M5b plan: task 01, the invalid-arguments detail

Task file and 13 given files (2 new: the wire fixture with a detail and brokerd/tests/invalid_detail.rs; 11 new versions of existing tests, most only adding `detail: None`). Checked against a reference in a scratch worktree with its own target/, on titan (rustc 1.98.1, clippy 0.1.98): make gate prints gate: ok with 956 tests (944 + 12 new); /tmp has no new entries after the gate. All 21 quoted before-blocks occur once in master, and applying the edits to master gives the reference byte for byte. The given files are the reference's, formatted, none over 500 lines; no chat-template tag in the plan. Before the edits, strict's envelopes_reject_unknown_keys_at_every_depth fails and ten test targets do not compile, as step 2 lists. Mutations: 21 one-line breaks of the reference; 20 caught by the given tests. The one not caught (dropping `default` from the detail attribute) cannot change behaviour: serde reads a missing Option field as None. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
2026-09-26 02:35:35 -07:00
parent 56b969d55b
commit fa5bcae503
14 changed files with 4338 additions and 0 deletions
@@ -0,0 +1,388 @@
+//! One `admin.sock` connection: list, approve, refuse, check the grants; and expiry, which
+//! answers an approval the same way. Do not edit.
+
+#[path = "support/client.rs"]
+mod client;
+#[path = "support/rig.rs"]
+mod rig;
+#[path = "support/runtime.rs"]
+mod runtime;
+#[path = "support/sink.rs"]
+mod sink;
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::os::unix::net::UnixStream;
+use std::sync::{Arc, Barrier};
+use std::time::{Duration, Instant};
+
+use brokerd::admin;
+use brokerd::broker::{self, Broker};
+use brokerd::policy::Label;
+use bxcore::{
+    ApprovalAnswer, Approve, AuditEvent, DataClass, DecisionRecord, DenyReason, Empty, ErrorCode,
+    Message, Refuse, SessionId, Timestamp, ToolResponse,
+};
+use client::{Serve, next, open};
+use rig::{Rig, grant_text, request};
+use runtime::Recording;
+
+fn ask_notes(rig: &Rig) {
+    rig.grant(
+        "notes",
+        &grant_text("read_file", "ask", "", "paths = [\"/n\"]"),
+    );
+}
+
+/// Sends a call that waits for approval; returns its connection once the pending frame is read.
+fn pending(broker: &Arc<Broker>, call: u64, arguments: &str) -> (UnixStream, u64) {
+    let req = request("s1", call, "read_file", arguments);
+    let mut stream = open(broker, broker::handle, call, Message::ToolRequest(req));
+    match next(&mut stream).msg {
+        Message::ToolResponse(ToolResponse::PendingApproval { approval, .. }) => (stream, approval),
+        other => panic!("{other:?}"),
+    }
+}
+
+/// One admin request; returns the answer's message after checking its id and `final`.
+fn admin(broker: &Arc<Broker>, msg: Message) -> Message {
+    let mut stream = open(broker, admin::handle, 5, msg);
+    let env = next(&mut stream);
+    assert_eq!((env.id, env.r#final), (5, true), "{env:?}");
+    env.msg
+}
+
+fn approve(broker: &Arc<Broker>, approval: u64) -> Message {
+    admin(broker, Message::Approve(Approve { approval }))
+}
+
+fn refuse(broker: &Arc<Broker>, approval: u64, reason: Option<&str>) -> Message {
+    let reason = reason.map(str::to_string);
+    admin(broker, Message::Refuse(Refuse { approval, reason }))
+}
+
+fn final_answer(stream: &mut UnixStream) -> ToolResponse {
+    match next(stream).msg {
+        Message::ToolResponse(r) => r,
+        other => panic!("{other:?}"),
+    }
+}
+
+fn outcome(msg: Message) -> DecisionRecord {
+    match msg {
+        Message::ApproveResult(r) => r.outcome,
+        other => panic!("{other:?}"),
+    }
+}
+
+fn error_code(msg: &Message) -> Option<ErrorCode> {
+    match msg {
+        Message::Error(e) => Some(e.code),
+        _ => None,
+    }
+}
+
+fn denied(reason: DenyReason) -> ToolResponse {
+    ToolResponse::Denied {
+        reason,
+        detail: None,
+    }
+}
+
+#[test]
+fn approvals_lists_the_arguments_as_policy_parsed_them() {
+    let rig = Rig::new("admin-list");
+    ask_notes(&rig);
+    let broker = rig.broker(&Recording::answering("x"));
+    assert!(matches!(
+        admin(&broker, Message::Approvals(Empty {})),
+        Message::ApprovalList(list) if list.items.is_empty()
+    ));
+    // A backslash-u escape of `/` is `/` in JSON; the owner sees the one spelling policy matched.
+    let escaped = format!(r#"{{"path":"{}n{}a"}}"#, "\\u002f", "\\u002f");
+    assert!(
+        escaped.contains("u002f"),
+        "the escape must survive: {escaped}"
+    );
+    let (_stream, approval) = pending(&broker, 1, &escaped);
+    match admin(&broker, Message::Approvals(Empty {})) {
+        Message::ApprovalList(list) => {
+            assert_eq!(list.items.len(), 1);
+            assert_eq!(list.items[0].approval, approval);
+            assert_eq!(list.items[0].arguments, r#"{"path":"/n/a"}"#);
+            assert_eq!(list.items[0].grant, "notes");
+        }
+        other => panic!("{other:?}"),
+    }
+    // The audit log keeps the raw string.
+    match &rig.events()[0] {
+        AuditEvent::Decision { arguments, .. } => assert_eq!(arguments, &escaped),
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn approve_runs_the_call_and_answers_with_the_re_decision() {
+    let rig = Rig::new("admin-approve");
+    ask_notes(&rig);
+    let rt = Recording::answering("the notes");
+    let broker = rig.broker(&rt);
+    let (mut stream, approval) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    assert_eq!(outcome(approve(&broker, approval)), DecisionRecord::Ask {});
+    assert!(matches!(
+        final_answer(&mut stream),
+        ToolResponse::Result { .. }
+    ));
+    assert_eq!(rt.count(), 1);
+    match &rig.events()[1] {
+        AuditEvent::Approval { answer, by, .. } => {
+            assert_eq!(*answer, ApprovalAnswer::Approved);
+            assert_eq!(by.as_deref(), Some("bxctl"));
+        }
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn refuse_denies_the_call_and_records_the_reason() {
+    let rig = Rig::new("admin-refuse");
+    ask_notes(&rig);
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, approval) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    assert!(matches!(
+        refuse(&broker, approval, Some("not today")),
+        Message::Ok(Empty {})
+    ));
+    assert_eq!(
+        final_answer(&mut stream),
+        denied(DenyReason::ApprovalRefused)
+    );
+    assert_eq!(rt.count(), 0);
+    match &rig.events()[1] {
+        AuditEvent::Approval { answer, reason, .. } => {
+            assert_eq!(*answer, ApprovalAnswer::Refused);
+            assert_eq!(reason.as_deref(), Some("not today"));
+        }
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn an_unknown_or_answered_id_is_no_such_approval() {
+    let rig = Rig::new("admin-unknown");
+    ask_notes(&rig);
+    let broker = rig.broker(&Recording::answering("x"));
+    assert_eq!(
+        error_code(&approve(&broker, 41)),
+        Some(ErrorCode::NoSuchApproval)
+    );
+    assert_eq!(
+        error_code(&refuse(&broker, 41, None)),
+        Some(ErrorCode::NoSuchApproval)
+    );
+    let (_stream, approval) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    refuse(&broker, approval, None);
+    assert_eq!(
+        error_code(&approve(&broker, approval)),
+        Some(ErrorCode::NoSuchApproval)
+    );
+    assert_eq!(rig.events().len(), 2, "one decision, one approval");
+}
+
+#[test]
+fn approve_and_refuse_at_once_give_exactly_one_answer() {
+    let rig = Rig::new("admin-race");
+    ask_notes(&rig);
+    let broker = rig.broker(&Recording::answering("x"));
+    for round in 0..100u64 {
+        let (mut stream, approval) = pending(&broker, round, r#"{"path":"/n/a"}"#);
+        let start = Arc::new(Barrier::new(2));
+        let racers: Vec<_> = [true, false]
+            .into_iter()
+            .map(|approving| {
+                let (broker, start) = (Arc::clone(&broker), Arc::clone(&start));
+                std::thread::spawn(move || {
+                    start.wait();
+                    if approving {
+                        approve(&broker, approval)
+                    } else {
+                        refuse(&broker, approval, None)
+                    }
+                })
+            })
+            .collect();
+        let answers: Vec<Message> = racers.into_iter().map(|r| r.join().unwrap()).collect();
+        let losers = answers
+            .iter()
+            .filter(|m| error_code(m) == Some(ErrorCode::NoSuchApproval))
+            .count();
+        assert_eq!(losers, 1, "round {round}: {answers:?}");
+        final_answer(&mut stream);
+        let approvals = rig
+            .events()
+            .iter()
+            .filter(|e| matches!(e, AuditEvent::Approval { decision, .. } if *decision == approval))
+            .count();
+        assert_eq!(approvals, 1, "round {round}");
+    }
+}
+
+#[test]
+fn approve_after_the_grant_file_is_removed_is_denied() {
+    let rig = Rig::new("admin-removed");
+    ask_notes(&rig);
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, approval) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    rig.remove_grant("notes");
+    let reason = DenyReason::NoGrant;
+    assert_eq!(
+        outcome(approve(&broker, approval)),
+        DecisionRecord::Denied { reason }
+    );
+    assert_eq!(final_answer(&mut stream), denied(reason));
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn approve_after_the_taint_rose_past_the_grant_is_denied() {
+    let rig = Rig::new("admin-taint");
+    let text = grant_text("read_file", "ask", "", "paths = [\"/n\"]")
+        .replace("max_taint = \"secret\"", "max_taint = \"private\"");
+    rig.grant("notes", &text);
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, approval) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    let s1 = SessionId::new("s1").unwrap();
+    let secret = Label {
+        class: DataClass::Secret,
+        untrusted: false,
+    };
+    rig.state()
+        .raise(&s1, rig.state().read(&s1).unwrap(), secret)
+        .unwrap();
+    let reason = DenyReason::TaintTooHigh;
+    assert_eq!(
+        outcome(approve(&broker, approval)),
+        DecisionRecord::Denied { reason }
+    );
+    assert_eq!(final_answer(&mut stream), denied(reason));
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn an_approval_that_cannot_be_recorded_is_denied_on_both_sides() {
+    let rig = Rig::new("admin-norecord");
+    ask_notes(&rig);
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, approval) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    rig.switch.fail(true);
+    let reason = DenyReason::AuditUnavailable;
+    assert_eq!(
+        outcome(approve(&broker, approval)),
+        DecisionRecord::Denied { reason }
+    );
+    assert_eq!(final_answer(&mut stream), denied(reason));
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn a_refusal_that_cannot_be_recorded_is_an_error_for_bxctl() {
+    let rig = Rig::new("admin-norefuse");
+    ask_notes(&rig);
+    let broker = rig.broker(&Recording::answering("x"));
+    let (mut stream, approval) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    rig.switch.fail(true);
+    match refuse(&broker, approval, None) {
+        Message::Error(e) => {
+            assert_eq!(e.code, ErrorCode::Internal);
+            assert!(
+                e.detail.ends_with("see docs/runbook.md#audit-unavailable"),
+                "{}",
+                e.detail
+            );
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(
+        final_answer(&mut stream),
+        denied(DenyReason::AuditUnavailable)
+    );
+}
+
+#[test]
+fn expiry_denies_an_approval_nobody_answered() {
+    let rig = Rig::with_ttl("admin-expire", 100);
+    ask_notes(&rig);
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, _) = pending(&broker, 1, r#"{"path":"/n/a"}"#);
+    let until = Instant::now() + Duration::from_secs(10);
+    while admin::expire_due(&broker, Timestamp::now()) == 0 {
+        assert!(Instant::now() < until, "never expired");
+        std::thread::sleep(Duration::from_millis(20));
+    }
+    assert_eq!(
+        final_answer(&mut stream),
+        denied(DenyReason::ApprovalExpired)
+    );
+    match &rig.events()[1] {
+        AuditEvent::Approval { answer, by, .. } => {
+            assert_eq!((*answer, by.as_deref()), (ApprovalAnswer::Expired, None));
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn check_grants_reports_every_problem_or_none() {
+    let rig = Rig::new("admin-grants");
+    ask_notes(&rig);
+    let broker = rig.broker(&Recording::answering("x"));
+    match admin(&broker, Message::CheckGrants(Empty {})) {
+        Message::GrantsReport(r) => assert!(r.problems.is_empty(), "{r:?}"),
+        other => panic!("{other:?}"),
+    }
+    rig.grant("bad", "tool = \"read_file\"\nmdoe = \"auto\"\n");
+    rig.grant(
+        "Worse",
+        &grant_text("read_file", "ask", "", "paths = [\"/n\"]"),
+    );
+    match admin(&broker, Message::CheckGrants(Empty {})) {
+        Message::GrantsReport(r) => {
+            let files: Vec<&str> = r.problems.iter().map(|p| p.file.as_str()).collect();
+            assert!(files.contains(&"bad.toml"), "{r:?}");
+            assert!(files.contains(&"Worse.toml"), "{r:?}");
+        }
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn every_other_kind_on_admin_sock_is_forbidden() {
+    let rig = Rig::new("admin-forbidden");
+    let broker = rig.broker(&Recording::answering("x"));
+    let req = request("s1", 1, "read_file", r#"{"path":"/n/a"}"#);
+    for msg in [
+        Message::ToolRequest(req),
+        Message::Ok(Empty {}),
+        Message::ToolResponse(denied(DenyReason::NoGrant)),
+    ] {
+        assert_eq!(error_code(&admin(&broker, msg)), Some(ErrorCode::Forbidden));
+    }
+    let lines = rig.lines.with("on admin.sock");
+    assert_eq!(lines.len(), 3, "{:?}", rig.lines.all());
+    assert!(lines[0].contains("tool_request"), "{}", lines[0]);
+    assert!(
+        lines
+            .iter()
+            .all(|l| l.ends_with("see docs/runbook.md#socket-forbidden"))
+    );
+    assert!(
+        rig.events().is_empty(),
+        "a refused tool request decides nothing"
+    );
+}
@@ -0,0 +1,327 @@
+//! One `broker.sock` connection: decision, record, run, answer; and every way it fails closed.
+//! Do not edit.
+
+#[path = "support/client.rs"]
+mod client;
+#[path = "support/rig.rs"]
+mod rig;
+#[path = "support/runtime.rs"]
+mod runtime;
+#[path = "support/sink.rs"]
+mod sink;
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::io::Write;
+use std::os::unix::fs::PermissionsExt;
+use std::os::unix::net::UnixStream;
+use std::sync::Arc;
+
+use brokerd::broker;
+use brokerd::ledger::NOT_RECORDED;
+use brokerd::runner::{REFUSING, RunError};
+use bxcore::{
+    ApprovalList, Approve, ApproveResult, AuditEvent, DataClass, DecisionRecord, DenyReason, Empty,
+    Envelope, ErrorCode, GrantsReport, Message, Refuse, ResultStatus, SessionId, ToolResponse,
+    Turn, TurnDone, TurnEvent, Usage, WireError,
+};
+use client::{Serve, call, next, open};
+use rig::{Rig, grant_text, request};
+use runtime::Recording;
+
+const NOTES: &str = r#"{"path":"/n/a"}"#;
+
+fn read_notes(call_id: u64) -> bxcore::ToolRequest {
+    request("s1", call_id, "read_file", NOTES)
+}
+
+fn only(frames: &[Envelope]) -> &ToolResponse {
+    assert_eq!(frames.len(), 1, "{frames:?}");
+    assert!(frames[0].r#final);
+    match &frames[0].msg {
+        Message::ToolResponse(r) => r,
+        other => panic!("{other:?}"),
+    }
+}
+
+fn denied(reason: DenyReason) -> ToolResponse {
+    ToolResponse::Denied {
+        reason,
+        detail: None,
+    }
+}
+
+fn auto_notes(rig: &Rig, class: &str) {
+    let text = grant_text("read_file", "auto", "", "paths = [\"/n\"]").replace(
+        "result_class = \"private\"",
+        &format!("result_class = \"{class}\""),
+    );
+    rig.grant("notes", &text);
+}
+
+#[test]
+fn a_call_no_grant_allows_is_denied_and_recorded() {
+    let rig = Rig::new("broker-nogrant");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let frames = call(&broker, read_notes(41));
+    assert_eq!(frames[0].id, 41, "the answer carries the request's id");
+    assert_eq!(only(&frames), &denied(DenyReason::NoGrant));
+    assert_eq!(rt.count(), 0);
+    match rig.events().as_slice() {
+        [AuditEvent::Decision { outcome, .. }] => assert_eq!(
+            *outcome,
+            DecisionRecord::Denied {
+                reason: DenyReason::NoGrant
+            }
+        ),
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn an_allowed_call_runs_and_its_result_raises_the_state() {
+    let rig = Rig::new("broker-allowed");
+    auto_notes(&rig, "secret");
+    let rt = Recording::answering("the notes");
+    let broker = rig.broker(&rt);
+    let frames = call(&broker, read_notes(7));
+    assert_eq!(
+        only(&frames),
+        &ToolResponse::Result {
+            content: "the notes".to_string(),
+            class: DataClass::Secret,
+            untrusted: false,
+            truncated: false,
+        }
+    );
+    assert_eq!(rt.count(), 1);
+    let events = rig.events();
+    assert!(matches!(
+        events.as_slice(),
+        [
+            AuditEvent::Decision { .. },
+            AuditEvent::Result {
+                decision: 0,
+                status: ResultStatus::Result,
+                taint_after: DataClass::Secret,
+                ..
+            }
+        ]
+    ));
+    let state = rig.state().read(&SessionId::new("s1").unwrap()).unwrap();
+    assert_eq!(state.taint, DataClass::Secret);
+}
+
+#[test]
+fn a_runtime_failure_is_passed_on_and_recorded_as_failed() {
+    let rig = Rig::new("broker-refusing");
+    auto_notes(&rig, "secret");
+    let rt = Recording::with(Err(RunError::Unavailable(REFUSING.to_string())));
+    let broker = rig.broker(&rt);
+    let frames = call(&broker, read_notes(1));
+    assert_eq!(
+        only(&frames),
+        &ToolResponse::Failed {
+            message: REFUSING.to_string()
+        }
+    );
+    match &rig.events()[1] {
+        AuditEvent::Result {
+            status,
+            taint_after,
+            ..
+        } => assert_eq!(
+            (*status, *taint_after),
+            (ResultStatus::Failed, DataClass::Private)
+        ),
+        other => panic!("{other:?}"),
+    }
+    assert!(!rig.state_file("s1").exists(), "a failure changes no state");
+}
+
+fn every_kind_but_tool_request() -> Vec<Message> {
+    let usage = Usage {
+        cache_n: 0,
+        prompt_n: 0,
+        predicted_n: 0,
+        reasoning_tokens: 0,
+        thinking_capped: false,
+    };
+    vec![
+        Message::ToolResponse(denied(DenyReason::NoGrant)),
+        Message::Error(WireError {
+            code: ErrorCode::Internal,
+            detail: String::new(),
+        }),
+        Message::Turn(Turn {
+            session: SessionId::new("s1").unwrap(),
+            content: String::new(),
+            resume: false,
+        }),
+        Message::TurnEvent(TurnEvent::Content {
+            text: String::new(),
+        }),
+        Message::TurnDone(TurnDone {
+            content: String::new(),
+            usage,
+        }),
+        Message::Approvals(Empty {}),
+        Message::ApprovalList(ApprovalList { items: Vec::new() }),
+        Message::Approve(Approve { approval: 0 }),
+        Message::ApproveResult(ApproveResult {
+            outcome: DecisionRecord::Allowed {},
+        }),
+        Message::Refuse(Refuse {
+            approval: 0,
+            reason: None,
+        }),
+        Message::Ok(Empty {}),
+        Message::CheckGrants(Empty {}),
+        Message::GrantsReport(GrantsReport {
+            problems: Vec::new(),
+        }),
+    ]
+}
+
+#[test]
+fn every_other_kind_on_broker_sock_is_forbidden() {
+    let rig = Rig::new("broker-forbidden");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let kinds = every_kind_but_tool_request();
+    assert_eq!(kinds.len(), 13, "every Message variant but tool_request");
+    for (i, msg) in kinds.into_iter().enumerate() {
+        let id = 100 + i as u64;
+        let mut stream = open(&broker, broker::handle, id, msg);
+        let env = next(&mut stream);
+        assert_eq!((env.id, env.r#final), (id, true));
+        match env.msg {
+            Message::Error(e) => assert_eq!(e.code, ErrorCode::Forbidden),
+            other => panic!("{other:?}"),
+        }
+    }
+    let lines = rig.lines.with("broker.sock");
+    assert_eq!(lines.len(), 13, "{:?}", rig.lines.all());
+    for line in &lines {
+        assert!(
+            line.ends_with("\nsee docs/runbook.md#socket-forbidden"),
+            "{line}"
+        );
+    }
+    assert_eq!(rig.lines.with("kind approve on broker.sock").len(), 1);
+    assert!(
+        rig.events().is_empty(),
+        "nothing is recorded for a refused kind"
+    );
+}
+
+#[test]
+fn a_frame_that_is_not_json_is_answered_with_bad_message() {
+    let rig = Rig::new("broker-badframe");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut client, server) = UnixStream::pair().unwrap();
+    let b = Arc::clone(&broker);
+    std::thread::spawn(move || broker::handle(server, &b));
+    client.write_all(&3u32.to_be_bytes()).unwrap();
+    client.write_all(b"{{{").unwrap();
+    let env = next(&mut client);
+    assert_eq!((env.id, env.r#final), (0, true));
+    match env.msg {
+        Message::Error(e) => assert_eq!(e.code, ErrorCode::BadMessage),
+        other => panic!("{other:?}"),
+    }
+    assert!(rig.events().is_empty());
+}
+
+#[test]
+fn one_invalid_grant_file_denies_a_call_a_valid_file_would_allow() {
+    let rig = Rig::new("broker-invalid");
+    auto_notes(&rig, "private");
+    rig.grant("bad", "tool = \"read_file\"\nmdoe = \"auto\"\n");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    for id in [1, 2] {
+        let frames = call(&broker, read_notes(id));
+        assert_eq!(only(&frames), &denied(DenyReason::GrantsInvalid));
+    }
+    assert_eq!(rt.count(), 0);
+    // Printed once for the two calls, with the pointer.
+    let printed = rig.lines.with("bad.toml");
+    assert_eq!(printed.len(), 1, "{:?}", rig.lines.all());
+    assert!(
+        printed[0].ends_with("see docs/runbook.md#grants-invalid"),
+        "{}",
+        printed[0]
+    );
+    rig.remove_grant("bad");
+    let frames = call(&broker, read_notes(3));
+    assert!(matches!(only(&frames), ToolResponse::Result { .. }));
+    assert_eq!(rt.count(), 1);
+}
+
+#[test]
+fn a_damaged_session_state_is_state_unreadable() {
+    let rig = Rig::new("broker-state");
+    auto_notes(&rig, "private");
+    std::fs::create_dir_all(rig.cfg.state_dir()).unwrap();
+    std::fs::write(rig.state_file("s1"), "{\"taint\":\"loud\"}").unwrap();
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let frames = call(&broker, read_notes(1));
+    assert_eq!(only(&frames), &denied(DenyReason::StateUnreadable));
+    assert_eq!(rt.count(), 0);
+    let lines = rig.lines.with("s1.json");
+    assert!(
+        lines
+            .iter()
+            .any(|l| l.ends_with("see docs/runbook.md#broker-state-damaged")),
+        "{lines:?}"
+    );
+}
+
+#[test]
+fn a_failed_audit_write_runs_nothing_now_or_later() {
+    let rig = Rig::new("broker-audit");
+    auto_notes(&rig, "private");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    rig.switch.fail(true);
+    let frames = call(&broker, read_notes(1));
+    assert_eq!(only(&frames), &denied(DenyReason::AuditUnavailable));
+    rig.switch.fail(false);
+    let frames = call(&broker, read_notes(2));
+    assert_eq!(only(&frames), &denied(DenyReason::AuditUnavailable));
+    assert_eq!(rt.count(), 0);
+    assert!(
+        !rig.lines
+            .with("see docs/runbook.md#audit-unavailable")
+            .is_empty()
+    );
+}
+
+#[test]
+fn a_state_that_cannot_be_written_withholds_the_content() {
+    if tmp::running_as_root("a_state_that_cannot_be_written_withholds_the_content") {
+        return;
+    }
+    let rig = Rig::new("broker-ro");
+    auto_notes(&rig, "secret");
+    let dir = rig.cfg.state_dir();
+    std::fs::create_dir_all(&dir).unwrap();
+    std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(0o500)).unwrap();
+    let rt = Recording::answering("the secret");
+    let broker = rig.broker(&rt);
+    let frames = call(&broker, read_notes(1));
+    std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(0o700)).unwrap();
+    assert_eq!(
+        only(&frames),
+        &ToolResponse::Failed {
+            message: NOT_RECORDED.to_string()
+        }
+    );
+    assert_eq!(rt.count(), 1, "it ran; its content is what is withheld");
+    let text = format!("{frames:?}");
+    assert!(!text.contains("the secret"), "{text}");
+}
@@ -0,0 +1,233 @@
+//! A call an `ask` grant matched: the pending frame, the table entry, the wait, and a requester
+//! that goes away. The tests answer entries by hand, as `admin` will. Do not edit.
+
+#[path = "support/client.rs"]
+mod client;
+#[path = "support/rig.rs"]
+mod rig;
+#[path = "support/runtime.rs"]
+mod runtime;
+#[path = "support/sink.rs"]
+mod sink;
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::os::unix::net::UnixStream;
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+use brokerd::broker::{self, Broker, GONE};
+use brokerd::ledger::Answer;
+use bxcore::{
+    AuditEvent, DataClass, DecisionRecord, DenyReason, Message, ResultStatus, Timestamp,
+    ToolResponse,
+};
+use client::{Serve, next, open};
+use rig::{Rig, grant_text, request};
+use runtime::Recording;
+
+fn ask_notes(rig: &Rig, extra: &str) {
+    rig.grant(
+        "notes",
+        &grant_text("read_file", "ask", extra, "paths = [\"/n\"]"),
+    );
+}
+
+/// Sends a call and reads its pending frame. Returns the connection and the frame's values.
+fn start(broker: &Arc<Broker>, arguments: &str) -> (UnixStream, u64, Timestamp) {
+    let req = request("s1", 9, "read_file", arguments);
+    let mut stream = open(broker, broker::handle, 9, Message::ToolRequest(req));
+    let env = next(&mut stream);
+    assert_eq!((env.id, env.r#final), (9, false), "{env:?}");
+    match env.msg {
+        Message::ToolResponse(ToolResponse::PendingApproval { approval, expires }) => {
+            (stream, approval, expires)
+        }
+        other => panic!("{other:?}"),
+    }
+}
+
+/// What `admin` does for an entry it took: record the answer, send the verdict.
+fn answer_by_hand(broker: &Broker, id: u64, answer: Answer) -> DecisionRecord {
+    let entry = broker.table().take(id).expect("the entry is pending");
+    answer_entry(broker, entry, answer)
+}
+
+fn answer_entry(
+    broker: &Broker,
+    entry: brokerd::approvals::Entry,
+    answer: Answer,
+) -> DecisionRecord {
+    let grants = broker.grants();
+    let done = broker.ledger().answer(
+        entry.ask,
+        entry.info.approval,
+        answer,
+        &grants,
+        Timestamp::now(),
+    );
+    entry.reply.send(done.verdict).unwrap();
+    done.outcome
+}
+
+fn approved() -> Answer {
+    Answer::Approved {
+        by: Some("bxctl".to_string()),
+        post: None,
+    }
+}
+
+fn final_answer(stream: &mut UnixStream) -> ToolResponse {
+    let env = next(stream);
+    assert!(env.r#final, "{env:?}");
+    match env.msg {
+        Message::ToolResponse(r) => r,
+        other => panic!("{other:?}"),
+    }
+}
+
+fn eventually(what: &str, mut done: impl FnMut() -> bool) {
+    let until = Instant::now() + Duration::from_secs(10);
+    while !done() {
+        assert!(Instant::now() < until, "never happened: {what}");
+        std::thread::sleep(Duration::from_millis(20));
+    }
+}
+
+#[test]
+fn an_ask_call_waits_is_listed_and_runs_when_approved() {
+    let rig = Rig::new("pending-approve");
+    ask_notes(&rig, "");
+    let rt = Recording::answering("the notes");
+    let broker = rig.broker(&rt);
+    let before = Timestamp::now();
+    // Spaced JSON: the table shows the arguments as policy parsed them, not as sent.
+    let (mut stream, approval, expires) = start(&broker, r#"{ "path" : "/n/a" }"#);
+    assert_eq!(approval, 0, "the approval id is the decision's seq");
+    let items = broker.table().list();
+    assert_eq!(items.len(), 1);
+    let item = &items[0];
+    assert_eq!(item.approval, 0);
+    assert_eq!((item.session.as_str(), item.call.0), ("s1", 9));
+    assert_eq!(item.tool, "read_file");
+    assert_eq!(item.arguments, r#"{"path":"/n/a"}"#);
+    assert_eq!(
+        (item.grant.as_str(), item.taint),
+        ("notes", DataClass::Private)
+    );
+    assert!(item.created >= before);
+    assert_eq!(item.expires, expires);
+    assert_eq!(
+        expires.unix_millis() - item.created.unix_millis(),
+        900_000,
+        "now plus ttl_ms"
+    );
+    assert_eq!(rt.count(), 0, "nothing runs while it waits");
+
+    assert_eq!(
+        answer_by_hand(&broker, 0, approved()),
+        DecisionRecord::Ask {}
+    );
+    assert!(matches!(
+        final_answer(&mut stream),
+        ToolResponse::Result { .. }
+    ));
+    assert_eq!(rt.count(), 1);
+    let events = rig.events();
+    assert!(
+        matches!(
+            events.as_slice(),
+            [
+                AuditEvent::Decision { .. },
+                AuditEvent::Approval { decision: 0, .. },
+                AuditEvent::Result { decision: 0, .. }
+            ]
+        ),
+        "{events:?}"
+    );
+}
+
+#[test]
+fn the_expiry_is_the_grants_when_that_is_earlier() {
+    let rig = Rig::new("pending-grant-expiry");
+    let soon = Timestamp::from_unix_millis(Timestamp::now().unix_millis() + 60_000).unwrap();
+    ask_notes(&rig, &format!("expires = \"{}\"", soon.to_rfc3339()));
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (_stream, _, expires) = start(&broker, r#"{"path":"/n/a"}"#);
+    assert_eq!(expires, soon);
+}
+
+#[test]
+fn a_denied_verdict_reaches_the_requester_and_nothing_runs() {
+    let rig = Rig::new("pending-refuse");
+    ask_notes(&rig, "");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, approval, _) = start(&broker, r#"{"path":"/n/a"}"#);
+    let refused = Answer::Refused {
+        by: Some("bxctl".to_string()),
+        post: None,
+        reason: None,
+    };
+    answer_by_hand(&broker, approval, refused);
+    assert_eq!(
+        final_answer(&mut stream),
+        ToolResponse::Denied {
+            reason: DenyReason::ApprovalRefused,
+            detail: None,
+        }
+    );
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn a_requester_that_leaves_takes_its_own_entry_and_nothing_is_written() {
+    let rig = Rig::new("pending-leave");
+    ask_notes(&rig, "");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (stream, _, _) = start(&broker, r#"{"path":"/n/a"}"#);
+    drop(stream);
+    // The waiting thread looks at its connection every second.
+    eventually("the entry is removed", || broker.table().list().is_empty());
+    std::thread::sleep(Duration::from_millis(200));
+    assert_eq!(rig.events().len(), 1, "only the decision");
+    let report = brokerd::audit::verify_dir(&rig.cfg.audit_dir()).unwrap();
+    assert_eq!(report.abandoned, [0]);
+    assert!(report.failure.is_none());
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn a_requester_gone_at_the_last_look_runs_nothing_and_closes_the_call() {
+    let rig = Rig::new("pending-lastlook");
+    ask_notes(&rig, "");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (stream, approval, _) = start(&broker, r#"{"path":"/n/a"}"#);
+    // Take the entry first, so the waiting thread cannot take it back when it sees the
+    // connection gone; then approve.
+    let entry = broker.table().take(approval).unwrap();
+    drop(stream);
+    answer_entry(&broker, entry, approved());
+    eventually("the call is closed", || rig.events().len() == 3);
+    match &rig.events()[2] {
+        AuditEvent::Result {
+            status,
+            decision,
+            sha256,
+            bytes,
+            ..
+        } => {
+            assert_eq!((*status, *decision), (ResultStatus::Failed, 0));
+            assert_eq!(*sha256, bxcore::sha256(GONE.as_bytes()).unwrap());
+            assert_eq!(*bytes, GONE.len() as u64);
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(GONE, "the requester went away");
+    assert_eq!(rt.count(), 0);
+    let report = brokerd::audit::verify_dir(&rig.cfg.audit_dir()).unwrap();
+    assert!(report.unfinished.is_empty() && report.abandoned.is_empty());
+}
@@ -0,0 +1,449 @@
+//! The courier carries a pending approval to `gatewayd` and its answer back, against a fake
+//! `gatewayd` on a real socket (M4b spec, section 5). Do not edit.
+
+#[path = "support/client.rs"]
+mod client;
+#[path = "support/fake_gateway.rs"]
+mod fake_gateway;
+#[path = "support/rig.rs"]
+mod rig;
+#[path = "support/runtime.rs"]
+mod runtime;
+#[path = "support/sink.rs"]
+mod sink;
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::os::unix::net::UnixStream;
+use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
+use std::time::{Duration, Instant};
+
+use brokerd::broker::{self, Broker};
+use brokerd::config::Gateway;
+use brokerd::courier::{self, Timing};
+use brokerd::ledger::Answer;
+use bxcore::{
+    ApprovalAnswer, ApprovalReply, AuditEvent, DenyReason, GatewayOutcome, Message, Timestamp,
+    ToolResponse,
+};
+use client::{Serve, next, open};
+use fake_gateway::{FakeGateway, KYLE, POST};
+use rig::{Rig, grant_text, request};
+use runtime::Recording;
+
+const WAIT: Duration = Duration::from_secs(5);
+
+/// A rig whose `[gateway]` is a fake gatewayd in its home, and an `ask` grant for /n.
+fn rig_with_gateway(tag: &str) -> (Rig, FakeGateway) {
+    let mut rig = Rig::new(tag);
+    let socket = rig.dir.path().join("gw.sock");
+    let fake = FakeGateway::start(&socket);
+    rig.cfg.gateway = Some(Gateway { socket });
+    rig.grant(
+        "notes",
+        &grant_text("read_file", "ask", "", "paths = [\"/n\"]"),
+    );
+    (rig, fake)
+}
+
+/// A call that waits for approval; its connection and the approval id.
+fn ask(broker: &Arc<Broker>) -> (UnixStream, u64) {
+    let mut stream = open(
+        broker,
+        broker::handle,
+        9,
+        Message::ToolRequest(request("s1", 9, "read_file", r#"{"path":"/n/a"}"#)),
+    );
+    match next(&mut stream).msg {
+        Message::ToolResponse(ToolResponse::PendingApproval { approval, .. }) => (stream, approval),
+        other => panic!("{other:?}"),
+    }
+}
+
+fn final_answer(stream: &mut UnixStream) -> ToolResponse {
+    let env = next(stream);
+    assert!(env.r#final, "{env:?}");
+    match env.msg {
+        Message::ToolResponse(r) => r,
+        other => panic!("{other:?}"),
+    }
+}
+
+/// The `Approval` record's (answer, by, post, reason).
+fn approval_record(
+    rig: &Rig,
+) -> (
+    ApprovalAnswer,
+    Option<String>,
+    Option<String>,
+    Option<String>,
+) {
+    let events = rig.events();
+    let found = events.iter().find_map(|e| match e {
+        AuditEvent::Approval {
+            answer,
+            by,
+            post,
+            reason,
+            ..
+        } => Some((*answer, by.clone(), post.clone(), reason.clone())),
+        _ => None,
+    });
+    found.expect("an Approval record")
+}
+
+fn some(s: &str) -> Option<String> {
+    Some(s.to_string())
+}
+
+/// Sets the courier's stop flag when dropped, so a failing assertion ends the courier too.
+struct StopOnDrop<'a>(&'a AtomicBool);
+
+impl Drop for StopOnDrop<'_> {
+    fn drop(&mut self) {
+        self.0.store(true, std::sync::atomic::Ordering::SeqCst);
+    }
+}
+
+/// The courier must end by itself within five seconds.
+fn ends_by_itself<T>(courier: &std::thread::ScopedJoinHandle<'_, T>) {
+    let until = Instant::now() + WAIT;
+    while !courier.is_finished() {
+        assert!(Instant::now() < until, "the courier did not stop");
+        std::thread::sleep(Duration::from_millis(20));
+    }
+}
+
+#[test]
+fn approved_in_mattermost_it_runs_and_the_record_names_who_and_where() {
+    let (rig, fake) = rig_with_gateway("courier-approve");
+    let rt = Recording::answering("the notes");
+    let broker = rig.broker(&rt);
+    let (mut stream, id) = ask(&broker);
+    let mut conn = fake.next(WAIT).expect("the approval reaches gatewayd");
+    assert_eq!(
+        (conn.request.approval, conn.request.tool.as_str()),
+        (id, "read_file")
+    );
+    assert_eq!(conn.request.arguments, r#"{"path":"/n/a"}"#);
+    conn.posted();
+    conn.approve();
+    assert_eq!(conn.outcome(), Some(GatewayOutcome::Runs {}));
+    assert!(matches!(
+        final_answer(&mut stream),
+        ToolResponse::Result { .. }
+    ));
+    assert_eq!(rt.count(), 1);
+    assert_eq!(
+        approval_record(&rig),
+        (ApprovalAnswer::Approved, some(KYLE), some(POST), None)
+    );
+}
+
+#[test]
+fn refused_in_mattermost_it_is_denied_with_the_reason() {
+    let (rig, fake) = rig_with_gateway("courier-refuse");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, _) = ask(&broker);
+    let mut conn = fake.next(WAIT).unwrap();
+    conn.posted();
+    conn.refuse("not on a Friday");
+    assert_eq!(conn.outcome(), Some(GatewayOutcome::Refused {}));
+    assert_eq!(
+        final_answer(&mut stream),
+        ToolResponse::Denied {
+            reason: DenyReason::ApprovalRefused,
+            detail: None,
+        }
+    );
+    assert_eq!(rt.count(), 0);
+    assert_eq!(
+        approval_record(&rig),
+        (
+            ApprovalAnswer::Refused,
+            some(KYLE),
+            some(POST),
+            some("not on a Friday")
+        )
+    );
+}
+
+#[test]
+fn approved_after_the_grant_is_gone_it_is_denied_on_the_recheck() {
+    let (rig, fake) = rig_with_gateway("courier-recheck");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, _) = ask(&broker);
+    let mut conn = fake.next(WAIT).unwrap();
+    conn.posted();
+    rig.remove_grant("notes");
+    conn.approve();
+    assert_eq!(
+        conn.outcome(),
+        Some(GatewayOutcome::Denied {
+            reason: DenyReason::NoGrant
+        })
+    );
+    assert_eq!(
+        final_answer(&mut stream),
+        ToolResponse::Denied {
+            reason: DenyReason::NoGrant,
+            detail: None,
+        }
+    );
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn answered_in_bxctl_the_courier_closes_without_an_outcome() {
+    let (rig, fake) = rig_with_gateway("courier-bxctl");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, id) = ask(&broker);
+    let mut conn = fake.next(WAIT).unwrap();
+    conn.posted();
+    let entry = broker.table().take(id).unwrap();
+    brokerd::admin::answer(
+        &broker,
+        entry,
+        Answer::Approved {
+            by: some("bxctl"),
+            post: None,
+        },
+        Timestamp::now(),
+    );
+    assert!(
+        conn.closed_within(WAIT),
+        "the courier closes, and sends nothing"
+    );
+    assert!(matches!(
+        final_answer(&mut stream),
+        ToolResponse::Result { .. }
+    ));
+    assert_eq!(approval_record(&rig).1, some("bxctl"));
+}
+
+#[test]
+fn an_answer_after_it_ended_is_gone() {
+    let rig = Rig::new("courier-gone");
+    rig.grant(
+        "notes",
+        &grant_text("read_file", "ask", "", "paths = [\"/n\"]"),
+    );
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (_stream, id) = ask(&broker);
+    // The courier is run by hand here, so that the answer can arrive after the entry is taken.
+    let socket = rig.dir.path().join("gw.sock");
+    let fake = FakeGateway::start(&socket);
+    let info = broker.table().list().remove(0);
+    let stop = AtomicBool::new(false);
+    std::thread::scope(|scope| {
+        let courier = scope.spawn(|| {
+            courier::carry(
+                &broker,
+                &socket,
+                &info,
+                &stop,
+                Timing {
+                    retry: Duration::from_millis(100),
+                    tick: Duration::from_secs(5),
+                },
+            )
+        });
+        let _guard = StopOnDrop(&stop);
+        let mut conn = fake.next(WAIT).unwrap();
+        conn.posted();
+        let entry = broker.table().take(id).unwrap();
+        brokerd::admin::answer(&broker, entry, Answer::Expired, Timestamp::now());
+        conn.approve();
+        assert_eq!(conn.outcome(), Some(GatewayOutcome::Gone {}));
+        ends_by_itself(&courier);
+    });
+}
+
+#[test]
+fn if_gatewayd_goes_away_the_approval_is_posted_again() {
+    let (rig, fake) = rig_with_gateway("courier-again");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, id) = ask(&broker);
+    let mut first = fake.next(WAIT).unwrap();
+    first.posted();
+    first.close();
+    let mut second = fake.next(Duration::from_secs(10)).expect("posted again");
+    assert_eq!(second.request.approval, id);
+    second.posted();
+    second.approve();
+    assert_eq!(second.outcome(), Some(GatewayOutcome::Runs {}));
+    assert!(matches!(
+        final_answer(&mut stream),
+        ToolResponse::Result { .. }
+    ));
+    assert!(
+        rig.lines.all().iter().any(|l| l.contains(&format!(
+            "approval {id}: gatewayd went away; posting it again"
+        ))),
+        "{:?}",
+        rig.lines.all()
+    );
+}
+
+#[test]
+fn a_bad_answer_answers_nothing() {
+    let (rig, fake) = rig_with_gateway("courier-bad");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (_stream, id) = ask(&broker);
+    for bad in [
+        ApprovalReply {
+            approval: id + 1,
+            approved: true,
+            by: KYLE.to_string(),
+            post: POST.to_string(),
+            reason: None,
+        },
+        ApprovalReply {
+            approval: id,
+            approved: true,
+            by: "kyle".to_string(),
+            post: POST.to_string(),
+            reason: None,
+        },
+        ApprovalReply {
+            approval: id,
+            approved: false,
+            by: KYLE.to_string(),
+            post: POST.to_string(),
+            reason: Some("x".repeat(501)),
+        },
+    ] {
+        let mut conn = fake.next(Duration::from_secs(10)).unwrap();
+        conn.posted();
+        conn.reply(bad);
+        assert_eq!(conn.outcome(), None, "no outcome for a bad answer");
+        assert!(broker.table().contains(id), "still pending");
+    }
+    assert_eq!(rt.count(), 0);
+}
+
+#[test]
+fn unreachable_at_first_then_posted_and_said_once() {
+    let rig = Rig::new("courier-late");
+    rig.grant(
+        "notes",
+        &grant_text("read_file", "ask", "", "paths = [\"/n\"]"),
+    );
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (_stream, id) = ask(&broker);
+    let socket = rig.dir.path().join("gw.sock");
+    let info = broker.table().list().remove(0);
+    let stop = AtomicBool::new(false);
+    let timing = Timing {
+        retry: Duration::from_millis(100),
+        tick: Duration::from_millis(20),
+    };
+    std::thread::scope(|scope| {
+        let courier = scope.spawn(|| courier::carry(&broker, &socket, &info, &stop, timing));
+        let _guard = StopOnDrop(&stop);
+        std::thread::sleep(Duration::from_millis(500));
+        let fake = FakeGateway::start(&socket);
+        let mut conn = fake.next(WAIT).unwrap();
+        conn.posted();
+        let until = Instant::now() + WAIT;
+        while !rig
+            .lines
+            .all()
+            .iter()
+            .any(|l| l.contains("posted in Mattermost"))
+        {
+            assert!(Instant::now() < until, "{:?}", rig.lines.all());
+            std::thread::sleep(Duration::from_millis(20));
+        }
+        broker.table().take(id);
+        ends_by_itself(&courier);
+    });
+    let lines = rig.lines.all();
+    let unreachable: Vec<&String> = lines
+        .iter()
+        .filter(|l| l.contains("gatewayd is not reachable"))
+        .collect();
+    assert_eq!(
+        unreachable.len(),
+        1,
+        "said once, not every retry: {lines:?}"
+    );
+    assert!(
+        unreachable[0].starts_with(&format!(
+            "brokerd: approval {id}: gatewayd is not reachable at "
+        )),
+        "{lines:?}"
+    );
+    assert!(
+        unreachable[0]
+            .ends_with("; bxctl can still answer it\nsee docs/runbook.md#gateway-unreachable"),
+        "{lines:?}"
+    );
+    assert!(
+        lines.contains(&format!("brokerd: approval {id} posted in Mattermost")),
+        "{lines:?}"
+    );
+}
+
+#[test]
+fn if_gatewayd_cannot_post_it_is_tried_again() {
+    let (rig, fake) = rig_with_gateway("courier-cannot");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, _) = ask(&broker);
+    fake.next(WAIT).unwrap().cannot_post();
+    let mut conn = fake.next(Duration::from_secs(10)).expect("tried again");
+    conn.posted();
+    conn.approve();
+    assert_eq!(conn.outcome(), Some(GatewayOutcome::Runs {}));
+    assert!(matches!(
+        final_answer(&mut stream),
+        ToolResponse::Result { .. }
+    ));
+    assert!(
+        rig.lines
+            .all()
+            .iter()
+            .any(|l| l.contains("gatewayd could not post it: Mattermost is down"))
+    );
+}
+
+#[test]
+fn without_a_gateway_nothing_is_carried() {
+    let rig = Rig::new("courier-none");
+    rig.grant(
+        "notes",
+        &grant_text("read_file", "ask", "", "paths = [\"/n\"]"),
+    );
+    let socket = rig.dir.path().join("gw.sock");
+    let fake = FakeGateway::start(&socket);
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let (mut stream, id) = ask(&broker);
+    assert!(
+        fake.next(Duration::from_millis(500)).is_none(),
+        "no [gateway]: no connection"
+    );
+    let entry = broker.table().take(id).unwrap();
+    brokerd::admin::answer(
+        &broker,
+        entry,
+        Answer::Approved {
+            by: some("bxctl"),
+            post: None,
+        },
+        Timestamp::now(),
+    );
+    assert!(matches!(
+        final_answer(&mut stream),
+        ToolResponse::Result { .. }
+    ));
+}
@@ -0,0 +1,258 @@
+//! An invalid call is denied with the parser's reason: `ArgsError::detail` makes it one short
+//! line, `policy::decide` keeps it, and `broker.sock` sends it. The audit record is unchanged.
+//! Do not edit.
+
+#[path = "support/build.rs"]
+mod build;
+#[path = "support/client.rs"]
+mod client;
+#[path = "support/rig.rs"]
+mod rig;
+#[path = "support/runtime.rs"]
+mod runtime;
+#[path = "support/sink.rs"]
+mod sink;
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use brokerd::args::{ArgsError, MAX_DETAIL};
+use brokerd::policy::decide;
+use build::{denied, fetch, grant, now, private, read, set, shell};
+use bxcore::{AuditEvent, DecisionRecord, DenyReason, Envelope, Message, Mode, ToolResponse};
+use client::{Serve, call};
+use rig::{Rig, grant_text};
+use runtime::Recording;
+
+#[test]
+fn the_detail_is_the_error_text() {
+    assert_eq!(MAX_DETAIL, 300);
+    let cases = [
+        (
+            ArgsError::Shape("missing field `path`".to_string()),
+            "invalid argument shape: missing field `path`",
+        ),
+        (
+            ArgsError::Path("notes/a.md".to_string()),
+            "invalid path: notes/a.md",
+        ),
+        (
+            ArgsError::Url("http://example.com/".to_string()),
+            "invalid url: http://example.com/",
+        ),
+    ];
+    for (error, want) in cases {
+        assert_eq!(error.detail(), want);
+        assert_eq!(
+            error.detail(),
+            error.to_string(),
+            "a short clean text is kept as it is"
+        );
+    }
+}
+
+/// Every control character, and U+2028 and U+2029 (the line and paragraph separators), becomes
+/// one space each; other characters, spaces and non-ASCII letters included, are kept.
+#[test]
+fn the_detail_is_one_line() {
+    let error = ArgsError::Path(
+        "a\nb\r\nc\td\u{7}e\u{1b}[31mf\u{7f}g\u{85}h\u{2028}i\u{2029}j k \u{e9}\u{65e5}"
+            .to_string(),
+    );
+    assert_eq!(
+        error.detail(),
+        "invalid path: a b  c d e [31mf g h i j k \u{e9}\u{65e5}"
+    );
+    let error = ArgsError::Shape("line one\nline two\n".to_string());
+    assert_eq!(error.detail(), "invalid argument shape: line one line two ");
+}
+
+/// At most `MAX_DETAIL` characters, counted as characters, not bytes: a cut never splits one.
+#[test]
+fn the_detail_is_cut_to_300_characters() {
+    // "invalid path: " is 14 characters.
+    let exact = ArgsError::Path("a".repeat(286));
+    assert_eq!(
+        exact.detail(),
+        exact.to_string(),
+        "300 characters are kept whole"
+    );
+    let over = ArgsError::Path("a".repeat(287));
+    assert_eq!(over.detail(), format!("invalid path: {}", "a".repeat(286)));
+
+    let wide = ArgsError::Path("\u{e9}".repeat(1000));
+    let detail = wide.detail();
+    assert_eq!(detail.chars().count(), 300);
+    assert_eq!(detail, format!("invalid path: {}", "\u{e9}".repeat(286)));
+
+    // Newlines past the cut do not matter; newlines before it are spaces.
+    let long = ArgsError::Shape(format!("{}\n{}", "x".repeat(10), "y\n".repeat(500)));
+    let detail = long.detail();
+    assert_eq!(detail.chars().count(), 300);
+    assert!(!detail.contains('\n'), "{detail:?}");
+    assert!(detail.starts_with("invalid argument shape: xxxxxxxxxx y y"));
+}
+
+/// Each kind of `ArgsError`, from the model's own argument text, reaches the denial as its detail,
+/// with no grant and with a grant that would cover the call: arguments are checked first. (The
+/// memory tools take no grant: their policy is built in.)
+#[test]
+fn each_kind_of_invalid_arguments_carries_its_detail() {
+    let covering = set(vec![
+        grant("r", "read_file", Mode::Auto).paths(&["/n"]),
+        grant("s", "shell", Mode::Auto),
+        grant("f", "http_fetch", Mode::Auto).hosts(&["example.com"]),
+    ]);
+    let none = set(vec![]);
+    let cases = [
+        (
+            build::request(
+                "memory",
+                r#"{"action":"note","file":"notes.md","text":"x"}"#,
+            ),
+            "invalid argument shape: unknown field `file`, expected `text`",
+        ),
+        (
+            build::request("read_file", r#"{"path":"/n/a","file":"x"}"#),
+            "invalid argument shape: unknown field `file`, expected `path` at line 1 column 21",
+        ),
+        (
+            build::request("read_file", "not json"),
+            "invalid argument shape: expected ident at line 1 column 2",
+        ),
+        (
+            build::request("read_file", "{}"),
+            "invalid argument shape: missing field `path` at line 1 column 2",
+        ),
+        (
+            build::request("recall", r#"{"query":""}"#),
+            "invalid argument shape: query must have 1 to 500 characters",
+        ),
+        (read("n/a"), "invalid path: n/a"),
+        (shell(Some("a\\nb")), "invalid path: a b"),
+        (
+            fetch("http://example.com/"),
+            "invalid url: http://example.com/",
+        ),
+    ];
+    for grants in [&none, &covering] {
+        for (req, want) in &cases {
+            let text = req.arguments.clone();
+            let denial = denied(decide(req.clone(), grants, private(), now()));
+            assert_eq!(denial.reason, DenyReason::InvalidArguments, "{text}");
+            assert_eq!(denial.detail.as_deref(), Some(*want), "{text}");
+            assert_eq!(denial.grant, None, "{text}");
+        }
+    }
+}
+
+#[test]
+fn a_long_invalid_path_is_cut_in_the_denial() {
+    let path = format!("/{}", "a".repeat(5000));
+    let denial = denied(decide(read(&path), &set(vec![]), private(), now()));
+    assert_eq!(denial.reason, DenyReason::InvalidArguments);
+    let detail = denial.detail.unwrap();
+    assert_eq!(detail.chars().count(), 300);
+    assert_eq!(detail, format!("invalid path: /{}", "a".repeat(285)));
+}
+
+/// Only an invalid-arguments denial has a detail.
+#[test]
+fn other_denials_have_no_detail() {
+    let grants = set(vec![
+        grant("no", "read_file", Mode::Deny).paths(&["/n"]),
+        grant("old", "shell", Mode::Auto).expires("2026-09-01T00:00:00.000Z"),
+    ]);
+    let cases = [
+        (build::request("echo", "not json"), DenyReason::NoGrant),
+        (fetch("https://example.com/"), DenyReason::NoGrant),
+        (read("/n/a"), DenyReason::DeniedByGrant),
+        (shell(None), DenyReason::GrantExpired),
+    ];
+    for (req, reason) in cases {
+        let text = req.arguments.clone();
+        let denial = denied(decide(req, &grants, private(), now()));
+        assert_eq!(denial.reason, reason, "{text}");
+        assert_eq!(denial.detail, None, "{text}");
+    }
+}
+
+fn only(frames: &[Envelope]) -> &ToolResponse {
+    assert_eq!(frames.len(), 1, "{frames:?}");
+    assert!(frames[0].r#final);
+    match &frames[0].msg {
+        Message::ToolResponse(r) => r,
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn broker_sock_sends_the_detail_and_the_audit_record_does_not_hold_it() {
+    let rig = Rig::new("broker-invalid-detail");
+    rig.grant(
+        "notes",
+        &grant_text("read_file", "auto", "", "paths = [\"/n\"]"),
+    );
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let arguments = r#"{"path":"/n/a","file":"x"}"#;
+    let frames = call(&broker, rig::request("s1", 5, "read_file", arguments));
+    let detail =
+        "invalid argument shape: unknown field `file`, expected `path` at line 1 column 21";
+    assert_eq!(
+        only(&frames),
+        &ToolResponse::Denied {
+            reason: DenyReason::InvalidArguments,
+            detail: Some(detail.to_string()),
+        }
+    );
+    assert_eq!(rt.count(), 0, "nothing ran");
+    match rig.events().as_slice() {
+        [
+            AuditEvent::Decision {
+                outcome,
+                arguments: recorded,
+                ..
+            },
+        ] => {
+            assert_eq!(
+                *outcome,
+                DecisionRecord::Denied {
+                    reason: DenyReason::InvalidArguments
+                }
+            );
+            assert_eq!(recorded, arguments, "the record keeps the arguments");
+        }
+        other => panic!("{other:?}"),
+    }
+    let audit = rig.cfg.audit_dir();
+    let mut logs = 0;
+    for entry in std::fs::read_dir(&audit).unwrap() {
+        let path = entry.unwrap().path();
+        if path.extension().is_none_or(|e| e != "jsonl") {
+            continue;
+        }
+        logs += 1;
+        let text = std::fs::read_to_string(&path).unwrap();
+        assert!(!text.contains("unknown field"), "{text}");
+        assert!(!text.contains("\"detail\""), "{text}");
+    }
+    assert_eq!(logs, 1, "one audit log file");
+}
+
+#[test]
+fn a_call_no_grant_allows_is_denied_on_broker_sock_without_a_detail() {
+    let rig = Rig::new("broker-no-detail");
+    let rt = Recording::answering("x");
+    let broker = rig.broker(&rt);
+    let frames = call(
+        &broker,
+        rig::request("s1", 6, "read_file", r#"{"path":"/n/a"}"#),
+    );
+    assert_eq!(
+        only(&frames),
+        &ToolResponse::Denied {
+            reason: DenyReason::NoGrant,
+            detail: None,
+        }
+    );
+}
@@ -0,0 +1,425 @@
+//! The ledger's first and third steps: decide and record; raise the state and record the result.
+//! And what happens after a failed append or a panic. Do not edit.
+
+#[path = "support/build.rs"]
+mod build;
+#[path = "support/rig.rs"]
+mod rig;
+#[path = "support/sink.rs"]
+mod sink;
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::os::unix::fs::PermissionsExt;
+use std::panic::AssertUnwindSafe;
+
+use brokerd::ledger::{Call, Decided, Grants, NOT_RECORDED, POISONED, STOPPED};
+use brokerd::policy::{Denial, Label, SessionState};
+use build::{grant, now, read, set};
+use bxcore::{
+    AuditEvent, CallId, DataClass, DecisionRecord, DenyReason, GrantProblem, Mode, ResultStatus,
+    SessionId, ToolResponse,
+};
+use rig::Rig;
+
+fn one(grants: Vec<build::Build>) -> Grants {
+    Ok(set(grants))
+}
+
+fn notes(mode: Mode) -> Grants {
+    one(vec![grant("n", "read_file", mode).paths(&["/n"])])
+}
+
+fn call(label: Label) -> Call {
+    Call {
+        session: SessionId::new("s1").unwrap(),
+        call: CallId(1),
+        decision: 0,
+        label,
+    }
+}
+
+fn secret() -> Label {
+    Label {
+        class: DataClass::Secret,
+        untrusted: true,
+    }
+}
+
+fn result(content: &str, label: Label) -> ToolResponse {
+    ToolResponse::Result {
+        content: content.to_string(),
+        class: label.class,
+        untrusted: label.untrusted,
+        truncated: false,
+    }
+}
+
+#[test]
+fn an_allowed_call_is_recorded_in_full() {
+    let rig = Rig::new("ledger-allowed");
+    let ledger = rig.ledger();
+    let request = read("/n/a");
+    let arguments = request.arguments.clone();
+    match ledger.decide(request, &notes(Mode::Auto), now()) {
+        Decided::Allowed { decision, seq } => {
+            assert_eq!(seq, 0);
+            assert_eq!(decision.grant(), "n");
+        }
+        other => panic!("{other:?}"),
+    }
+    let records = rig.records();
+    assert_eq!(records.len(), 1);
+    assert_eq!(records[0].time, now());
+    assert_eq!(
+        records[0].event,
+        AuditEvent::Decision {
+            session: SessionId::new("s1").unwrap(),
+            call: CallId(1),
+            tool: "read_file".to_string(),
+            arguments,
+            outcome: DecisionRecord::Allowed {},
+            grant: Some("n".to_string()),
+            grant_sha256: Some(bxcore::sha256(b"n").unwrap()),
+            taint: DataClass::Private,
+            untrusted: false,
+        }
+    );
+}
+
+#[test]
+fn an_ask_is_recorded_as_ask_with_the_state_it_was_decided_at() {
+    let rig = Rig::new("ledger-ask");
+    let ledger = rig.ledger();
+    match ledger.decide(read("/n/a"), &notes(Mode::Ask), now()) {
+        Decided::Ask { ask, seq, state } => {
+            assert_eq!((seq, ask.grant()), (0, "n"));
+            assert_eq!(state, SessionState::default());
+        }
+        other => panic!("{other:?}"),
+    }
+    match &rig.events()[0] {
+        AuditEvent::Decision { outcome, grant, .. } => {
+            assert_eq!(*outcome, DecisionRecord::Ask {});
+            assert_eq!(grant.as_deref(), Some("n"));
+        }
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn a_denial_names_a_grant_only_when_a_grant_denied() {
+    let rig = Rig::new("ledger-denied");
+    let ledger = rig.ledger();
+    let none = ledger.decide(read("/n/a"), &one(Vec::new()), now());
+    assert!(
+        matches!(
+            none,
+            Decided::Denied(Denial {
+                reason: DenyReason::NoGrant,
+                ..
+            })
+        ),
+        "{none:?}"
+    );
+    let deny = one(vec![grant("d", "read_file", Mode::Deny).paths(&["/n"])]);
+    let by = ledger.decide(read("/n/a"), &deny, now());
+    assert!(
+        matches!(
+            by,
+            Decided::Denied(Denial {
+                reason: DenyReason::DeniedByGrant,
+                ..
+            })
+        ),
+        "{by:?}"
+    );
+    let events = rig.events();
+    let named: Vec<(DecisionRecord, Option<String>)> = events
+        .iter()
+        .map(|e| match e {
+            AuditEvent::Decision { outcome, grant, .. } => (outcome.clone(), grant.clone()),
+            other => panic!("{other:?}"),
+        })
+        .collect();
+    assert_eq!(
+        named,
+        [
+            (
+                DecisionRecord::Denied {
+                    reason: DenyReason::NoGrant
+                },
+                None
+            ),
+            (
+                DecisionRecord::Denied {
+                    reason: DenyReason::DeniedByGrant
+                },
+                Some("d".to_string())
+            ),
+        ]
+    );
+}
+
+#[test]
+fn invalid_grants_come_first_then_an_unreadable_state() {
+    let rig = Rig::new("ledger-order");
+    let ledger = rig.ledger();
+    std::fs::create_dir_all(rig.cfg.state_dir()).unwrap();
+    std::fs::write(rig.state_file("s1"), "not json").unwrap();
+    let invalid: Grants = Err(vec![GrantProblem {
+        file: "x.toml".to_string(),
+        line: None,
+        problem: "bad".to_string(),
+    }]);
+    let first = ledger.decide(read("/n/a"), &invalid, now());
+    assert!(
+        matches!(
+            first,
+            Decided::Denied(Denial {
+                reason: DenyReason::GrantsInvalid,
+                ..
+            })
+        ),
+        "{first:?}"
+    );
+    let second = ledger.decide(read("/n/a"), &notes(Mode::Auto), now());
+    assert!(
+        matches!(
+            second,
+            Decided::Denied(Denial {
+                reason: DenyReason::StateUnreadable,
+                ..
+            })
+        ),
+        "{second:?}"
+    );
+    // A state brokerd cannot read is recorded as the most sensitive one.
+    for event in rig.events() {
+        match event {
+            AuditEvent::Decision {
+                taint, untrusted, ..
+            } => assert_eq!((taint, untrusted), (DataClass::Secret, true)),
+            other => panic!("{other:?}"),
+        }
+    }
+    let damaged = rig.lines.with("see docs/runbook.md#broker-state-damaged");
+    assert!(!damaged.is_empty(), "{:?}", rig.lines.all());
+}
+
+#[test]
+fn a_failed_append_denies_this_call_and_every_later_one() {
+    let rig = Rig::new("ledger-stop");
+    let ledger = rig.ledger();
+    rig.switch.fail(true);
+    let first = ledger.decide(read("/n/a"), &notes(Mode::Auto), now());
+    assert!(
+        matches!(
+            first,
+            Decided::Denied(Denial {
+                reason: DenyReason::AuditUnavailable,
+                ..
+            })
+        ),
+        "{first:?}"
+    );
+    assert_eq!(rig.switch.attempts(), 1);
+    // The sink would work again; the ledger does not try it.
+    rig.switch.fail(false);
+    let later = ledger.decide(read("/n/a"), &notes(Mode::Auto), now());
+    assert!(
+        matches!(
+            later,
+            Decided::Denied(Denial {
+                reason: DenyReason::AuditUnavailable,
+                ..
+            })
+        ),
+        "{later:?}"
+    );
+    let finished = ledger.finish(&call(secret()), result("x", secret()), now());
+    assert_eq!(
+        finished,
+        ToolResponse::Failed {
+            message: NOT_RECORDED.to_string()
+        }
+    );
+    assert_eq!(rig.switch.attempts(), 1, "nothing more was written");
+    assert!(rig.records().is_empty());
+    assert!(!rig.state_file("s1").exists(), "the state was not raised");
+    assert!(!rig.lines.with(STOPPED).is_empty());
+    assert!(
+        rig.lines
+            .all()
+            .iter()
+            .all(|l| !l.contains("runbook") || l.ends_with("see docs/runbook.md#audit-unavailable")),
+        "{:?}",
+        rig.lines.all()
+    );
+}
+
+#[test]
+fn a_panic_while_holding_the_ledger_denies_every_later_call() {
+    let rig = Rig::new("ledger-poison");
+    let ledger = rig.ledger();
+    rig.switch.panic_next();
+    let panicked = std::panic::catch_unwind(AssertUnwindSafe(|| {
+        ledger.decide(read("/n/a"), &notes(Mode::Auto), now())
+    }));
+    assert!(panicked.is_err());
+    let later = ledger.decide(read("/n/a"), &notes(Mode::Auto), now());
+    assert!(
+        matches!(
+            later,
+            Decided::Denied(Denial {
+                reason: DenyReason::AuditUnavailable,
+                ..
+            })
+        ),
+        "{later:?}"
+    );
+    assert_eq!(rig.switch.attempts(), 1);
+    assert!(
+        !rig.lines.with(POISONED).is_empty(),
+        "{:?}",
+        rig.lines.all()
+    );
+    assert!(POISONED.ends_with("see docs/runbook.md#audit-unavailable"));
+}
+
+#[test]
+fn a_result_raises_the_state_then_is_recorded() {
+    let rig = Rig::new("ledger-result");
+    let ledger = rig.ledger();
+    let answer = ledger.finish(&call(secret()), result("key", secret()), now());
+    assert_eq!(answer, result("key", secret()), "passed on unchanged");
+    let state = rig.state().read(&SessionId::new("s1").unwrap()).unwrap();
+    assert_eq!(
+        state,
+        SessionState {
+            taint: DataClass::Secret,
+            untrusted: true
+        }
+    );
+    assert_eq!(
+        rig.events(),
+        [AuditEvent::Result {
+            session: SessionId::new("s1").unwrap(),
+            call: CallId(1),
+            decision: 0,
+            status: ResultStatus::Result,
+            class: DataClass::Secret,
+            untrusted: true,
+            truncated: false,
+            bytes: 3,
+            sha256: bxcore::sha256(b"key").unwrap(),
+            taint_after: DataClass::Secret,
+        }]
+    );
+}
+
+#[test]
+fn taint_never_goes_down() {
+    let rig = Rig::new("ledger-down");
+    let ledger = rig.ledger();
+    let public = Label {
+        class: DataClass::Public,
+        untrusted: false,
+    };
+    ledger.finish(&call(secret()), result("a", secret()), now());
+    ledger.finish(&call(public), result("b", public), now());
+    let after: Vec<DataClass> = rig
+        .events()
+        .iter()
+        .map(|e| match e {
+            AuditEvent::Result { taint_after, .. } => *taint_after,
+            other => panic!("{other:?}"),
+        })
+        .collect();
+    assert_eq!(after, [DataClass::Secret, DataClass::Secret]);
+    let state = rig.state().read(&SessionId::new("s1").unwrap()).unwrap();
+    assert_eq!((state.taint, state.untrusted), (DataClass::Secret, true));
+}
+
+#[test]
+fn a_failure_changes_no_state_and_is_recorded_by_its_message() {
+    let rig = Rig::new("ledger-failed");
+    let ledger = rig.ledger();
+    let failed = ToolResponse::Failed {
+        message: "the tool timed out".to_string(),
+    };
+    assert_eq!(
+        ledger.finish(&call(secret()), failed.clone(), now()),
+        failed
+    );
+    assert!(!rig.state_file("s1").exists());
+    match &rig.events()[0] {
+        AuditEvent::Result {
+            status,
+            class,
+            bytes,
+            sha256,
+            taint_after,
+            ..
+        } => {
+            assert_eq!(*status, ResultStatus::Failed);
+            assert_eq!(
+                *class,
+                DataClass::Secret,
+                "the label the result would have had"
+            );
+            assert_eq!(*bytes, 18);
+            assert_eq!(*sha256, bxcore::sha256(b"the tool timed out").unwrap());
+            assert_eq!(*taint_after, DataClass::Private, "the taint before");
+        }
+        other => panic!("{other:?}"),
+    }
+}
+
+#[test]
+fn a_state_that_cannot_be_written_withholds_the_content() {
+    if tmp::running_as_root("a_state_that_cannot_be_written_withholds_the_content") {
+        return;
+    }
+    let rig = Rig::new("ledger-ro");
+    let ledger = rig.ledger();
+    let dir = rig.cfg.state_dir();
+    std::fs::create_dir_all(&dir).unwrap();
+    std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(0o500)).unwrap();
+    let answer = ledger.finish(&call(secret()), result("key", secret()), now());
+    std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(0o700)).unwrap();
+    assert_eq!(
+        answer,
+        ToolResponse::Failed {
+            message: NOT_RECORDED.to_string()
+        }
+    );
+    assert!(
+        rig.records().is_empty(),
+        "no Result for a state not on disk"
+    );
+    assert!(
+        !rig.lines
+            .with("see docs/runbook.md#broker-state-damaged")
+            .is_empty()
+    );
+}
+
+#[test]
+fn a_result_record_that_cannot_be_written_withholds_the_content() {
+    let rig = Rig::new("ledger-norecord");
+    let ledger = rig.ledger();
+    rig.switch.fail(true);
+    let answer = ledger.finish(&call(secret()), result("key", secret()), now());
+    assert_eq!(
+        answer,
+        ToolResponse::Failed {
+            message: NOT_RECORDED.to_string()
+        }
+    );
+    assert!(
+        !rig.lines
+            .with("see docs/runbook.md#audit-unavailable")
+            .is_empty()
+    );
+}
@@ -0,0 +1,411 @@
+//! `brokerd serve` as a process: its startup, both sockets, and the expiry thread. Do not edit.
+
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use std::os::unix::fs::PermissionsExt;
+use std::os::unix::net::{UnixListener, UnixStream};
+use std::path::{Path, PathBuf};
+use std::process::{Child, Command, Output, Stdio};
+use std::time::{Duration, Instant};
+
+use brokerd::audit::RECOVERED_NOTICE;
+use brokerd::runner::REFUSING;
+use bxcore::{
+    AuditEvent, CallId, DenyReason, Empty, Envelope, ErrorCode, Message, PROTOCOL_VERSION,
+    ResultStatus, SessionId, ToolRequest, ToolResponse,
+};
+use tmp::TempDir;
+
+struct Home {
+    dir: TempDir,
+    config: PathBuf,
+}
+
+impl Home {
+    fn new(tag: &str, ttl_ms: u64) -> Home {
+        let dir = TempDir::new(tag);
+        std::fs::create_dir_all(dir.path().join("grants")).unwrap();
+        let text = format!(
+            "[paths]\nhome = \"{home}\"\ngrants = \"{home}/grants\"\n[approvals]\nttl_ms = {ttl_ms}\n",
+            home = dir.path().display()
+        );
+        let config = dir.write("brokerd.toml", &text);
+        Home { dir, config }
+    }
+
+    fn path(&self, relative: &str) -> PathBuf {
+        self.dir.path().join(relative)
+    }
+
+    fn tools(&self) -> PathBuf {
+        self.path("run/loop-broker/broker.sock")
+    }
+
+    fn admin(&self) -> PathBuf {
+        self.path("run/owner-broker/admin.sock")
+    }
+
+    fn grant(&self, id: &str, mode: &str) {
+        let text = format!(
+            "tool = \"read_file\"\nmode = \"{mode}\"\nmax_taint = \"secret\"\n\
+             result_class = \"private\"\nuntrusted = false\n[constraints]\npaths = [\"/n\"]\n"
+        );
+        std::fs::write(self.path(&format!("grants/{id}.toml")), text).unwrap();
+    }
+
+    fn command(&self, extra: &[&str]) -> Command {
+        let mut command = Command::new(env!("CARGO_BIN_EXE_brokerd"));
+        command
+            .args(["serve", "--config"])
+            .arg(&self.config)
+            .args(extra);
+        command
+    }
+
+    /// Starts `brokerd serve` and waits until both sockets answer.
+    fn serve(&self) -> Running {
+        let child = self
+            .command(&[])
+            .stdout(Stdio::null())
+            .stderr(Stdio::piped())
+            .spawn()
+            .unwrap();
+        let running = Running(Some(child));
+        let until = Instant::now() + Duration::from_secs(10);
+        while UnixStream::connect(self.tools()).is_err()
+            || UnixStream::connect(self.admin()).is_err()
+        {
+            assert!(Instant::now() < until, "brokerd never listened");
+            std::thread::sleep(Duration::from_millis(20));
+        }
+        running
+    }
+
+    /// Runs `brokerd serve` expecting it to exit by itself.
+    fn run(&self, extra: &[&str]) -> Output {
+        self.command(extra).output().unwrap()
+    }
+
+    fn events(&self) -> Vec<AuditEvent> {
+        let dir = self.path("audit");
+        let mut names: Vec<String> = std::fs::read_dir(&dir)
+            .unwrap()
+            .map(|e| e.unwrap().file_name().into_string().unwrap())
+            .filter(|n| n.ends_with(".jsonl"))
+            .collect();
+        names.sort();
+        names
+            .iter()
+            .flat_map(|n| {
+                let text = std::fs::read_to_string(dir.join(n)).unwrap();
+                text.lines()
+                    .map(|l| {
+                        serde_json::from_str::<bxcore::AuditRecord>(l)
+                            .unwrap()
+                            .event
+                    })
+                    .collect::<Vec<_>>()
+            })
+            .collect()
+    }
+}
+
+/// Kills the daemon when dropped; `stop` returns what it printed.
+struct Running(Option<Child>);
+
+impl Running {
+    fn stop(mut self) -> String {
+        let mut child = self.0.take().unwrap();
+        child.kill().unwrap();
+        let output = child.wait_with_output().unwrap();
+        String::from_utf8_lossy(&output.stderr).to_string()
+    }
+}
+
+impl Drop for Running {
+    fn drop(&mut self) {
+        if let Some(child) = &mut self.0 {
+            let _ = child.kill();
+            let _ = child.wait();
+        }
+    }
+}
+
+fn mode(path: &Path) -> u32 {
+    std::fs::metadata(path).unwrap().permissions().mode() & 0o777
+}
+
+fn exchange(socket: &Path, id: u64, msg: Message) -> Vec<Envelope> {
+    let mut stream = UnixStream::connect(socket).unwrap();
+    stream
+        .set_read_timeout(Some(Duration::from_secs(10)))
+        .unwrap();
+    let env = Envelope {
+        v: PROTOCOL_VERSION,
+        id,
+        r#final: true,
+        msg,
+    };
+    bxcore::write_frame(&mut stream, &env).unwrap();
+    let mut frames = Vec::new();
+    loop {
+        let env = bxcore::read_frame(&mut stream).unwrap();
+        let last = env.r#final;
+        frames.push(env);
+        if last {
+            return frames;
+        }
+    }
+}
+
+fn read_notes(call: u64) -> Message {
+    Message::ToolRequest(ToolRequest {
+        session: SessionId::new("s1").unwrap(),
+        call: CallId(call),
+        tool: "read_file".to_string(),
+        arguments: r#"{"path":"/n/a"}"#.to_string(),
+    })
+}
+
+fn last_response(frames: &[Envelope]) -> &ToolResponse {
+    match &frames.last().unwrap().msg {
+        Message::ToolResponse(r) => r,
+        other => panic!("{other:?}"),
+    }
+}
+
+fn stderr(output: &Output) -> String {
+    String::from_utf8_lossy(&output.stderr).to_string()
+}
+
+#[test]
+fn it_makes_its_directories_0700_and_its_sockets_0600() {
+    let home = Home::new("serve-modes", 900_000);
+    // One directory found too open, one made.
+    std::fs::create_dir_all(home.path("run/owner-broker")).unwrap();
+    std::fs::set_permissions(
+        home.path("run/owner-broker"),
+        std::fs::Permissions::from_mode(0o755),
+    )
+    .unwrap();
+    let running = home.serve();
+    assert_eq!(mode(&home.path("run/loop-broker")), 0o700);
+    assert_eq!(mode(&home.path("run/owner-broker")), 0o700);
+    assert_eq!(mode(&home.tools()), 0o600);
+    assert_eq!(mode(&home.admin()), 0o600);
+    assert_eq!(mode(&home.path("audit")), 0o700);
+    let printed = running.stop();
+    assert!(printed.contains("serving"), "{printed}");
+}
+
+#[test]
+fn a_stale_socket_is_replaced() {
+    let home = Home::new("serve-stale", 900_000);
+    std::fs::create_dir_all(home.path("run/loop-broker")).unwrap();
+    drop(UnixListener::bind(home.tools()).unwrap());
+    assert!(home.tools().exists(), "the stale socket file is there");
+    let _running = home.serve();
+    let frames = exchange(&home.tools(), 3, read_notes(3));
+    assert_eq!(
+        last_response(&frames),
+        &ToolResponse::Denied {
+            reason: DenyReason::NoGrant,
+            detail: None,
+        }
+    );
+}
+
+#[test]
+fn a_second_brokerd_on_the_same_home_refuses_to_start() {
+    let home = Home::new("serve-twice", 900_000);
+    let _running = home.serve();
+    let second = home.run(&[]);
+    assert_eq!(second.status.code(), Some(1));
+    let text = stderr(&second);
+    assert!(text.contains("brokerd is already running"), "{text}");
+    assert!(
+        text.trim_end()
+            .ends_with("see docs/runbook.md#brokerd-already-running"),
+        "{text}"
+    );
+    // The first still has its sockets.
+    let frames = exchange(&home.admin(), 1, Message::Approvals(Empty {}));
+    assert!(matches!(frames[0].msg, Message::ApprovalList(_)));
+}
+
+#[test]
+fn it_answers_each_socket_and_refuses_the_other_kinds() {
+    let home = Home::new("serve-kinds", 900_000);
+    home.grant("notes", "auto");
+    let running = home.serve();
+    let frames = exchange(&home.tools(), 7, read_notes(7));
+    assert_eq!(frames[0].id, 7);
+    // The production runtime runs nothing.
+    assert_eq!(
+        last_response(&frames),
+        &ToolResponse::Failed {
+            message: REFUSING.to_string()
+        }
+    );
+    let wrong = exchange(&home.tools(), 8, Message::Approvals(Empty {}));
+    assert!(matches!(&wrong[0].msg, Message::Error(e) if e.code == ErrorCode::Forbidden));
+    let wrong = exchange(&home.admin(), 9, read_notes(9));
+    assert!(matches!(&wrong[0].msg, Message::Error(e) if e.code == ErrorCode::Forbidden));
+    let printed = running.stop();
+    assert!(
+        printed.contains("approvals on broker.sock\nsee docs/runbook.md#socket-forbidden"),
+        "{printed}"
+    );
+    assert!(
+        printed.contains("tool_request on admin.sock\nsee docs/runbook.md#socket-forbidden"),
+        "{printed}"
+    );
+    assert!(matches!(
+        home.events().as_slice(),
+        [
+            AuditEvent::Decision { .. },
+            AuditEvent::Result {
+                status: ResultStatus::Failed,
+                ..
+            }
+        ]
+    ));
+}
+
+#[test]
+fn an_approval_nobody_answers_expires() {
+    let home = Home::new("serve-expire", 100);
+    home.grant("notes", "ask");
+    let _running = home.serve();
+    let started = Instant::now();
+    let frames = exchange(&home.tools(), 2, read_notes(2));
+    assert_eq!(frames.len(), 2, "{frames:?}");
+    assert!(matches!(
+        &frames[0].msg,
+        Message::ToolResponse(ToolResponse::PendingApproval { approval: 0, .. })
+    ));
+    assert_eq!(
+        last_response(&frames),
+        &ToolResponse::Denied {
+            reason: DenyReason::ApprovalExpired,
+            detail: None,
+        }
+    );
+    // The expiry thread looks every second.
+    assert!(
+        started.elapsed() < Duration::from_secs(5),
+        "{:?}",
+        started.elapsed()
+    );
+}
+
+fn copy_case(home: &Home, case: &str, only: &[&str]) {
+    let from = format!(
+        "{}/../bxcore/tests/fixtures/audit/{case}",
+        env!("CARGO_MANIFEST_DIR")
+    );
+    std::fs::create_dir_all(home.path("audit")).unwrap();
+    for name in only {
+        std::fs::copy(
+            format!("{from}/{name}"),
+            home.path(&format!("audit/{name}")),
+        )
+        .unwrap();
+    }
+}
+
+fn snapshot(dir: &Path) -> Vec<(String, Vec<u8>)> {
+    let mut all: Vec<(String, Vec<u8>)> = std::fs::read_dir(dir)
+        .unwrap()
+        .map(|e| e.unwrap())
+        .filter(|e| e.file_name().to_string_lossy().ends_with(".jsonl"))
+        .map(|e| {
+            (
+                e.file_name().into_string().unwrap(),
+                std::fs::read(e.path()).unwrap(),
+            )
+        })
+        .collect();
+    all.sort();
+    all
+}
+
+#[test]
+fn a_broken_chain_stops_it_before_any_socket_and_nothing_is_written() {
+    let home = Home::new("serve-broken", 900_000);
+    copy_case(&home, "changed-byte", &["2026-09-17.jsonl"]);
+    let before = snapshot(&home.path("audit"));
+    let output = home.run(&[]);
+    assert_eq!(output.status.code(), Some(1));
+    let text = stderr(&output);
+    assert!(text.contains("2026-09-17.jsonl:4: "), "{text}");
+    assert!(
+        text.trim_end()
+            .ends_with("see docs/runbook.md#audit-chain-broken"),
+        "{text}"
+    );
+    assert_eq!(snapshot(&home.path("audit")), before);
+    assert!(!home.tools().exists() && !home.admin().exists());
+}
+
+#[test]
+fn a_torn_tail_is_recovered_and_it_serves() {
+    let home = Home::new("serve-torn", 900_000);
+    copy_case(
+        &home,
+        "torn-tail",
+        &["2026-09-17.jsonl", "2026-09-18.jsonl"],
+    );
+    let running = home.serve();
+    let printed = running.stop();
+    assert!(printed.contains(RECOVERED_NOTICE), "{printed}");
+    assert!(RECOVERED_NOTICE.ends_with("see docs/runbook.md#audit-recovered"));
+}
+
+#[test]
+fn accept_break_with_nothing_to_accept_exits_2() {
+    let home = Home::new("serve-nothing", 900_000);
+    let output = home.run(&["--accept-break"]);
+    assert_eq!(output.status.code(), Some(2));
+    assert!(
+        stderr(&output).contains("nothing to accept"),
+        "{}",
+        stderr(&output)
+    );
+}
+
+#[test]
+fn bad_arguments_and_bad_configs_do_not_start() {
+    let brokerd = env!("CARGO_BIN_EXE_brokerd");
+    for args in [
+        &[][..],
+        &["serve"][..],
+        &["serve", "--config"][..],
+        &["serve", "--config", "a", "--config", "b"][..],
+        &["serve", "--config", "a", "--loud"][..],
+        &["run", "--config", "a"][..],
+    ] {
+        let output = Command::new(brokerd).args(args).output().unwrap();
+        assert_eq!(output.status.code(), Some(2), "{args:?}");
+        assert!(
+            stderr(&output).starts_with("usage: brokerd serve"),
+            "{args:?}"
+        );
+    }
+    let home = Home::new("serve-config", 900_000);
+    std::fs::write(&home.config, "[paths]\nhoem = \"/x\"\n").unwrap();
+    let output = home.run(&[]);
+    assert_eq!(output.status.code(), Some(1));
+    assert!(
+        stderr(&output).contains("brokerd.toml"),
+        "{}",
+        stderr(&output)
+    );
+    std::fs::remove_file(&home.config).unwrap();
+    assert_eq!(home.run(&[]).status.code(), Some(1));
+    assert!(
+        !home.path("audit").exists(),
+        "nothing made before the config is read"
+    );
+}
@@ -0,0 +1 @@
+{"v":1,"id":9,"final":true,"msg":{"kind":"tool_response","body":{"status":"denied","reason":"invalid_arguments","detail":"invalid argument shape: unknown field `file`, expected `text`"}}}
@@ -0,0 +1,151 @@
+//! Every JSON object in every fixture must reject an unknown key. Do not edit.
+//!
+//! The other test files check unknown fields in a few hand-picked places. This one checks all of
+//! them: it walks each fixture, adds one unknown key to one object at a time, at every depth, and
+//! requires that the result no longer decodes.
+
+use bxcore::{AuditRecord, Envelope, Grant, LogRecord};
+use serde::de::DeserializeOwned;
+use serde_json::Value;
+
+/// Every copy of `value` that has exactly one extra key in exactly one object.
+fn with_one_unknown_key(value: &Value) -> Vec<Value> {
+    let mut out = Vec::new();
+    match value {
+        Value::Object(map) => {
+            let mut extended = map.clone();
+            extended.insert("zz_unknown".to_string(), Value::Bool(true));
+            out.push(Value::Object(extended));
+            for (key, child) in map {
+                for changed in with_one_unknown_key(child) {
+                    let mut copy = map.clone();
+                    copy.insert(key.clone(), changed);
+                    out.push(Value::Object(copy));
+                }
+            }
+        }
+        Value::Array(items) => {
+            for (i, child) in items.iter().enumerate() {
+                for changed in with_one_unknown_key(child) {
+                    let mut copy = items.clone();
+                    copy[i] = changed;
+                    out.push(Value::Array(copy));
+                }
+            }
+        }
+        _ => {}
+    }
+    out
+}
+
+/// Returns how many variations were tried, so callers can check the walk reached nested objects.
+fn check<T: DeserializeOwned>(what: &str, text: &str) -> usize {
+    let value: Value = serde_json::from_str(text).unwrap_or_else(|e| panic!("{what}: {e}"));
+    assert!(
+        serde_json::from_value::<T>(value.clone()).is_ok(),
+        "{what}: fixture must decode"
+    );
+    let variations = with_one_unknown_key(&value);
+    for changed in &variations {
+        assert!(
+            serde_json::from_value::<T>(changed.clone()).is_err(),
+            "{what}: accepted an unknown key: {changed}"
+        );
+    }
+    variations.len()
+}
+
+fn fixture(path: &str) -> String {
+    let full = format!("{}/tests/fixtures/{path}", env!("CARGO_MANIFEST_DIR"));
+    std::fs::read_to_string(&full).unwrap_or_else(|e| panic!("{full}: {e}"))
+}
+
+#[test]
+fn envelopes_reject_unknown_keys_at_every_depth() {
+    for name in [
+        "tool_request.json",
+        "tool_response_pending.json",
+        "tool_response_result.json",
+        "tool_response_failed.json",
+        "tool_response_denied.json",
+        "tool_response_denied_detail.json",
+        "error.json",
+        "turn.json",
+        "turn_event_tool_result.json",
+        "turn_event_content.json",
+        "turn_event_retrying.json",
+        "turn_done.json",
+        "error_session_full.json",
+        "approvals.json",
+        "approval_list.json",
+        "approval_list_empty.json",
+        "approve.json",
+        "approve_result_allowed.json",
+        "approve_result_denied.json",
+        "refuse.json",
+        "refuse_no_reason.json",
+        "ok.json",
+        "check_grants.json",
+        "grants_report.json",
+        "grants_report_ok.json",
+        "error_forbidden.json",
+        "error_no_such_approval.json",
+        "turn_event_approval_pending.json",
+        "turn_event_tool_denied.json",
+    ] {
+        // Envelope, msg and body: three objects. Some bodies hold more: turn_done a usage
+        // object, approval_list one item, approve_result an outcome, grants_report two problems.
+        let want = match name {
+            "turn_done.json" | "approval_list.json" => 4,
+            "approve_result_allowed.json" | "approve_result_denied.json" => 4,
+            "grants_report.json" => 5,
+            _ => 3,
+        };
+        assert_eq!(
+            check::<Envelope>(name, &fixture(&format!("wire/{name}"))),
+            want,
+            "{name}"
+        );
+    }
+}
+
+#[test]
+fn audit_records_reject_unknown_keys_at_every_depth() {
+    for (i, line) in fixture("records/audit.jsonl").lines().enumerate() {
+        // The record and its event: two objects. Decision and approval events also hold an
+        // outcome object.
+        let want = if line.contains("\"outcome\"") { 3 } else { 2 };
+        assert_eq!(
+            check::<AuditRecord>(&format!("audit.jsonl:{}", i + 1), line),
+            want
+        );
+    }
+}
+
+#[test]
+fn log_records_reject_unknown_keys_at_every_depth() {
+    let mut tried = 0;
+    for (i, line) in fixture("records/session.jsonl").lines().enumerate() {
+        tried += check::<LogRecord>(&format!("session.jsonl:{}", i + 1), line);
+    }
+    // Seven records, plus the one tool call inside the first assistant record.
+    assert_eq!(tried, 8);
+}
+
+#[test]
+fn usage_log_records_reject_unknown_keys_at_every_depth() {
+    let mut tried = 0;
+    for (i, line) in fixture("records/session_usage.jsonl").lines().enumerate() {
+        tried += check::<LogRecord>(&format!("session_usage.jsonl:{}", i + 1), line);
+    }
+    // Seven records, plus the one tool call inside the first assistant record.
+    assert_eq!(tried, 8);
+}
+
+#[test]
+fn grants_reject_unknown_keys_at_every_depth() {
+    let grant: Grant = toml::from_str(&fixture("grant/full.toml")).unwrap();
+    let text = serde_json::to_string(&grant).unwrap();
+    // The grant and its constraints: two objects.
+    assert_eq!(check::<Grant>("full.toml as JSON", &text), 2);
+}
@@ -0,0 +1,219 @@
+//! Tests for IPC messages against byte-exact fixtures. Do not edit these or the fixtures.
+
+use bxcore::{
+    CallId, DataClass, DenyReason, Envelope, ErrorCode, Message, SessionId, Timestamp, ToolRequest,
+    ToolResponse, WireError,
+};
+
+fn fixture(name: &str) -> String {
+    let path = format!("{}/tests/fixtures/wire/{name}", env!("CARGO_MANIFEST_DIR"));
+    let text = std::fs::read_to_string(&path).unwrap_or_else(|e| panic!("{path}: {e}"));
+    text.trim_end_matches('\n').to_string()
+}
+
+/// The fixture must decode to `want`, and `want` must encode to exactly the fixture's bytes.
+fn check(name: &str, want: Envelope) {
+    let text = fixture(name);
+    let got: Envelope = serde_json::from_str(&text).unwrap_or_else(|e| panic!("{name}: {e}"));
+    assert_eq!(got, want, "{name}: decoded value");
+    assert_eq!(
+        serde_json::to_string(&want).unwrap(),
+        text,
+        "{name}: encoded bytes"
+    );
+}
+
+fn response(id: u64, r#final: bool, body: ToolResponse) -> Envelope {
+    Envelope {
+        v: 1,
+        id,
+        r#final,
+        msg: Message::ToolResponse(body),
+    }
+}
+
+#[test]
+fn tool_request() {
+    let body = ToolRequest {
+        session: SessionId::new("mm-thread-42").unwrap(),
+        call: CallId(3),
+        tool: "read_file".to_string(),
+        arguments: r#"{"path":"/etc/hosts"}"#.to_string(),
+    };
+    check(
+        "tool_request.json",
+        Envelope {
+            v: 1,
+            id: 7,
+            r#final: true,
+            msg: Message::ToolRequest(body),
+        },
+    );
+}
+
+#[test]
+fn tool_response_pending() {
+    let body = ToolResponse::PendingApproval {
+        approval: 41,
+        expires: Timestamp::parse("2026-09-17T08:35:00.000Z").unwrap(),
+    };
+    check("tool_response_pending.json", response(7, false, body));
+}
+
+#[test]
+fn an_approval_id_is_a_number_not_a_string() {
+    let good = fixture("tool_response_pending.json");
+    let bad = good.replacen("\"approval\":41", "\"approval\":\"41\"", 1);
+    assert_ne!(good, bad);
+    assert!(serde_json::from_str::<Envelope>(&good).is_ok());
+    assert!(serde_json::from_str::<Envelope>(&bad).is_err());
+    let negative = good.replacen("\"approval\":41", "\"approval\":-1", 1);
+    assert!(serde_json::from_str::<Envelope>(&negative).is_err());
+}
+
+#[test]
+fn tool_response_result() {
+    let body = ToolResponse::Result {
+        content: "127.0.0.1 localhost\n".to_string(),
+        class: DataClass::Private,
+        untrusted: true,
+        truncated: false,
+    };
+    check("tool_response_result.json", response(7, true, body));
+}
+
+#[test]
+fn tool_response_failed() {
+    let body = ToolResponse::Failed {
+        message: "exit status 2".to_string(),
+    };
+    check("tool_response_failed.json", response(8, true, body));
+}
+
+#[test]
+fn tool_response_denied() {
+    let body = ToolResponse::Denied {
+        reason: DenyReason::TaintTooHigh,
+        detail: None,
+    };
+    check("tool_response_denied.json", response(9, true, body));
+}
+
+/// A detail is written only when there is one, so the fixture above has no `detail` key.
+#[test]
+fn tool_response_denied_with_a_detail() {
+    let body = ToolResponse::Denied {
+        reason: DenyReason::InvalidArguments,
+        detail: Some("invalid argument shape: unknown field `file`, expected `text`".to_string()),
+    };
+    check("tool_response_denied_detail.json", response(9, true, body));
+}
+
+#[test]
+fn a_detail_must_be_a_string() {
+    let good = fixture("tool_response_denied_detail.json");
+    assert!(serde_json::from_str::<Envelope>(&good).is_ok());
+    let at = good.find("\"detail\":").unwrap();
+    for value in ["7", "[\"a\"]", "{}", "true"] {
+        let bad = format!("{}\"detail\":{value}}}}}}}", &good[..at]);
+        assert!(
+            serde_json::from_str::<Envelope>(&bad).is_err(),
+            "accepted {bad}"
+        );
+    }
+}
+
+#[test]
+fn error_message() {
+    let body = WireError {
+        code: ErrorCode::BadVersion,
+        detail: "expected 1".to_string(),
+    };
+    check(
+        "error.json",
+        Envelope {
+            v: 1,
+            id: 0,
+            r#final: true,
+            msg: Message::Error(body),
+        },
+    );
+}
+
+#[test]
+fn deny_reasons_and_error_codes_are_snake_case() {
+    let reasons = [
+        (DenyReason::NoGrant, "no_grant"),
+        (DenyReason::GrantExpired, "grant_expired"),
+        (DenyReason::TaintTooHigh, "taint_too_high"),
+        (DenyReason::DeniedByGrant, "denied_by_grant"),
+        (DenyReason::ApprovalRefused, "approval_refused"),
+        (DenyReason::ApprovalExpired, "approval_expired"),
+        (DenyReason::GrantsInvalid, "grants_invalid"),
+        (DenyReason::AuditUnavailable, "audit_unavailable"),
+        (DenyReason::InvalidArguments, "invalid_arguments"),
+        (DenyReason::StateUnreadable, "state_unreadable"),
+    ];
+    for (value, text) in reasons {
+        assert_eq!(
+            serde_json::to_string(&value).unwrap(),
+            format!("\"{text}\"")
+        );
+    }
+    let codes = [
+        (ErrorCode::BadFrame, "bad_frame"),
+        (ErrorCode::BadVersion, "bad_version"),
+        (ErrorCode::BadMessage, "bad_message"),
+        (ErrorCode::Internal, "internal"),
+        (ErrorCode::Forbidden, "forbidden"),
+        (ErrorCode::NoSuchApproval, "no_such_approval"),
+    ];
+    for (value, text) in codes {
+        assert_eq!(
+            serde_json::to_string(&value).unwrap(),
+            format!("\"{text}\"")
+        );
+    }
+}
+
+#[test]
+fn unknown_and_missing_fields_are_rejected() {
+    let good = fixture("tool_request.json");
+    assert!(serde_json::from_str::<Envelope>(&good).is_ok());
+    let bad = [
+        // extra field in the envelope
+        good.replacen("{\"v\":1,", "{\"v\":1,\"extra\":0,", 1),
+        // extra field beside kind and body
+        good.replacen(
+            "\"kind\":\"tool_request\",",
+            "\"kind\":\"tool_request\",\"x\":1,",
+            1,
+        ),
+        // extra field in the body
+        good.replacen("\"call\":3,", "\"call\":3,\"priority\":9,", 1),
+        // missing field in the body
+        good.replacen("\"call\":3,", "", 1),
+        // missing `final`
+        good.replacen("\"final\":true,", "", 1),
+        // unknown kind
+        good.replacen("tool_request", "tool_demand", 1),
+        // invalid session id inside a message
+        good.replacen("mm-thread-42", "../../etc", 1),
+    ];
+    for text in bad {
+        assert!(
+            serde_json::from_str::<Envelope>(&text).is_err(),
+            "accepted {text}"
+        );
+    }
+}
+
+#[test]
+fn unknown_field_in_a_response_variant_is_rejected() {
+    let good = fixture("tool_response_denied.json");
+    let bad = good.replacen("\"reason\":", "\"note\":\"x\",\"reason\":", 1);
+    assert!(serde_json::from_str::<Envelope>(&good).is_ok());
+    assert!(serde_json::from_str::<Envelope>(&bad).is_err());
+    let bad_status = good.replacen("denied", "refused", 1);
+    assert!(serde_json::from_str::<Envelope>(&bad_status).is_err());
+}
@@ -0,0 +1,321 @@
+//! Tests for `BrokerPort` against a fake `brokerd` on a temporary socket: answers, approvals and
+//! timeouts. Frames that break the protocol are in `broker_port_bad.rs`. Do not edit.
+
+#[path = "support/broker.rs"]
+mod fake;
+
+use std::io::Write;
+use std::os::unix::net::UnixListener;
+use std::thread;
+use std::time::Duration;
+
+use bxcore::{
+    DenyReason, Message, PROTOCOL_VERSION, Timestamp, ToolResponse, read_frame, write_frame,
+};
+use fake::{assert_unavailable, broker, call, frame, in_ms, request, result, send, socket_path};
+use loopd::broker_port::{BrokerPort, NoBroker, POINTER, TOO_LARGE};
+use loopd::tools::{Pending, ToolPort};
+
+#[test]
+fn a_result_comes_back_as_it_was_sent() {
+    let (socket, broker) = broker(|stream, request| {
+        send(stream, request.id, true, result("hello\n"));
+    });
+    let got = call(socket, 2_000, &request());
+    assert_eq!(got.response, result("hello\n"));
+    assert!(got.pending.is_empty());
+    assert!(got.lines.is_empty(), "nothing to report: {:?}", got.lines);
+
+    // What the broker received: one final frame holding exactly the request.
+    let sent = broker.join().unwrap();
+    assert_eq!(sent.v, PROTOCOL_VERSION);
+    assert!(sent.r#final, "a request is a single final frame");
+    assert_eq!(sent.msg, Message::ToolRequest(request()));
+}
+
+#[test]
+fn every_denial_and_a_failure_come_back_as_they_were_sent() {
+    let mut answers: Vec<ToolResponse> = [
+        DenyReason::NoGrant,
+        DenyReason::GrantExpired,
+        DenyReason::TaintTooHigh,
+        DenyReason::DeniedByGrant,
+        DenyReason::ApprovalRefused,
+        DenyReason::ApprovalExpired,
+        DenyReason::GrantsInvalid,
+        DenyReason::AuditUnavailable,
+        DenyReason::InvalidArguments,
+        DenyReason::StateUnreadable,
+    ]
+    .into_iter()
+    .map(|reason| ToolResponse::Denied {
+        reason,
+        detail: None,
+    })
+    .collect();
+    answers.push(ToolResponse::Denied {
+        reason: DenyReason::InvalidArguments,
+        detail: Some("invalid path: n/a".to_string()),
+    });
+    answers.push(ToolResponse::Failed {
+        message: "the runner arrives in M3b".to_string(),
+    });
+    for answer in answers {
+        let reply = answer.clone();
+        let (socket, broker) = broker(move |stream, request| {
+            send(stream, request.id, true, reply);
+        });
+        let got = call(socket, 2_000, &request());
+        assert_eq!(got.response, answer);
+        assert!(
+            got.lines.is_empty(),
+            "a denial is not an outage: {:?}",
+            got.lines
+        );
+        broker.join().unwrap();
+    }
+}
+
+#[test]
+fn a_pending_frame_reaches_the_callback_and_the_final_frame_is_the_answer() {
+    let expires = in_ms(60_000);
+    let (socket, broker) = broker(move |stream, request| {
+        let pending = ToolResponse::PendingApproval {
+            approval: 41,
+            expires,
+        };
+        send(stream, request.id, false, pending);
+        thread::sleep(Duration::from_millis(150));
+        send(stream, request.id, true, result("approved"));
+    });
+    let got = call(socket, 2_000, &request());
+    assert_eq!(got.response, result("approved"));
+    assert_eq!(
+        got.pending,
+        [Pending {
+            approval: 41,
+            expires
+        }],
+        "called once, with the frame's values"
+    );
+    assert!(got.lines.is_empty(), "{:?}", got.lines);
+    broker.join().unwrap();
+}
+
+#[test]
+fn an_answer_after_expires_but_within_the_timeout_of_it_is_delivered() {
+    // expires in 100 ms, timeout 1,500 ms: the port waits until 1,600 ms. The answer comes at
+    // about 400 ms, well after `expires`: an approval given at the last moment still gets the
+    // whole timeout to run.
+    let (socket, broker) = broker(|stream, request| {
+        let pending = ToolResponse::PendingApproval {
+            approval: 1,
+            expires: in_ms(100),
+        };
+        send(stream, request.id, false, pending);
+        thread::sleep(Duration::from_millis(400));
+        send(stream, request.id, true, result("late but good"));
+    });
+    let got = call(socket, 1_500, &request());
+    assert_eq!(got.response, result("late but good"));
+    assert!(got.lines.is_empty(), "{:?}", got.lines);
+    broker.join().unwrap();
+}
+
+#[test]
+fn an_expiry_that_has_already_passed_still_leaves_the_timeout() {
+    let (socket, broker) = broker(|stream, request| {
+        let pending = ToolResponse::PendingApproval {
+            approval: 1,
+            expires: Timestamp::parse("2020-01-01T00:00:00.000Z").unwrap(),
+        };
+        send(stream, request.id, false, pending);
+        thread::sleep(Duration::from_millis(200));
+        send(stream, request.id, true, result("fine"));
+    });
+    let got = call(socket, 1_500, &request());
+    assert_eq!(got.response, result("fine"));
+    broker.join().unwrap();
+}
+
+#[test]
+fn no_socket_is_unavailable() {
+    let socket = socket_path();
+    let _ = std::fs::remove_file(&socket);
+    let got = call(socket.to_path_buf(), 2_000, &request());
+    assert_unavailable(&got, "no socket file");
+    assert!(
+        got.lines[0].contains(&socket.display().to_string()),
+        "the line names the socket: {}",
+        got.lines[0]
+    );
+    assert!(got.took < Duration::from_millis(1_500), "{:?}", got.took);
+}
+
+#[test]
+fn a_broker_that_closes_without_answering_is_unavailable() {
+    let (socket, broker) = broker(|_, _| {});
+    let got = call(socket, 2_000, &request());
+    assert_unavailable(&got, "closed before any frame");
+    assert!(
+        got.took < Duration::from_millis(1_500),
+        "a close is seen at once: {:?}",
+        got.took
+    );
+    broker.join().unwrap();
+}
+
+#[test]
+fn a_broker_that_closes_while_pending_is_unavailable() {
+    let (socket, broker) = broker(|stream, request| {
+        let pending = ToolResponse::PendingApproval {
+            approval: 3,
+            expires: in_ms(60_000),
+        };
+        send(stream, request.id, false, pending);
+        // brokerd was restarted: the connection just ends.
+    });
+    let got = call(socket, 2_000, &request());
+    assert_unavailable(&got, "closed while pending");
+    assert_eq!(got.pending.len(), 1, "the pending frame was reported first");
+    assert!(got.took < Duration::from_millis(1_500), "{:?}", got.took);
+    broker.join().unwrap();
+}
+
+#[test]
+fn a_broker_that_never_answers_is_unavailable_after_the_timeout() {
+    let (socket, broker) = broker(|_, _| thread::sleep(Duration::from_millis(1_200)));
+    let got = call(socket, 300, &request());
+    assert_unavailable(&got, "silence");
+    assert!(
+        got.took >= Duration::from_millis(250),
+        "gave up early: {:?}",
+        got.took
+    );
+    assert!(
+        got.took < Duration::from_millis(1_100),
+        "gave up late: {:?}",
+        got.took
+    );
+    broker.join().unwrap();
+}
+
+#[test]
+fn a_pending_call_nobody_answers_ends_at_expires_plus_the_timeout() {
+    let (socket, broker) = broker(|stream, request| {
+        let pending = ToolResponse::PendingApproval {
+            approval: 3,
+            expires: in_ms(300),
+        };
+        send(stream, request.id, false, pending);
+        thread::sleep(Duration::from_millis(1_800));
+    });
+    let got = call(socket, 300, &request());
+    assert_unavailable(&got, "silence while pending");
+    assert!(
+        got.took >= Duration::from_millis(550),
+        "it must wait for expires (300) plus the timeout (300): {:?}",
+        got.took
+    );
+    assert!(
+        got.took < Duration::from_millis(1_700),
+        "gave up late: {:?}",
+        got.took
+    );
+    broker.join().unwrap();
+}
+
+#[test]
+fn the_timeout_is_a_deadline_for_the_frame_not_for_each_read() {
+    // The four header bytes arrive 250 ms apart, the last at 1,000 ms. A port that only sets a
+    // 600 ms read timeout sees every single read succeed and returns the result; a port with a
+    // deadline gives up at 600 ms. The answer tells them apart, so no timing is asserted.
+    let (socket, broker) = broker(|stream, request| {
+        let mut bytes = Vec::new();
+        write_frame(
+            &mut bytes,
+            &frame(request.id, true, Message::ToolResponse(result("slow"))),
+        )
+        .unwrap();
+        for byte in bytes.iter().take(4) {
+            thread::sleep(Duration::from_millis(250));
+            if stream.write_all(&[*byte]).is_err() {
+                return;
+            }
+        }
+        let _ = stream.write_all(&bytes[4..]);
+    });
+    let got = call(socket, 600, &request());
+    assert_unavailable(&got, "a trickled frame");
+    broker.join().unwrap();
+}
+
+#[test]
+fn a_zero_timeout_fails_closed_and_does_not_panic() {
+    let (socket, _broker) = broker(|stream, request| {
+        send(stream, request.id, true, result("too late"));
+    });
+    let got = call(socket, 0, &request());
+    assert_unavailable(&got, "timeout_ms = 0");
+}
+
+#[test]
+fn a_request_too_large_for_a_frame_is_its_own_failure() {
+    // Nothing is sent, so the fake broker sees a connection that closes or none at all.
+    let path = socket_path();
+    let _ = std::fs::remove_file(&path);
+    let _listener = UnixListener::bind(&path).unwrap();
+    let mut big = request();
+    big.arguments = format!(r#"{{"path":"/{}"}}"#, "a".repeat(bxcore::MAX_FRAME));
+    let got = call(path.to_path_buf(), 1_000, &big);
+    assert_eq!(
+        got.response,
+        ToolResponse::Failed {
+            message: TOO_LARGE.to_string()
+        }
+    );
+    assert!(
+        got.lines.is_empty(),
+        "the broker is fine; this is not an outage: {:?}",
+        got.lines
+    );
+}
+
+#[test]
+fn every_call_is_its_own_connection() {
+    let path = socket_path();
+    let _ = std::fs::remove_file(&path);
+    let listener = UnixListener::bind(&path).unwrap();
+    let server = thread::spawn(move || {
+        for n in 0..3u64 {
+            let (mut stream, _) = listener.accept().unwrap();
+            let request = read_frame(&mut stream).unwrap();
+            send(
+                &mut stream,
+                request.id,
+                true,
+                result(&format!("answer {n}")),
+            );
+        }
+    });
+    let port = BrokerPort::new(path.to_path_buf(), Duration::from_millis(2_000));
+    for n in 0..3 {
+        let got = port.call(&request(), &mut |_| {});
+        assert_eq!(got, result(&format!("answer {n}")));
+    }
+    server.join().unwrap();
+}
+
+#[test]
+fn without_a_broker_every_call_fails_in_words_the_model_can_repeat() {
+    let mut seen = 0;
+    let got = NoBroker.call(&request(), &mut |_| seen += 1);
+    assert_eq!(
+        got,
+        ToolResponse::Failed {
+            message: "no tool broker is configured".to_string()
+        }
+    );
+    assert_eq!(seen, 0);
+    assert_eq!(POINTER, "see docs/runbook.md#broker-unavailable");
+}
@@ -0,0 +1,335 @@
+//! Tests for the registry, dispatch, the denial sentences, the result cap and the fake tools.
+//! Do not edit.
+
+mod support;
+
+use bxcore::{CallId, DenyReason, SessionId, ToolRequest, ToolResponse};
+use loopd::tools::{
+    Dispatch, FakeTools, Pending, Registry, ToolPort, cap_result, denial_message, denial_text,
+    dispatch,
+};
+
+fn req(tool: &str, arguments: &str) -> ToolRequest {
+    ToolRequest {
+        session: SessionId::new("s").unwrap(),
+        call: CallId(1),
+        tool: tool.to_string(),
+        arguments: arguments.to_string(),
+    }
+}
+
+#[test]
+fn the_core_schemas_are_the_fixed_tools_array() {
+    let names: Vec<String> = Registry::m2b()
+        .core_schemas()
+        .into_iter()
+        .map(|s| s.name)
+        .collect();
+    assert_eq!(names, ["clock", "find_tool", "call_tool"]);
+    let find = &Registry::m2b().core_schemas()[1];
+    assert_eq!(find.parameters["required"], serde_json::json!(["query"]));
+    let call = &Registry::m2b().core_schemas()[2];
+    assert_eq!(
+        call.parameters["required"],
+        serde_json::json!(["name", "arguments"])
+    );
+}
+
+#[test]
+fn find_matches_name_or_description_case_insensitively() {
+    let r = Registry::m2b();
+    let names = |q: &str| r.find(q).iter().map(|s| s.name.clone()).collect::<Vec<_>>();
+    assert_eq!(names("echo"), ["echo"]);
+    assert_eq!(names("ECHO"), ["echo"]);
+    assert_eq!(names("unchanged"), ["echo"], "matches the description too");
+    assert_eq!(names("time"), ["clock"]);
+    assert!(names("weather").is_empty());
+    assert!(
+        names("").is_empty(),
+        "an empty query matches nothing, not everything"
+    );
+    assert!(names("   ").is_empty());
+}
+
+#[test]
+fn find_tool_is_answered_locally() {
+    let r = Registry::m2b();
+    match dispatch(&r, "find_tool", r#"{"query":"echo"}"#) {
+        Dispatch::Local(text) => {
+            assert!(text.starts_with("1 tool(s) match:\n"), "{text}");
+            assert!(text.contains("\"name\":\"echo\""), "{text}");
+            assert!(
+                text.contains("\"parameters\""),
+                "the schema is included: {text}"
+            );
+            assert!(text.ends_with("Call it with call_tool."), "{text}");
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(
+        dispatch(&r, "find_tool", r#"{"query":"weather"}"#),
+        Dispatch::Local("No tool matches.".to_string())
+    );
+    assert!(
+        matches!(dispatch(&r, "find_tool", "not json"), Dispatch::Local(t) if t == "No tool matches.")
+    );
+}
+
+#[test]
+fn call_tool_unwraps_a_known_non_core_tool_and_nothing_else() {
+    let r = Registry::m2b();
+    let args = r#"{"name":"echo","arguments":{"text": "box"}}"#;
+    match dispatch(&r, "call_tool", args) {
+        Dispatch::Port { tool, arguments } => {
+            assert_eq!(tool, "echo");
+            assert_eq!(
+                serde_json::from_str::<serde_json::Value>(&arguments).unwrap(),
+                serde_json::json!({"text": "box"})
+            );
+        }
+        other => panic!("{other:?}"),
+    }
+    let cases = [
+        (
+            r#"{"name":"weather","arguments":{}}"#,
+            "No tool named \"weather\"",
+        ),
+        (r#"{"name":"clock","arguments":{}}"#, "core tool"),
+        (r#"{"arguments":{}}"#, "No tool named \"\""),
+        ("not json", "needs a JSON object"),
+    ];
+    for (args, want) in cases {
+        match dispatch(&r, "call_tool", args) {
+            Dispatch::Local(text) => assert!(text.contains(want), "{args}: {text}"),
+            other => panic!("{args}: {other:?}, must never reach the port"),
+        }
+    }
+}
+
+#[test]
+fn any_other_tool_goes_to_the_port_as_it_is() {
+    let r = Registry::m2b();
+    // Even one the registry does not know: the port (brokerd) decides, not loopd.
+    let want = Dispatch::Port {
+        tool: "read_file".to_string(),
+        arguments: r#"{"path":"/x"}"#.to_string(),
+    };
+    assert_eq!(dispatch(&r, "read_file", r#"{"path":"/x"}"#), want);
+    let want = Dispatch::Port {
+        tool: "weather".to_string(),
+        arguments: "not json".to_string(),
+    };
+    assert_eq!(dispatch(&r, "weather", "not json"), want);
+}
+
+#[test]
+fn the_clock_is_answered_locally_whatever_its_arguments() {
+    for registry in [Registry::m2b(), Registry::m3a(), Registry::new(vec![])] {
+        for arguments in ["{}", r#"{"zone":"UTC"}"#, "not json", ""] {
+            let before = bxcore::Timestamp::now();
+            match dispatch(&registry, "clock", arguments) {
+                Dispatch::Local(text) => {
+                    let time = bxcore::Timestamp::parse(&text)
+                        .unwrap_or_else(|e| panic!("an RFC 3339 time, got {text:?}: {e:?}"));
+                    assert!(time >= before, "{text}");
+                    assert!(text.ends_with('Z'), "UTC: {text}");
+                }
+                other => panic!("{arguments:?}: {other:?}, the clock must not reach the port"),
+            }
+        }
+    }
+}
+
+#[test]
+fn the_m3a_registry_has_the_same_core_and_the_four_broker_tools() {
+    let r = Registry::m3a();
+    assert_eq!(
+        r.core_schemas(),
+        Registry::m2b().core_schemas(),
+        "the tools array is part of the baseline: it must not change"
+    );
+    let names = |q: &str| r.find(q).iter().map(|s| s.name.clone()).collect::<Vec<_>>();
+    assert_eq!(names("file"), ["read_file", "write_file"]);
+    assert_eq!(names("shell"), ["shell"]);
+    assert_eq!(names("fetch"), ["http_fetch"]);
+    assert_eq!(names("https"), ["http_fetch"]);
+    assert!(names("echo").is_empty(), "echo is a test tool only");
+
+    // The argument schemas are section 3's table: exactly these properties, all strings.
+    let table: [(&str, &[&str], &[&str]); 4] = [
+        ("read_file", &["path"], &["path"]),
+        ("write_file", &["content", "path"], &["path", "content"]),
+        ("shell", &["command", "cwd"], &["command"]),
+        ("http_fetch", &["url"], &["url"]),
+    ];
+    for (name, properties, required) in table {
+        let entry = r.get(name).unwrap_or_else(|| panic!("{name} is missing"));
+        assert!(!entry.core, "{name} is found with find_tool, not declared");
+        let p = &entry.schema.parameters;
+        assert_eq!(p["type"], "object", "{name}");
+        let mut got: Vec<&str> = p["properties"]
+            .as_object()
+            .unwrap()
+            .keys()
+            .map(String::as_str)
+            .collect();
+        got.sort_unstable();
+        assert_eq!(got, properties, "{name}: properties");
+        for property in properties {
+            assert_eq!(
+                p["properties"][property]["type"], "string",
+                "{name}.{property}"
+            );
+            assert!(
+                p["properties"][property]["description"].is_string(),
+                "{name}.{property} needs a description"
+            );
+        }
+        assert_eq!(
+            p["required"],
+            serde_json::json!(required),
+            "{name}: required"
+        );
+    }
+
+    // call_tool lets the four through to the port and nothing else.
+    match dispatch(
+        &r,
+        "call_tool",
+        r#"{"name":"shell","arguments":{"command":"ls"}}"#,
+    ) {
+        Dispatch::Port { tool, arguments } => {
+            assert_eq!(tool, "shell");
+            assert_eq!(arguments, r#"{"command":"ls"}"#);
+        }
+        other => panic!("{other:?}"),
+    }
+    assert!(matches!(
+        dispatch(&r, "call_tool", r#"{"name":"echo","arguments":{}}"#),
+        Dispatch::Local(t) if t.contains("No tool named \"echo\"")
+    ));
+}
+
+#[test]
+fn every_deny_reason_has_its_sentence() {
+    let table = [
+        (DenyReason::NoGrant, "Denied: no grant allows this call."),
+        (
+            DenyReason::GrantExpired,
+            "Denied: the grant for this call has expired.",
+        ),
+        (
+            DenyReason::TaintTooHigh,
+            "Denied: this session has seen data too sensitive for this call.",
+        ),
+        (
+            DenyReason::DeniedByGrant,
+            "Denied: a grant forbids this call.",
+        ),
+        (
+            DenyReason::ApprovalRefused,
+            "Denied: the owner refused this call.",
+        ),
+        (
+            DenyReason::ApprovalExpired,
+            "Denied: the approval request expired without an answer.",
+        ),
+        (
+            DenyReason::InvalidArguments,
+            "Denied: the arguments are not valid for this tool.",
+        ),
+        (
+            DenyReason::GrantsInvalid,
+            "Denied: the grant files have an error; the owner has been told.",
+        ),
+        (
+            DenyReason::AuditUnavailable,
+            "Denied: the audit log cannot be written; the owner has been told.",
+        ),
+        (
+            DenyReason::StateUnreadable,
+            "Denied: this session's broker state is damaged; the owner has been told.",
+        ),
+    ];
+    for (reason, want) in table {
+        assert_eq!(denial_text(reason), want, "{reason:?}");
+        assert_eq!(
+            denial_message(reason, None),
+            want,
+            "{reason:?}: without a detail, the sentence alone"
+        );
+    }
+}
+
+/// The broker's detail goes after the sentence, before its full stop.
+#[test]
+fn a_denial_with_a_detail_appends_it_to_the_sentence() {
+    assert_eq!(
+        denial_message(
+            DenyReason::InvalidArguments,
+            Some("invalid argument shape: unknown field `file`, expected `text`")
+        ),
+        "Denied: the arguments are not valid for this tool: invalid argument shape: unknown field `file`, expected `text`."
+    );
+    assert_eq!(
+        denial_message(DenyReason::InvalidArguments, Some("invalid path: n/a")),
+        "Denied: the arguments are not valid for this tool: invalid path: n/a."
+    );
+    assert_eq!(
+        denial_message(DenyReason::NoGrant, Some("x")),
+        "Denied: no grant allows this call: x.",
+        "any reason's detail is shown the same way"
+    );
+}
+
+#[test]
+fn results_are_cut_on_a_character_boundary_and_marked() {
+    assert_eq!(cap_result("short", 100), ("short".to_string(), false));
+    assert_eq!(cap_result("exactly", 7), ("exactly".to_string(), false));
+    let (text, truncated) = cap_result("abcdefghij", 4);
+    assert_eq!(text, "abcd\n[truncated]");
+    assert!(truncated);
+    // 3 ASCII bytes, then 2-byte characters: byte 4 is inside a character.
+    let (text, truncated) = cap_result("abc\u{e9}\u{e9}\u{e9}", 4);
+    assert_eq!(text, "abc\n[truncated]");
+    assert!(truncated);
+    assert_eq!(cap_result("", 0), (String::new(), false));
+    assert_eq!(cap_result("x", 0), ("\n[truncated]".to_string(), true));
+}
+
+#[test]
+fn fake_tools_answer_echo_deny_the_rest_and_record_calls() {
+    let fake = FakeTools::new();
+    let mut seen = 0;
+    let mut on_pending = |_: &Pending| seen += 1;
+    match fake.call(&req("echo", r#"{"text":"box"}"#), &mut on_pending) {
+        ToolResponse::Result {
+            content,
+            class,
+            untrusted,
+            truncated,
+        } => {
+            assert_eq!(content, "box");
+            assert_eq!(class, bxcore::DataClass::Public);
+            assert!(!untrusted && !truncated);
+        }
+        other => panic!("{other:?}"),
+    }
+    assert!(matches!(
+        fake.call(&req("echo", r#"{"tex":"box"}"#), &mut on_pending),
+        ToolResponse::Failed { .. }
+    ));
+    for tool in ["weather", "read_file", "clock"] {
+        assert_eq!(
+            fake.call(&req(tool, "{}"), &mut on_pending),
+            ToolResponse::Denied {
+                reason: DenyReason::NoGrant,
+                detail: None,
+            },
+            "{tool}: the clock is loopd's own now, not the port's"
+        );
+    }
+    assert_eq!(seen, 0, "the fake never asks for approval");
+    assert_eq!(fake.calls().len(), 5);
+    assert_eq!(fake.calls()[0].tool, "echo");
+}
@@ -0,0 +1,269 @@
+//! Tests for what the turn loop does with the broker's answers: denials, pending approvals, and
+//! a port that misbehaves. Do not edit.
+
+mod support;
+#[path = "support/turn.rs"]
+mod turn_support;
+
+use bxcore::{DataClass, DenyReason, LogRecord, Timestamp, ToolResponse, TurnEvent};
+use support::{Reply, ReturnsPendingPort, ok_result, pending};
+use turn_support::{setup, types};
+
+const CHAT: &str = "/v1/chat/completions";
+
+/// The tool events of a turn, in order, as short strings.
+fn tool_events(events: &[TurnEvent]) -> Vec<String> {
+    events
+        .iter()
+        .filter_map(|e| match e {
+            TurnEvent::ToolCallStarted { name } => Some(format!("started {name}")),
+            TurnEvent::ApprovalPending { approval, tool, .. } => {
+                Some(format!("pending {approval} {tool}"))
+            }
+            TurnEvent::ToolDenied { name, reason } => Some(format!("denied {name} {reason:?}")),
+            TurnEvent::ToolResult { name, .. } => Some(format!("result {name}")),
+            _ => None,
+        })
+        .collect()
+}
+
+#[test]
+fn a_denial_is_a_fixed_sentence_for_the_model_and_an_event_for_the_owner() {
+    let s = setup(vec![ToolResponse::Denied {
+        reason: DenyReason::TaintTooHigh,
+        detail: None,
+    }]);
+    s.server.route(
+        CHAT,
+        vec![Reply::fixture("tool_call"), Reply::fixture("plain")],
+    );
+    let mut session = s.session("a");
+    let (result, events) = s.turn(&mut session, "x");
+    assert!(
+        result.is_ok(),
+        "the turn goes on after a denial: {result:?}"
+    );
+    match &session.records()[4] {
+        LogRecord::ToolResult {
+            content,
+            class,
+            untrusted,
+            truncated,
+            ..
+        } => {
+            assert_eq!(
+                content,
+                "Denied: this session has seen data too sensitive for this call."
+            );
+            assert_eq!(
+                (*class, *untrusted, *truncated),
+                (DataClass::Public, false, false)
+            );
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(
+        tool_events(&events),
+        [
+            "started read_file",
+            "denied read_file TaintTooHigh",
+            "result read_file"
+        ],
+        "the denial comes before the result"
+    );
+    // The model reads the sentence in the next request.
+    let m2 = s.server.requests_to(CHAT)[1].json();
+    assert_eq!(
+        m2["messages"][3]["content"],
+        "Denied: this session has seen data too sensitive for this call."
+    );
+}
+
+/// The broker's detail reaches the model, in the log and in the next request; the owner's event
+/// is the same as for any denial.
+#[test]
+fn an_invalid_call_tells_the_model_why() {
+    let s = setup(vec![ToolResponse::Denied {
+        reason: DenyReason::InvalidArguments,
+        detail: Some("invalid path: n/a".to_string()),
+    }]);
+    s.server.route(
+        CHAT,
+        vec![Reply::fixture("tool_call"), Reply::fixture("plain")],
+    );
+    let mut session = s.session("a");
+    let (result, events) = s.turn(&mut session, "x");
+    assert!(result.is_ok(), "{result:?}");
+    let want = "Denied: the arguments are not valid for this tool: invalid path: n/a.";
+    match &session.records()[4] {
+        LogRecord::ToolResult {
+            content,
+            class,
+            untrusted,
+            truncated,
+            ..
+        } => {
+            assert_eq!(content, want);
+            assert_eq!(
+                (*class, *untrusted, *truncated),
+                (DataClass::Public, false, false)
+            );
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(
+        tool_events(&events),
+        [
+            "started read_file",
+            "denied read_file InvalidArguments",
+            "result read_file"
+        ]
+    );
+    let m2 = s.server.requests_to(CHAT)[1].json();
+    assert_eq!(m2["messages"][3]["content"], want);
+}
+
+#[test]
+fn a_denied_call_tool_names_the_target_tool_in_the_denial() {
+    let s = setup(vec![ToolResponse::Denied {
+        reason: DenyReason::NoGrant,
+        detail: None,
+    }]);
+    s.server.route(
+        CHAT,
+        vec![Reply::fixture("call_tool"), Reply::fixture("plain")],
+    );
+    let mut session = s.session("a");
+    let (result, events) = s.turn(&mut session, "echo box");
+    assert!(result.is_ok(), "{result:?}");
+    assert_eq!(
+        tool_events(&events),
+        [
+            "started call_tool",
+            "denied echo NoGrant",
+            "result call_tool"
+        ],
+        "the owner writes grants for `echo`, not for `call_tool`"
+    );
+    assert!(
+        matches!(&session.records()[4], LogRecord::ToolResult { content, .. } if content == "Denied: no grant allows this call.")
+    );
+}
+
+#[test]
+fn a_pending_approval_is_an_event_and_the_answer_after_it_is_the_result() {
+    let s = setup(vec![
+        pending(41, "2026-09-18T12:15:00.000Z"),
+        ok_result("straylight\n"),
+    ]);
+    s.server.route(
+        CHAT,
+        vec![Reply::fixture("tool_call"), Reply::fixture("plain")],
+    );
+    let mut session = s.session("a");
+    let (result, events) = s.turn(&mut session, "x");
+    assert!(result.is_ok(), "{result:?}");
+    assert_eq!(
+        tool_events(&events),
+        [
+            "started read_file",
+            "pending 41 read_file",
+            "result read_file"
+        ]
+    );
+    let expires = events.iter().find_map(|e| match e {
+        TurnEvent::ApprovalPending { expires, .. } => Some(*expires),
+        _ => None,
+    });
+    assert_eq!(
+        expires,
+        Some(Timestamp::parse("2026-09-18T12:15:00.000Z").unwrap())
+    );
+    assert_eq!(s.port.calls().len(), 1, "one call, however long it waited");
+    assert_eq!(
+        types(session.records()),
+        [
+            "start",
+            "user",
+            "assistant",
+            "usage",
+            "tool_result",
+            "assistant",
+            "usage"
+        ],
+        "waiting writes nothing to the log"
+    );
+    assert!(
+        matches!(&session.records()[4], LogRecord::ToolResult { content, .. } if content == "straylight\n")
+    );
+}
+
+#[test]
+fn a_pending_approval_that_ends_in_a_refusal() {
+    let s = setup(vec![
+        pending(7, "2026-09-18T12:15:00.000Z"),
+        ToolResponse::Denied {
+            reason: DenyReason::ApprovalRefused,
+            detail: None,
+        },
+    ]);
+    s.server.route(
+        CHAT,
+        vec![Reply::fixture("tool_call"), Reply::fixture("plain")],
+    );
+    let mut session = s.session("a");
+    let (result, events) = s.turn(&mut session, "x");
+    assert!(result.is_ok(), "{result:?}");
+    assert_eq!(
+        tool_events(&events),
+        [
+            "started read_file",
+            "pending 7 read_file",
+            "denied read_file ApprovalRefused",
+            "result read_file"
+        ]
+    );
+    assert!(
+        matches!(&session.records()[4], LogRecord::ToolResult { content, .. } if content == "Denied: the owner refused this call.")
+    );
+}
+
+#[test]
+fn a_port_that_returns_a_pending_frame_as_its_answer_is_a_failure_not_a_decision() {
+    let s = setup(vec![]);
+    s.server.route(
+        CHAT,
+        vec![Reply::fixture("tool_call"), Reply::fixture("plain")],
+    );
+    let mut session = s.session("a");
+    let runtime = loopd::turn::Runtime {
+        cfg: &s.cfg,
+        client: &s.client,
+        port: &ReturnsPendingPort,
+        registry: &s.registry,
+    };
+    let mut events = Vec::new();
+    let result =
+        loopd::turn::run_turn(&mut session, &runtime, "x", &mut |e| events.push(e.clone()));
+    assert!(result.is_ok(), "{result:?}");
+    match &session.records()[4] {
+        LogRecord::ToolResult {
+            content,
+            class,
+            untrusted,
+            ..
+        } => {
+            assert_eq!(
+                content,
+                "The tool failed: the tool broker gave no final answer"
+            );
+            assert_eq!((*class, *untrusted), (DataClass::Public, false));
+        }
+        other => panic!("{other:?}"),
+    }
+    assert_eq!(
+        tool_events(&events),
+        ["started read_file", "result read_file"],
+        "neither pending nor denied: the port said neither"
+    );
+}
				`@@ -0,0 +1 @@`
				{"v":1,"id":9,"final":true,"msg":{"kind":"tool_response","body":{"status":"denied","reason":"invalid_arguments","detail":"invalid argument shape: unknown field `file`, expected `text`"}}}