carry the declared token");
        let rest = &prompt[start + TOKEN_PREFIX.len()..];
        let suffix: String = rest.chars().take_while(char::is_ascii_hexdigit).collect();
        assert!(
            !suffix.is_empty(),
            "the token in the prompt must have a body after its prefix"
        );
        format!("{TOKEN_PREFIX}{suffix}")
    }

    /// A launcher that writes whatever `lines` the test asked for, given the
    /// token it found in the prompt. Records the prompt it was handed and how
    /// many times it ran.
    struct CannedLauncher<F: Fn(&str) -> Vec<String>> {
        lines: F,
    }

    impl<F: Fn(&str) -> Vec<String>> CanaryLauncher for CannedLauncher<F> {
        fn run(&self, prompt: &str, capture: &Path) -> Result<(), String> {
            let token = token_in(prompt);
            let body = (self.lines)(&token).join("\n");
            std::fs::write(capture, format!("{body}\n")).map_err(|err| err.to_string())?;
            Ok(())
        }
    }

    /// A launcher that could not run at all — a missing binary, a permission
    /// error, a spawn failure. It writes no capture.
    struct FailingLauncher(&'static str);

    impl CanaryLauncher for FailingLauncher {
        fn run(&self, _prompt: &str, _capture: &Path) -> Result<(), String> {
            Err(self.0.to_string())
        }
    }

    /// The token came back inside a top-level `result` — the notification path
    /// is alive.
    #[test]
    fn canary_confirmed_when_token_returns_in_a_top_level_result() {
        let dir = tempfile::tempdir().unwrap();

        let launcher = CannedLauncher {
            lines: |token| {
                vec![
                    INIT_LINE.to_string(),
                    top_level_result_line(&format!(
                        "The background task finished.\n{token}\nDEVFLOW_RESULT: {{\"status\":\"success\"}}"
                    )),
                ]
            },
        };

        let outcome = run_delivery_canary(&launcher, dir.path());

        assert_eq!(
            outcome,
            CanaryOutcome::Confirmed,
            "a token inside a top-level result is the whole point of the guard"
        );
    }

    /// D-13 trap 1, and the single most important test in this module: the CLI
    /// echoes the prompt back, so the planted token appears in the capture
    /// whether or not anything was delivered. A canary that scanned the capture
    /// would certify delivery that never happened.
    #[test]
    fn canary_absent_when_token_appears_only_as_a_prompt_echo() {
        let dir = tempfile::tempdir().unwrap();

        let launcher = CannedLauncher {
            lines: |token| {
                vec![
                    INIT_LINE.to_string(),
                    // The echo carries the token verbatim …
                    echoed_prompt_line(&canary_prompt(token)),
                    // … while the agent's own final word does not.
                    top_level_result_line("I could not dispatch a background task."),
                ]
            },
        };

        let outcome = run_delivery_canary(&launcher, dir.path());

        // Negative control for the assertion below: if the capture did not
        // contain the token at all, `Absent` would be true for an entirely
        // uninteresting reason and this test would be measuring nothing.
        // Checked per LINE and by parsing, not by slicing the raw text —
        // `serde_json` writes object keys in sorted order, so `"type":"user"`
        // lands AFTER the message body that carries the token and a
        // position-based check reads backwards.
        let capture = std::fs::read_to_string(canary_capture_path(dir.path())).unwrap();
        let carrying: Vec<serde_json::Value> = capture
            .lines()
            .filter(|line| line.contains(TOKEN_PREFIX))
            .map(|line| serde_json::from_str(line).expect("fixture lines are JSON"))
            .collect();
        assert!(
            !carrying.is_empty(),
            "fixture must actually contain the echoed token"
        );
        assert!(
            carrying.iter().all(|event| event["type"] == "user"),
            "fixture must place the echoed token ONLY inside a `user` event — \
             if any result event carries it, this test is not exercising the echo case"
        );

        assert_eq!(
            outcome,
            CanaryOutcome::Absent,
            "an echoed token must never satisfy the guard (30-05's false positive)"
        );
    }

    /// Provenance, the second half of trap 1: a `result` forwarded from a
    /// subagent is the right event TYPE and the wrong AUTHOR.
    #[test]
    fn canary_absent_when_token_appears_only_in_a_non_top_level_event() {
        let dir = tempfile::tempdir().unwrap();

        let launcher = CannedLauncher {
            lines: |token| {
                vec![
                    INIT_LINE.to_string(),
                    subagent_result_line(&format!("child reporting: {token}")),
                    top_level_result_line("Done."),
                ]
            },
        };

        let outcome = run_delivery_canary(&launcher, dir.path());

        // Same negative control: prove the token is present before concluding
        // anything from its not being honoured.
        let capture = std::fs::read_to_string(canary_capture_path(dir.path())).unwrap();
        assert!(
            capture.contains(TOKEN_PREFIX),
            "fixture must actually contain the token inside the subagent result"
        );
        assert!(
            capture.contains("parent_tool_use_id"),
            "fixture must actually mark that result as subagent-authored"
        );

        assert_eq!(
            outcome,
            CanaryOutcome::Absent,
            "a subagent-authored result must not certify orchestrator-level delivery"
        );
    }

    /// "The CLI could not be run" is not "the CLI ran and the behaviour is
    /// gone". Collapsing the two would report a missing binary as a broken
    /// premise and send the operator after the wrong problem entirely.
    #[test]
    fn canary_unverified_when_the_launcher_fails() {
        let dir = tempfile::tempdir().unwrap();

        let outcome = run_delivery_canary(
            &FailingLauncher("could not run `claude`: No such file or directory (os error 2)"),
            dir.path(),
        );

        match outcome {
            CanaryOutcome::Unverified(reason) => {
                assert!(
                    reason.contains("No such file or directory"),
                    "the reason the guard could not run must survive into the outcome, \
                     got: {reason}"
                );
            }
            other => panic!("a launcher failure must be Unverified, not {other:?}"),
        }
    }

    /// A token reused across runs would let a stale capture satisfy a later
    /// guard.
    #[test]
    fn declared_tokens_differ_between_runs() {
        let first = declare_token();
        let second = declare_token();

        assert_ne!(
            first, second,
            "each canary run must declare its own token, or a stale capture could satisfy it"
        );
        assert!(
            first.starts_with(TOKEN_PREFIX) && second.starts_with(TOKEN_PREFIX),
            "both tokens must carry the greppable prefix"
        );
    }
}