fix(maestro): drop an unused instant from report_activity

crates/maestro-cli/src/sessions.rs

@@ -176,6 +176,7 @@ mod tests {
             created_at: Utc.with_ymd_and_hms(2026, 5, 25, 12, 0, 0).unwrap(),
             foreground: None,
             activity: Default::default(),
+            activity_detail: None,
             last_activity_at: chrono::Utc::now(),
             activity_since: chrono::Utc::now(),
             current_cwd: None,

crates/maestro-daemon/src/activity.rs

@@ -15,6 +15,44 @@ use maestro_protocol::ActivityState;
 /// Active↔Idle. This window *is* the debounce the issue calls for.
 pub const ACTIVE_WINDOW: Duration = Duration::from_secs(3);
 
+/// How long an agent-reported state (#105) stays in force before inference
+/// resumes. Long enough for a working agent's heartbeat to hold between
+/// reports, short enough that an agent dying mid-run cannot pin its session
+/// at `Error` forever.
+pub const REPORT_TTL: Duration = Duration::from_secs(300);
+
+/// Server-side cap (#105) on a reported `detail`, in characters. The meta
+/// line renders it inline, so an unbounded string would own the whole card.
+pub const MAX_REPORT_DETAIL_CHARS: usize = 256;
+
+/// An agent-reported activity state (#105) still within its TTL. While one
+/// is [`Self::in_force`], it outranks [`decide`]'s inference; when it
+/// lapses, inference resumes and any reported detail is cleared with it.
+#[derive(Debug, Clone)]
+pub struct ReportedActivity {
+    pub state: ActivityState,
+    pub expires_at: std::time::Instant,
+}
+
+impl ReportedActivity {
+    pub fn new(state: ActivityState, ttl: Duration) -> Self {
+        Self {
+            state,
+            expires_at: std::time::Instant::now() + ttl,
+        }
+    }
+
+    pub fn in_force(&self, now: std::time::Instant) -> bool {
+        now < self.expires_at
+    }
+}
+
+/// Cap a reported detail to [`MAX_REPORT_DETAIL_CHARS`] characters,
+/// char-boundary safe.
+pub fn truncate_detail(detail: Option<String>) -> Option<String> {
+    detail.map(|d| d.chars().take(MAX_REPORT_DETAIL_CHARS).collect())
+}
+
 /// Combine the observed signals into an [`ActivityState`]:
 /// recent output ⇒ `Active`; else blocked on a terminal read ⇒ `AwaitingInput`;
 /// else `Idle`.
@@ -59,4 +97,25 @@ mod tests {
             ActivityState::Idle
         );
     }
+
+    #[test]
+    fn report_is_in_force_until_exactly_its_expiry() {
+        let ttl = Duration::from_secs(300);
+        let report = ReportedActivity::new(ActivityState::Error, ttl);
+        assert!(report.in_force(report.expires_at - Duration::from_millis(1)));
+        assert!(!report.in_force(report.expires_at));
+    }
+
+    #[test]
+    fn detail_truncates_to_the_cap_without_panic_on_multibyte_input() {
+        let long = "é".repeat(MAX_REPORT_DETAIL_CHARS + 10);
+        let capped = truncate_detail(Some(long)).unwrap();
+        assert_eq!(capped.chars().count(), MAX_REPORT_DETAIL_CHARS);
+
+        assert_eq!(truncate_detail(None), None);
+        assert_eq!(
+            truncate_detail(Some("short".into())).as_deref(),
+            Some("short")
+        );
+    }
 }

crates/maestro-daemon/src/lib.rs

@@ -27,5 +27,6 @@ pub mod tmux;
 pub mod worktree;
 pub mod worktree_probe;
 
+pub use activity::MAX_REPORT_DETAIL_CHARS;
 pub use config::{default_config_path, default_state_dir, load_config, DaemonConfig, EnvOverrides};
 pub use session::{SessionError, SessionManager};

crates/maestro-daemon/src/local_api.rs

@@ -15,12 +15,12 @@ use std::sync::Arc;
 use axum::extract::{Path, Query, State};
 use axum::http::StatusCode;
 use axum::response::{IntoResponse, Response};
-use axum::routing::{any, delete, get};
+use axum::routing::{any, delete, get, post};
 use axum::{Json, Router};
 use maestro_config::SpawnPreset;
 use maestro_protocol::{
-    routes, ApiError, CatalogEvent, CreateSessionRequest, DaemonInfo, GitRoot, NameBatch, Session,
-    PROTOCOL_VERSION,
+    routes, ApiError, CatalogEvent, CreateSessionRequest, DaemonInfo, GitRoot, NameBatch,
+    ReportActivityRequest, Session, PROTOCOL_VERSION,
 };
 use serde::Deserialize;
 use uuid::Uuid;
@@ -46,6 +46,7 @@ pub struct LocalApiState {
 pub fn router(state: LocalApiState) -> Router {
     Router::new()
         .route(routes::SESSIONS, get(list_sessions).post(create_session))
+        .route(routes::patterns::SESSION_ACTIVITY, post(report_activity))
         .route(routes::patterns::SESSION, delete(kill_session))
         .route(routes::DIRS, get(list_dir))
         .route(routes::RECENT_DIRS, get(recent_dirs))
@@ -318,6 +319,29 @@ async fn reclaim_worktree(State(s): State<LocalApiState>, Path(id): Path<Uuid>)
     }
 }
 
+/// `POST /api/sessions/:id/activity` (#105): an agent reports its own state.
+/// The report holds above the daemon's inference for a TTL, then inference
+/// resumes — an agent that dies mid-run cannot pin its session at `Error`
+/// forever. Unknown session is 404; an unknown state value is rejected by the
+/// JSON decoder (422) rather than coerced. Answers `204 No Content`.
+async fn report_activity(
+    State(s): State<LocalApiState>,
+    Path(id): Path<Uuid>,
+    Json(body): Json<ReportActivityRequest>,
+) -> Response {
+    match s.manager.report_activity(id, body.state, body.detail).await {
+        Ok(()) => StatusCode::NO_CONTENT.into_response(),
+        Err(e) => {
+            let info = e.to_error_info();
+            error_response(
+                session_error_status(&info.code),
+                info.code.as_str(),
+                info.message.as_str(),
+            )
+        }
+    }
+}
+
 fn error_response(status: StatusCode, code: &str, message: &str) -> Response {
     (status, Json(ApiError::new(code, message))).into_response()
 }

crates/maestro-daemon/src/session.rs

@@ -57,6 +57,11 @@ pub struct SessionRecord {
     /// fall back to a probe.
     pub worktree_branch: Option<String>,
     pub bridge: PtyBridge,
+    /// Agent-reported activity (#105) still within its TTL. While in force it
+    /// outranks the activity pass's own inference; when it lapses, inference
+    /// resumes and [`Session::activity_detail`] is cleared with it. This
+    /// field is daemon-local bookkeeping only — it never rides the wire.
+    pub reported_activity: Option<crate::activity::ReportedActivity>,
 }
 
 #[derive(Debug, thiserror::Error)]
@@ -136,6 +141,9 @@ struct Inner {
     /// the variable. A mutex because the runner sets it after construction,
     /// possibly while clones are already serving.
     daemon_url: std::sync::Mutex<Option<String>>,
+    /// TTL an agent-reported activity state (#105) stays in force. A const in
+    /// production; overridable for tests that watch the lapse happen.
+    report_ttl: Duration,
     sessions: Mutex<HashMap<Uuid, SessionRecord>>,
     events: broadcast::Sender<CatalogEvent>,
 }
@@ -176,6 +184,7 @@ impl SessionManager {
                 spawn_history: None,
                 retained: None,
                 daemon_url: std::sync::Mutex::new(None),
+                report_ttl: crate::activity::REPORT_TTL,
                 sessions: Mutex::new(HashMap::new()),
                 events,
             }),
@@ -208,6 +217,15 @@ impl SessionManager {
         self
     }
 
+    /// Override the agent-report TTL (#105). Tests pass a short one to watch
+    /// inference resume without waiting out [`crate::activity::REPORT_TTL`].
+    pub fn with_activity_report_ttl(mut self, ttl: Duration) -> Self {
+        let inner = Arc::get_mut(&mut self.inner)
+            .expect("with_activity_report_ttl must be called before the SessionManager is cloned");
+        inner.report_ttl = ttl;
+        self
+    }
+
     /// Set the daemon's own HTTP base URL (#103), exported to every session
     /// spawned afterwards as `MAESTRO_URL`. Unlike the `with_*` builders this
     /// is a plain setter: the daemon runner learns its bound address after
@@ -379,6 +397,7 @@ impl SessionManager {
             // activity watcher refines `foreground`/`activity` on its next tick.
             foreground: None,
             activity: ActivityState::Active,
+            activity_detail: None,
             last_activity_at: Utc::now(),
             activity_since: chrono::Utc::now(),
 
@@ -409,6 +428,7 @@ impl SessionManager {
                     worktree_base_repo,
                     worktree_branch: req.worktree.as_ref().map(|w| w.branch.clone()),
                     bridge,
+                    reported_activity: None,
                 },
             );
         }
@@ -602,6 +622,7 @@ impl SessionManager {
                     // recovered session had already been sitting idle.
                     foreground: None,
                     activity: ActivityState::Idle,
+                    activity_detail: None,
                     last_activity_at: Utc::now(),
                     activity_since: chrono::Utc::now(),
 
@@ -635,6 +656,7 @@ impl SessionManager {
                     // recovered session probes git for it at retain time.
                     worktree_branch: None,
                     bridge,
+                    reported_activity: None,
                 },
             );
             let _ = self
@@ -675,6 +697,47 @@ impl SessionManager {
             .await
             .map_err(SessionError::Tmux)
     }
+
+    /// Accept an agent-reported activity state (#105), the only path that can
+    /// produce `ActivityState::Error`. The report outranks the activity pass's
+    /// own inference until its TTL lapses — so an agent that dies mid-run
+    /// cannot pin its session at `Error` forever. An unknown session is
+    /// [`SessionError::NotFound`]; the reported `detail` is capped
+    /// server-side.
+    ///
+    /// A report that changes the effective state restarts `activity_since`
+    /// (#104's clock); re-reporting an unchanged state only extends the TTL
+    /// and stays silent on the event stream at steady state.
+    pub async fn report_activity(
+        &self,
+        id: Uuid,
+        state: ActivityState,
+        detail: Option<String>,
+    ) -> Result<(), SessionError> {
+        let updated = {
+            let mut sessions = self.inner.sessions.lock().await;
+            let rec = sessions.get_mut(&id).ok_or(SessionError::NotFound(id))?;
+            let detail = crate::activity::truncate_detail(detail);
+            let changed = rec.session.activity != state || rec.session.activity_detail != detail;
+            if rec.session.activity != state {
+                rec.session.activity_since = Utc::now();
+            }
+            rec.session.activity = state;
+            rec.session.activity_detail = detail;
+            rec.reported_activity = Some(crate::activity::ReportedActivity::new(
+                state,
+                self.inner.report_ttl,
+            ));
+            changed.then(|| rec.session.clone())
+        };
+        if let Some(session) = updated {
+            let _ = self
+                .inner
+                .events
+                .send(CatalogEvent::SessionUpdated { session });
+        }
+        Ok(())
+    }
 }
 
 /// Run a blocking `tmux` subprocess call on the blocking thread pool so it
@@ -845,8 +908,22 @@ async fn run_activity_pass(inner: &Arc<Inner>) {
             let Some(rec) = sessions.get_mut(&id) else {
                 continue;
             };
+            // #105: an agent report lapses at its TTL — drop it and its
+            // detail so this tick's inference takes over again.
+            if let Some(report) = &rec.reported_activity {
+                if !report.in_force(std::time::Instant::now()) {
+                    rec.reported_activity = None;
+                    rec.session.activity_detail = None;
+                }
+            }
             let idle = rec.bridge.idle_for();
-            let activity = crate::activity::decide(idle, awaiting);
+            // #105: while a report is in force it supplies the state and
+            // nothing here may overwrite it; only an unreported session is
+            // decided by inference.
+            let activity = match &rec.reported_activity {
+                Some(report) => report.state,
+                None => crate::activity::decide(idle, awaiting),
+            };
             let foreground = Some(pane.command.clone());
             // The pane's live cwd, drives the dir-based session tree; empty means
             // tmux reported none.

crates/maestro-daemon/tests/activity_report.rs

@@ -0,0 +1,164 @@
+//! Agent-reported activity (#105): `POST /api/sessions/:id/activity`
+//! semantics at the SessionManager level — precedence over the daemon's own
+//! inference, detail handling, and lapse back to inference at the TTL.
+
+mod common;
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use common::TmuxFixture;
+use maestro_daemon::sidecar::SidecarStore;
+use maestro_daemon::worktree::WorktreeManager;
+use maestro_daemon::SessionManager;
+use maestro_protocol::{ActivityState, CreateSessionRequest};
+use tempfile::TempDir;
+use tokio::time::sleep;
+use uuid::Uuid;
+
+struct Fixture {
+    manager: SessionManager,
+    _tmux: TmuxFixture,
+    _state_dir: TempDir,
+}
+
+async fn fixture(ttl: Duration) -> Fixture {
+    let tmux = TmuxFixture::new();
+    let state_dir = TempDir::new().unwrap();
+    let manager = SessionManager::new(
+        Arc::new(tmux.tmux.clone()),
+        SidecarStore::new(state_dir.path().join("sessions")),
+        WorktreeManager::new(None),
+        state_dir.path().join("fifos"),
+    )
+    .with_activity_report_ttl(ttl);
+    Fixture {
+        manager,
+        _tmux: tmux,
+        _state_dir: state_dir,
+    }
+}
+
+async fn spawn_sleeping(manager: &SessionManager) -> Uuid {
+    manager
+        .create(CreateSessionRequest {
+            cwd: "/tmp".into(),
+            command: "sleep 60".into(),
+            env: Default::default(),
+            metadata: Default::default(),
+            worktree: None,
+        })
+        .await
+        .unwrap()
+        .id
+}
+
+async fn observed(manager: &SessionManager, id: Uuid) -> maestro_protocol::Session {
+    manager
+        .snapshot()
+        .await
+        .into_iter()
+        .find(|s| s.id == id)
+        .expect("session still tracked")
+}
+
+#[tokio::test]
+async fn report_on_unknown_session_is_not_found() {
+    let fx = fixture(Duration::from_secs(300)).await;
+    let err = fx
+        .manager
+        .report_activity(Uuid::new_v4(), ActivityState::Idle, None)
+        .await
+        .unwrap_err();
+    assert!(matches!(err, maestro_daemon::SessionError::NotFound(_)));
+}
+
+#[tokio::test]
+async fn report_sets_error_state_and_detail() {
+    let fx = fixture(Duration::from_secs(300)).await;
+    let id = spawn_sleeping(&fx.manager).await;
+
+    fx.manager
+        .report_activity(
+            id,
+            ActivityState::Error,
+            Some("cargo build failed: link error".into()),
+        )
+        .await
+        .unwrap();
+
+    let s = observed(&fx.manager, id).await;
+    assert_eq!(s.activity, ActivityState::Error);
+    assert_eq!(
+        s.activity_detail.as_deref(),
+        Some("cargo build failed: link error")
+    );
+}
+
+#[tokio::test]
+async fn replacing_a_report_replaces_both_state_and_detail() {
+    let fx = fixture(Duration::from_secs(300)).await;
+    let id = spawn_sleeping(&fx.manager).await;
+
+    fx.manager
+        .report_activity(id, ActivityState::Error, Some("first failure".into()))
+        .await
+        .unwrap();
+    fx.manager
+        .report_activity(
+            id,
+            ActivityState::AwaitingInput,
+            Some("needs approval".into()),
+        )
+        .await
+        .unwrap();
+
+    let s = observed(&fx.manager, id).await;
+    assert_eq!(s.activity, ActivityState::AwaitingInput);
+    assert_eq!(s.activity_detail.as_deref(), Some("needs approval"));
+}
+
+#[tokio::test]
+async fn detail_is_capped_server_side() {
+    let fx = fixture(Duration::from_secs(300)).await;
+    let id = spawn_sleeping(&fx.manager).await;
+
+    let long = "x".repeat(10_000);
+    fx.manager
+        .report_activity(id, ActivityState::Error, Some(long))
+        .await
+        .unwrap();
+
+    let s = observed(&fx.manager, id).await;
+    let detail = s.activity_detail.expect("detail present");
+    assert_eq!(
+        detail.chars().count(),
+        maestro_daemon::MAX_REPORT_DETAIL_CHARS
+    );
+}
+
+#[tokio::test]
+async fn reported_state_survives_watcher_ticks_until_the_ttl_then_inference_resumes() {
+    let fx = fixture(Duration::from_millis(150)).await;
+    let id = spawn_sleeping(&fx.manager).await;
+    fx.manager.start_liveness_watcher(Duration::from_millis(50));
+
+    fx.manager
+        .report_activity(id, ActivityState::Error, Some("agent died mid-run".into()))
+        .await
+        .unwrap();
+
+    // Several watcher ticks inside the TTL: nothing infers `Error`, so the
+    // report must hold above inference.
+    sleep(Duration::from_millis(80)).await;
+    let s = observed(&fx.manager, id).await;
+    assert_eq!(s.activity, ActivityState::Error);
+    assert_eq!(s.activity_detail.as_deref(), Some("agent died mid-run"));
+
+    // After the TTL lapses, inference resumes and the detail goes with it.
+    common::wait_for_async(Duration::from_secs(5), || async {
+        let s = observed(&fx.manager, id).await;
+        s.activity != ActivityState::Error && s.activity_detail.is_none()
+    })
+    .await;
+}

crates/maestro-daemon/tests/local_api.rs

@@ -346,6 +346,109 @@ async fn delete_owned_session_kills_it() {
     assert!(!h.manager.has(session.id).await);
 }
 
+/// Spawn a session through the manager and return its id.
+async fn spawn_via_manager(h: &Harness) -> Uuid {
+    h.manager
+        .create(CreateSessionRequest {
+            cwd: "/tmp".into(),
+            command: "sleep 60".into(),
+            env: HashMap::new(),
+            metadata: HashMap::new(),
+            worktree: None,
+        })
+        .await
+        .unwrap()
+        .id
+}
+
+/// The agent activity endpoint on the wire (#105): a report lands in the
+/// session snapshot with its detail.
+#[tokio::test]
+async fn post_activity_reports_error_with_detail() {
+    let h = spawn("naps-laptop").await;
+    let id = spawn_via_manager(&h).await;
+
+    let resp = reqwest::Client::new()
+        .post(url(h.addr, &format!("/api/sessions/{id}/activity")))
+        .json(&serde_json::json!({
+            "state": "Error",
+            "detail": "cargo build failed"
+        }))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), reqwest::StatusCode::NO_CONTENT);
+
+    let sessions: Vec<Session> = reqwest::get(url(h.addr, "/api/sessions"))
+        .await
+        .unwrap()
+        .json()
+        .await
+        .unwrap();
+    let s = sessions.iter().find(|s| s.id == id).unwrap();
+    assert_eq!(s.activity, maestro_protocol::ActivityState::Error);
+    assert_eq!(s.activity_detail.as_deref(), Some("cargo build failed"));
+}
+
+#[tokio::test]
+async fn post_activity_to_unknown_session_returns_404() {
+    let h = spawn("naps-laptop").await;
+    let resp = reqwest::Client::new()
+        .post(url(
+            h.addr,
+            &format!("/api/sessions/{}/activity", Uuid::new_v4()),
+        ))
+        .json(&serde_json::json!({ "state": "Idle" }))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), reqwest::StatusCode::NOT_FOUND);
+}
+
+/// Unknown state values are rejected rather than coerced (#105).
+#[tokio::test]
+async fn post_activity_rejects_an_unknown_state_value() {
+    let h = spawn("naps-laptop").await;
+    let id = spawn_via_manager(&h).await;
+
+    let resp = reqwest::Client::new()
+        .post(url(h.addr, &format!("/api/sessions/{id}/activity")))
+        .json(&serde_json::json!({ "state": "Teleported" }))
+        .send()
+        .await
+        .unwrap();
+    assert!(resp.status().is_client_error(), "got {}", resp.status());
+}
+
+#[tokio::test]
+async fn post_activity_caps_an_overlong_detail_server_side() {
+    let h = spawn("naps-laptop").await;
+    let id = spawn_via_manager(&h).await;
+
+    let resp = reqwest::Client::new()
+        .post(url(h.addr, &format!("/api/sessions/{id}/activity")))
+        .json(&serde_json::json!({
+            "state": "Error",
+            "detail": "x".repeat(10_000)
+        }))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), reqwest::StatusCode::NO_CONTENT);
+
+    let sessions: Vec<Session> = reqwest::get(url(h.addr, "/api/sessions"))
+        .await
+        .unwrap()
+        .json()
+        .await
+        .unwrap();
+    let s = sessions.iter().find(|s| s.id == id).unwrap();
+    assert_eq!(
+        s.activity_detail.as_ref().map(|d| d.chars().count()),
+        Some(maestro_daemon::MAX_REPORT_DETAIL_CHARS)
+    );
+}
+
 /// The kill guard on the wire (#79): `DELETE /api/sessions/:id` answers 409
 /// with the structured breakdown, and the session keeps running.
 #[tokio::test]

crates/maestro-daemon/tests/sidecar.rs

@@ -26,6 +26,7 @@ fn sample(id: Uuid) -> Session {
             .with_timezone(&Utc),
         foreground: None,
         activity: Default::default(),
+        activity_detail: None,
         last_activity_at: chrono::Utc::now(),
         activity_since: chrono::Utc::now(),
         current_cwd: Some(PathBuf::from("/tmp/work/crates")),

crates/maestro-protocol/src/client.rs

@@ -16,7 +16,7 @@ use uuid::Uuid;
 use crate::routes::{self, UrlError};
 use crate::{
     ApiError, BranchInfo, CreateSessionRequest, DaemonInfo, DirListing, ErrorInfo, GitRoot,
-    KillOutcome, Project, RecentDirs, RetainedWorktree, Session,
+    KillOutcome, Project, RecentDirs, ReportActivityRequest, RetainedWorktree, Session,
 };
 
 /// Time budget for establishing a TCP connection to the daemon.
@@ -134,6 +134,21 @@ impl DaemonClient {
         json(resp).await
     }
 
+    /// `POST /api/sessions/:id/activity` (#105): report an agent-observed
+    /// state. The daemon holds it above its own inference for a TTL, then
+    /// resumes inference. Answers `204 No Content`.
+    pub async fn report_activity(
+        &self,
+        id: Uuid,
+        req: &ReportActivityRequest,
+    ) -> Result<(), ClientError> {
+        let resp = self
+            .send(Method::POST, &routes::session_activity(id), &[], Some(req))
+            .await?;
+        check(resp).await?;
+        Ok(())
+    }
+
     pub async fn daemon_info(&self) -> Result<DaemonInfo, ClientError> {
         self.get(routes::INFO, &[]).await
     }

crates/maestro-protocol/src/lib.rs

@@ -75,6 +75,12 @@ pub struct Session {
     /// byte stream.
     #[serde(default)]
     pub activity: ActivityState,
+    /// Short human-readable string an agent reported alongside its activity
+    /// (#105) — e.g. why it is in `Error`. Shown on a sidebar card's meta line
+    /// while the report is in force; cleared when the report lapses or is
+    /// replaced. `None` unless an agent reported one.
+    #[serde(default)]
+    pub activity_detail: Option<String>,
     /// Wall-clock time of the session's most recent observed PTY output.
     /// Drives "idle for Ns" displays. Defaults to the Unix epoch on payloads
     /// from daemons that predate the field.
@@ -131,6 +137,18 @@ pub enum ActivityState {
     Error,
 }
 
+/// HTTP `POST /api/sessions/:id/activity` body (#105): an agent's report of
+/// its own state. The daemon holds the reported state above its own inference
+/// for a fixed TTL, then resumes inference.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct ReportActivityRequest {
+    pub state: ActivityState,
+    /// Short human string shown next to the state (e.g. an error message).
+    /// The daemon caps its length server-side.
+    #[serde(default)]
+    pub detail: Option<String>,
+}
+
 /// Optional worktree request attached to a `CreateSessionRequest`.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct WorktreeSpec {
@@ -544,6 +562,7 @@ mod tests {
                 .with_timezone(&Utc),
             foreground: Some("claude".into()),
             activity: ActivityState::Active,
+            activity_detail: None,
             last_activity_at: DateTime::parse_from_rfc3339("2026-05-24T12:05:00Z")
                 .unwrap()
                 .with_timezone(&Utc),
@@ -639,6 +658,49 @@ mod tests {
         assert_eq!(ActivityState::default(), ActivityState::Idle);
     }
 
+    #[test]
+    fn activity_detail_defaults_to_none_on_legacy_payload() {
+        // #105: a daemon predating the detail field omits it; it must
+        // deserialize to `None` rather than failing, and a present value
+        // round-trips.
+        let json = serde_json::json!({
+            "id": Uuid::from_u128(1),
+            "cwd": "/tmp",
+            "command": "bash",
+            "status": "Running",
+            "created_at": "2026-05-24T12:00:00Z"
+        });
+        let s: Session = serde_json::from_value(json).unwrap();
+        assert_eq!(s.activity_detail, None);
+
+        let mut reported = sample_session();
+        reported.activity = ActivityState::Error;
+        reported.activity_detail = Some("build failed".into());
+        assert_eq!(reported.clone(), round_trip(&reported));
+    }
+
+    #[test]
+    fn report_activity_request_defaults_detail_and_rejects_unknown_state() {
+        let bare: ReportActivityRequest =
+            serde_json::from_value(serde_json::json!({ "state": "Error" })).unwrap();
+        assert_eq!(bare.state, ActivityState::Error);
+        assert_eq!(bare.detail, None);
+
+        let full = ReportActivityRequest {
+            state: ActivityState::AwaitingInput,
+            detail: Some("waiting on permission".into()),
+        };
+        assert_eq!(full, round_trip(&full));
+
+        // Unknown state values are rejected rather than coerced (#105).
+        assert!(
+            serde_json::from_value::<ReportActivityRequest>(serde_json::json!({
+                "state": "Teleported"
+            }))
+            .is_err()
+        );
+    }
+
     #[test]
     fn session_round_trips() {
         let s = sample_session();

crates/maestro-protocol/src/routes.rs

@@ -47,6 +47,11 @@ pub fn session(id: Uuid) -> String {
     format!("{SESSIONS}/{id}")
 }
 
+/// `POST /api/sessions/:id/activity` (#105): an agent reports its own state.
+pub fn session_activity(id: Uuid) -> String {
+    format!("{SESSIONS}/{id}/activity")
+}
+
 /// `DELETE /api/worktrees/:id` — reclaim one retained worktree (#80).
 pub fn worktree(id: Uuid) -> String {
     format!("{WORKTREES}/{id}")
@@ -61,6 +66,7 @@ pub fn attach(id: Uuid) -> String {
 /// registers the same paths [`session`] and [`attach`] build.
 pub mod patterns {
     pub const SESSION: &str = "/api/sessions/:id";
+    pub const SESSION_ACTIVITY: &str = "/api/sessions/:id/activity";
     pub const ATTACH: &str = "/api/attach/:id";
     pub const WORKTREE: &str = "/api/worktrees/:id";
 }
@@ -169,6 +175,10 @@ mod tests {
             session(id()),
             "/api/sessions/6f1a7c0e-0f5e-4a2b-9a3d-6b1c2d3e4f50"
         );
+        assert_eq!(
+            session_activity(id()),
+            "/api/sessions/6f1a7c0e-0f5e-4a2b-9a3d-6b1c2d3e4f50/activity"
+        );
         assert_eq!(
             attach(id()),
             "/api/attach/6f1a7c0e-0f5e-4a2b-9a3d-6b1c2d3e4f50"
@@ -191,6 +201,10 @@ mod tests {
             patterns::SESSION.trim_end_matches(":id"),
             session(id()).trim_end_matches(&id().to_string())
         );
+        assert_eq!(
+            patterns::SESSION_ACTIVITY.trim_end_matches(":id/activity"),
+            session(id()).trim_end_matches(&id().to_string())
+        );
         assert_eq!(
             patterns::ATTACH.trim_end_matches(":id"),
             attach(id()).trim_end_matches(&id().to_string())

crates/maestro-testing/src/lib.rs

@@ -41,6 +41,7 @@ pub fn session(id: Uuid, cwd: &str, command: &str) -> Session {
         created_at: Utc::now(),
         foreground: None,
         activity: Default::default(),
+        activity_detail: None,
         last_activity_at: Utc::now(),
         activity_since: chrono::Utc::now(),
         current_cwd: None,

crates/maestro-tui/src/app.rs

@@ -1126,6 +1126,7 @@ mod tests {
                 created_at: Utc::now(),
                 foreground: None,
                 activity: Default::default(),
+                activity_detail: None,
                 last_activity_at: Utc::now(),
                 activity_since: chrono::Utc::now(),
                 current_cwd: None,
@@ -1644,6 +1645,7 @@ mod tests {
                 created_at: Utc::now(),
                 foreground: None,
                 activity: Default::default(),
+                activity_detail: None,
                 last_activity_at: chrono::Utc::now(),
                 activity_since: chrono::Utc::now(),
 

crates/maestro-tui/src/catalog.rs

@@ -193,6 +193,7 @@ mod tests {
             created_at: Utc::now(),
             foreground: None,
             activity: Default::default(),
+            activity_detail: None,
             last_activity_at: chrono::Utc::now(),
             activity_since: chrono::Utc::now(),
             current_cwd: None,

crates/maestro-tui/src/sidebar.rs

@@ -271,6 +271,7 @@ mod tests {
             created_at: Utc::now(),
             foreground: None,
             activity: Default::default(),
+            activity_detail: None,
             last_activity_at: Utc::now(),
             activity_since: Utc::now(),
             current_cwd: None,
@@ -579,6 +580,7 @@ mod tests {
             created_at: Utc::now(),
             foreground: None,
             activity: Default::default(),
+            activity_detail: None,
             last_activity_at: Utc::now(),
             activity_since: Utc::now(),
             current_cwd: None,

docs/SIDEBAR.md

@@ -41,7 +41,7 @@ Otherwise line 2 is absent — never an empty placeholder row.
 When present, line 2 reads left to right:
 
 ```
-<dot> <duration> [<blocked work> | exited] [<command>]
+<dot> <duration> [<detail>] [<blocked work> | exited] [<command>]
 ```
 
 `<dot>` is a single filled glyph (`●` on both clients) carrying the activity
@@ -52,6 +52,11 @@ and duration are unconditional parts of line 2; the bracketed fragments appear
 only when their trigger holds. A payload that predates #102 (no usable
 `activity_since`) contributes no duration — the dot still renders.
 
+`<detail>` is an agent-reported string (#105) — e.g. why a session is in
+`Error`. It sits right after the duration, in the muted style, and shows only
+while an agent-reported state is in force (see Precedence): when the report
+lapses at its TTL or is replaced, the detail goes with it.
+
 `<command>` is optional and muted: the session's command line, shown only when
 the rendered name came from `metadata.name` and differs from the command (the
 web has always surfaced this; the TUI may omit it).
@@ -94,9 +99,10 @@ cutover points land within one tick.
 ## Precedence
 
 An agent-reported `ActivityState` (#105) overrides the daemon's inference for
-the TTL the report carries; when the TTL expires, inference resumes. Clients
-never blend the two: whichever source is in force supplies both the colour and
-the clock for `activity_since`.
+**five minutes** (the TTL is fixed by the daemon; re-reporting extends it).
+When the TTL expires, inference resumes and the session's reported detail —
+if any — clears with the report. Clients never blend the two: whichever source
+is in force supplies both the colour and the clock for `activity_since`.
 
 If a client receives an `ActivityState` variant it does not know, it must not
 fail rendering: treat it as `Idle` (neutral/dim, line-2 rules apply as usual)

web/src/types.ts

@@ -26,6 +26,10 @@ export interface Session {
   foreground?: string | null;
   /** Inferred activity. Defaults to `Idle` on the wire when omitted. */
   activity?: ActivityState;
+  /** Short human string an agent reported alongside its state (#105) — e.g.
+   * why it is in `Error`. Shown on a sidebar card's meta line while the
+   * report is in force; cleared when the report lapses or is replaced. */
+  activity_detail?: string | null;
   /** Wall-clock of the most recent observed PTY output (NAP-340). */
   last_activity_at?: IsoTimestamp;
   /** Wall-clock of when the session entered its current `activity` state