feat(maestro): report StopFailure as Error with the error spelling as detail

A newer daemon's unknown activity variant failed the whole Session
deserialization on older clients. Hand-written Deserialize maps any
unknown string to Idle; non-string values still error. (#110)

Cargo.lock

@@ -1065,6 +1065,7 @@ dependencies = [
  "futures-util",
  "libc",
  "maestro-config",
+ "maestro-hooks",
  "maestro-protocol",
  "maestro-testing",
  "nix",
@@ -1102,6 +1103,7 @@ dependencies = [
  "futures-util",
  "libc",
  "maestro-config",
+ "maestro-hooks",
  "maestro-names",
  "maestro-protocol",
  "reqwest",
@@ -1137,6 +1139,15 @@ dependencies = [
  "uuid",
 ]
 
+[[package]]
+name = "maestro-hooks"
+version = "0.1.0"
+dependencies = [
+ "serde_json",
+ "tempfile",
+ "thiserror 1.0.69",
+]
+
 [[package]]
 name = "maestro-installer"
 version = "0.1.0"

Cargo.toml

@@ -18,6 +18,7 @@ dirs = "5"
 futures-util = { version = "0.3", default-features = false, features = ["sink", "std"] }
 libc = "0.2"
 maestro-config = { path = "crates/maestro-config" }
+maestro-hooks = { path = "crates/maestro-hooks" }
 maestro-names = { path = "crates/maestro-names" }
 maestro-protocol = { path = "crates/maestro-protocol" }
 maestro-testing = { path = "crates/maestro-testing" }

crates/maestro-cli/Cargo.toml

@@ -17,6 +17,7 @@ dirs.workspace = true
 futures-util.workspace = true
 libc.workspace = true
 maestro-config.workspace = true
+maestro-hooks.workspace = true
 maestro-protocol = { workspace = true, features = ["client"] }
 nix.workspace = true
 serde_json.workspace = true

crates/maestro-cli/src/cli.rs

@@ -43,6 +43,23 @@ pub enum Command {
     Kill(KillArgs),
     /// List retained worktrees, or reclaim one by ID (#86).
     Worktrees(WorktreesArgs),
+    /// Install or update the agent activity hook (#106).
+    Hooks(HooksArgs),
+}
+
+/// `maestro hooks <install>`
+#[derive(Debug, clap::Args)]
+pub struct HooksArgs {
+    #[command(subcommand)]
+    pub command: HooksCommand,
+}
+
+#[derive(Debug, Subcommand)]
+pub enum HooksCommand {
+    /// Write the Claude Code activity hook into ~/.claude/settings.json.
+    /// Idempotent: an older maestro hook is updated in place, every other
+    /// hook is left untouched.
+    Install,
 }
 
 /// `maestro worktrees <list|reclaim>`
@@ -275,6 +292,23 @@ mod tests {
         assert!(err.to_string().contains("KEY=VALUE"));
     }
 
+    #[test]
+    fn parses_hooks_install() {
+        let cli = Cli::try_parse_from(["maestro", "hooks", "install"]).unwrap();
+        match cli.command {
+            Command::Hooks(h) => {
+                assert!(matches!(h.command, HooksCommand::Install));
+            }
+            _ => panic!("wrong subcommand"),
+        }
+        // No subcommand → help, like `worktrees`.
+        let err = Cli::try_parse_from(["maestro", "hooks"]).unwrap_err();
+        assert!(matches!(
+            err.kind(),
+            clap::error::ErrorKind::DisplayHelpOnMissingArgumentOrSubcommand
+        ));
+    }
+
     #[test]
     fn parses_worktrees_list() {
         let cli = Cli::try_parse_from(["maestro", "worktrees", "list", "--json"]).unwrap();

crates/maestro-cli/src/dispatch.rs

@@ -24,6 +24,10 @@ pub enum CliError {
     #[error("{0}")]
     Attach(#[from] crate::attach::AttachError),
     #[error("{0}")]
+    Hooks(#[from] maestro_hooks::HooksError),
+    #[error("could not determine the home directory ($HOME unset?)")]
+    NoHome,
+    #[error("{0}")]
     Io(#[from] std::io::Error),
 }
 
@@ -40,6 +44,15 @@ pub async fn dispatch(
         None => default_config_path()?,
     };
 
+    // `hooks install` is purely local: it must work with the daemon down, so
+    // it never touches the client.
+    if let Command::Hooks(args) = &command {
+        match args.command {
+            crate::cli::HooksCommand::Install => run_hooks_install()?,
+        }
+        return Ok(());
+    }
+
     let daemon = client(&config_path, daemon_url_override)?;
 
     match command {
@@ -77,6 +90,7 @@ pub async fn dispatch(
                 );
             }
         }
+        Command::Hooks(_) => unreachable!("hooks handled before the daemon client exists"),
         Command::Worktrees(args) => match args.command {
             WorktreesCommand::List(list_args) => {
                 let mut stdout = std::io::stdout().lock();
@@ -101,6 +115,25 @@ pub async fn dispatch(
     Ok(())
 }
 
+/// `maestro hooks install` (#106): write the shipped hook script under the
+/// user's data dir and register it in their Claude Code settings. Idempotent
+/// and foreign-hook-preserving — the guarantees live in [`maestro_hooks`].
+#[allow(clippy::result_large_err)]
+fn run_hooks_install() -> Result<(), CliError> {
+    let home = dirs::home_dir().ok_or(CliError::NoHome)?;
+    let changed = maestro_hooks::install_for_home(&home)?;
+    if changed {
+        println!(
+            "installed claude-code activity hook: {} → {}",
+            maestro_hooks::script_path(&home).display(),
+            maestro_hooks::settings_path(&home).display()
+        );
+    } else {
+        println!("claude-code activity hook already up to date");
+    }
+    Ok(())
+}
+
 /// Default detach byte, matching `maestro attach --detach-key`'s `^q`.
 const CTRL_Q: u8 = 0x11;
 

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-config/src/lib.rs

@@ -64,6 +64,24 @@ pub struct MaestroConfigFile {
     pub tui: TuiSection,
     #[serde(default)]
     pub spawn: SpawnSection,
+    #[serde(default)]
+    pub hooks: HooksSection,
+}
+
+/// `[hooks]` — agent-hook installation settings (#106).
+#[derive(Debug, Clone, Default, PartialEq, Eq, Deserialize)]
+pub struct HooksSection {
+    /// Whether the daemon installs the Claude Code activity hook into
+    /// `~/.claude/settings.json` on first start when no maestro hook is
+    /// there yet. Default `true`; set `auto_install = false` to opt out.
+    #[serde(default)]
+    pub auto_install: Option<bool>,
+}
+
+impl HooksSection {
+    pub fn auto_install(&self) -> bool {
+        self.auto_install.unwrap_or(true)
+    }
 }
 
 #[derive(Debug, Clone, Default, PartialEq, Eq, Deserialize)]
@@ -375,6 +393,26 @@ mod tests {
         assert_eq!(cfg, MaestroConfigFile::default());
     }
 
+    #[test]
+    fn hooks_auto_install_defaults_true_and_is_opt_out() {
+        // Absent file / absent [hooks] ⇒ auto_install on.
+        assert!(HooksSection::default().auto_install());
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("config.toml");
+        std::fs::write(&path, "").unwrap();
+        assert!(MaestroConfigFile::load(&path).unwrap().hooks.auto_install());
+
+        std::fs::write(
+            &path,
+            r#"
+[hooks]
+auto_install = false
+"#,
+        )
+        .unwrap();
+        assert!(!MaestroConfigFile::load(&path).unwrap().hooks.auto_install());
+    }
+
     #[test]
     fn spawn_presets_default_to_builtins_plus_custom() {
         // Absent [spawn].presets → built-ins, with the implicit custom tile last.

crates/maestro-daemon/Cargo.toml

@@ -18,6 +18,7 @@ dirs.workspace = true
 futures-util.workspace = true
 libc.workspace = true
 maestro-config.workspace = true
+maestro-hooks.workspace = true
 maestro-names.workspace = true
 maestro-protocol.workspace = true
 serde.workspace = true

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/cli.rs

@@ -47,6 +47,20 @@ async fn run(config_override: Option<PathBuf>) -> Result<(), Box<dyn Error>> {
     let _ = default_state_dir();
     std::fs::create_dir_all(config.sidecar_dir())?;
     std::fs::create_dir_all(config.fifo_dir())?;
+
+    // First-start hook auto-install (#106). Best-effort, before serving: a
+    // session spawned right away should already carry the hook.
+    if config.hooks_auto_install {
+        match crate::hooks::auto_install(&config.home_dir) {
+            crate::hooks::AutoInstallOutcome::Installed => {
+                tracing::info!("installed claude-code activity hook (#106)");
+            }
+            crate::hooks::AutoInstallOutcome::Failed => {
+                tracing::warn!("hooks auto-install failed; run `maestro hooks install` manually");
+            }
+            _ => {}
+        }
+    }
     // No worktree dir to pre-create: without an override the worktrees live
     // under their base repos, created on demand (#56).
 

crates/maestro-daemon/src/config.rs

@@ -88,6 +88,10 @@ pub struct DaemonConfig {
     /// Project-discovery depth (#54): `[spawn].project_scan_depth`, default
     /// [`DEFAULT_PROJECT_SCAN_DEPTH`].
     pub project_scan_depth: u32,
+    /// Whether the daemon installs the Claude Code activity hook on first
+    /// start when none is present (#106): `[hooks].auto_install`, default
+    /// `true`.
+    pub hooks_auto_install: bool,
 }
 
 impl DaemonConfig {
@@ -197,6 +201,7 @@ pub fn load_config(path: &Path, env: EnvOverrides) -> Result<DaemonConfig, Confi
         spawn_presets,
         project_roots,
         project_scan_depth,
+        hooks_auto_install: file.hooks.auto_install(),
     })
 }
 
@@ -233,6 +238,27 @@ pub fn system_hostname() -> String {
 mod tests {
     use super::*;
 
+    #[test]
+    fn hooks_auto_install_defaults_true_and_is_opt_out() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("config.toml");
+
+        std::fs::write(&path, "").unwrap();
+        let cfg = load_config(&path, EnvOverrides::default()).unwrap();
+        assert!(cfg.hooks_auto_install);
+
+        std::fs::write(
+            &path,
+            r#"
+[hooks]
+auto_install = false
+"#,
+        )
+        .unwrap();
+        let cfg = load_config(&path, EnvOverrides::default()).unwrap();
+        assert!(!cfg.hooks_auto_install);
+    }
+
     #[test]
     fn empty_file_defaults_everything() {
         let dir = tempfile::tempdir().unwrap();

crates/maestro-daemon/src/hooks.rs

@@ -0,0 +1,76 @@
+//! First-start hook auto-install (#106).
+//!
+//! The daemon runs [`auto_install`] once at startup: when the user's
+//! `~/.claude/settings.json` carries no maestro hook yet (and
+//! `[hooks].auto_install` hasn't opted out — checked by the caller), the same
+//! installer `maestro hooks install` uses writes it. Best-effort: a failure is
+//! warned about and never blocks startup. The same global hook serves every
+//! Claude Code session on the machine, so nothing here targets repositories
+//! or worktrees.
+
+use std::path::Path;
+
+/// Install the Claude Code activity hook for `home_dir` unless one is already
+/// present or the home dir is unresolvable (`""`). Returns what happened.
+pub fn auto_install(home_dir: &str) -> AutoInstallOutcome {
+    if home_dir.is_empty() {
+        return AutoInstallOutcome::Skipped;
+    }
+    let settings = maestro_hooks::settings_path(Path::new(home_dir));
+    if maestro_hooks::is_installed(&settings) {
+        return AutoInstallOutcome::AlreadyPresent;
+    }
+    match maestro_hooks::install_for_home(Path::new(home_dir)) {
+        Ok(_) => AutoInstallOutcome::Installed,
+        Err(e) => {
+            tracing::warn!(error = %e, "hooks auto-install failed (non-fatal)");
+            AutoInstallOutcome::Failed
+        }
+    }
+}
+
+/// What [`auto_install`] did, so callers can log and tests can assert.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum AutoInstallOutcome {
+    Installed,
+    AlreadyPresent,
+    Failed,
+    Skipped,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Everything happens under a temp `$HOME`; the developer's own
+    /// `~/.claude/settings.json` is never read or written.
+    #[test]
+    fn installs_when_settings_carry_no_maestro_hook() {
+        let dir = tempfile::tempdir().unwrap();
+        let outcome = auto_install(dir.path().to_str().unwrap());
+        assert_eq!(outcome, AutoInstallOutcome::Installed);
+        let settings = dir.path().join(".claude/settings.json");
+        assert!(maestro_hooks::is_installed(&settings));
+    }
+
+    #[test]
+    fn second_start_is_a_noop() {
+        let dir = tempfile::tempdir().unwrap();
+        let home = dir.path().to_str().unwrap();
+        auto_install(home);
+        // A foreign edit between starts survives; presence alone skips.
+        let settings = dir.path().join(".claude/settings.json");
+        let before = std::fs::read_to_string(&settings).unwrap();
+        assert_eq!(auto_install(home), AutoInstallOutcome::AlreadyPresent);
+        assert_eq!(
+            before,
+            std::fs::read_to_string(&settings).unwrap(),
+            "already-installed settings must not be rewritten"
+        );
+    }
+
+    #[test]
+    fn unresolvable_home_is_skipped() {
+        assert_eq!(auto_install(""), AutoInstallOutcome::Skipped);
+    }
+}

crates/maestro-daemon/src/lib.rs

@@ -14,6 +14,7 @@ pub mod cli;
 pub mod config;
 pub mod dirlist;
 pub mod gitroot;
+pub mod hooks;
 pub mod local_api;
 pub mod panes;
 pub mod proc;
@@ -27,5 +28,6 @@ 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-hooks/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "maestro-hooks"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+publish.workspace = true
+description = "Agent activity hooks (#106): shipped script + idempotent settings.json installer"
+
+[dependencies]
+serde_json.workspace = true
+thiserror.workspace = true
+
+[dev-dependencies]
+tempfile.workspace = true

crates/maestro-hooks/src/lib.rs

@@ -0,0 +1,261 @@
+//! Claude Code activity hook (#106).
+//!
+//! Two halves, both shared by the `maestro hooks install` CLI subcommand and
+//! the daemon's first-start auto-install:
+//!
+//! * [`HOOK_SCRIPT`] — the hook itself, written to disk at install time. It
+//!   posts an [`ActivityState`](maestro-protocol) to
+//!   `$MAESTRO_URL/api/sessions/$MAESTRO_SESSION_ID/activity` (#105). The same
+//!   global hook runs in every Claude Code session on the machine, so it exits
+//!   0 and silently when either #103 env var is unset, and it never lets a
+//!   daemon failure escape (short curl timeout, status ignored).
+//! * [`install`] — idempotent registration of that script in
+//!   `~/.claude/settings.json`. Only entries maestro itself wrote (identified
+//!   by [`HOOK_MARKER`]) are touched; every other hook and every other field
+//!   in the file is preserved verbatim.
+
+use std::path::Path;
+
+/// Marker substring every maestro-installed hook command carries. The
+/// installer rewrites or removes exactly these entries — nothing else in the
+/// user's settings file.
+pub const HOOK_MARKER: &str = "maestro-claude-hook";
+
+/// File name of the installed script (carries [`HOOK_MARKER`]).
+pub const SCRIPT_FILE_NAME: &str = "maestro-claude-hook.sh";
+
+/// Claude Code hook event → `ActivityState` spelling posted to the daemon.
+///
+/// `PreToolUse` is what makes a session `Active` *during* a long tool call —
+/// `PostToolUse` alone only fires once it ends (#114). `StopFailure` is
+/// Claude Code's own name for "the turn ended on an API error"; there is no
+/// failure signal in `Stop` or `SessionEnd` payloads, so those never report
+/// `Error`.
+pub const HOOK_EVENTS: [(&str, &str); 5] = [
+    // Tool starting: the agent is demonstrably producing work, even if the
+    // tool itself runs for minutes.
+    ("PreToolUse", "Active"),
+    // Tool finished successfully.
+    ("PostToolUse", "Active"),
+    // Permission / idle notices: Claude is blocked on the user.
+    ("Notification", "AwaitingInput"),
+    // Turn ended: waiting on the user again.
+    ("Stop", "AwaitingInput"),
+    // Turn ended on an API error; the payload's `error` field is forwarded
+    // as the report's `detail` (#114).
+    ("StopFailure", "Error"),
+];
+
+/// The shipped hook script. POSIX sh so any `/bin/sh` works.
+pub const HOOK_SCRIPT: &str = r#"#!/bin/sh
+# Installed by maestro (#106): report Claude Code activity to its daemon.
+# Exits silently unless running inside a session maestro spawned (#103 env).
+# Event -> ActivityState mapping:
+#   PreToolUse    -> Active         (tool started; agent is working)
+#   PostToolUse   -> Active         (tool finished)
+#   Notification  -> AwaitingInput  (permission / idle notice)
+#   Stop          -> AwaitingInput  (turn ended; waiting on the user)
+#   StopFailure   -> Error          (turn ended on an API error)
+state="$1"
+[ -n "$MAESTRO_SESSION_ID" ] || exit 0
+[ -n "$MAESTRO_URL" ] || exit 0
+payload=$(cat 2>/dev/null) || true
+body="{\"state\":\"$state\"}"
+# StopFailure payloads carry the outcome in "error". Only a plain enum
+# spelling is reported as the detail — anything else (quotes, backslashes,
+# prose) would risk breaking the JSON body, and the spelling already names
+# the failure.
+detail=$(printf '%s' "$payload" | sed -n 's/.*"error"[[:space:]]*:[[:space:]]*"\([a-z_]*\)".*/\1/p')
+if [ -n "$detail" ]; then
+    body="{\"state\":\"$state\",\"detail\":\"$detail\"}"
+fi
+curl -fsS --connect-timeout 1 --max-time 2 \
+    -H 'Content-Type: application/json' \
+    -d "$body" \
+    "$MAESTRO_URL/api/sessions/$MAESTRO_SESSION_ID/activity" \
+    >/dev/null 2>&1 || true
+exit 0
+"#;
+
+#[derive(Debug, thiserror::Error)]
+pub enum HooksError {
+    #[error("io: {0}")]
+    Io(#[from] std::io::Error),
+    #[error("json: {0}")]
+    Json(#[from] serde_json::Error),
+}
+
+/// Where the script is written for a given home directory:
+/// `<home>/.local/share/maestro/maestro-claude-hook.sh`.
+pub fn script_path(home: &Path) -> std::path::PathBuf {
+    home.join(".local/share/maestro").join(SCRIPT_FILE_NAME)
+}
+
+/// The user-level Claude Code settings file for a given home directory.
+pub fn settings_path(home: &Path) -> std::path::PathBuf {
+    home.join(".claude/settings.json")
+}
+
+/// Whether `settings_path` already carries a maestro hook (#106).
+pub fn is_installed(settings_path: &Path) -> bool {
+    settings_json(settings_path)
+        .map(|root| find_maestro_commands(&root).next().is_some())
+        .unwrap_or(false)
+}
+
+/// Write [`HOOK_SCRIPT`] to `path`, executable. Parent directories are
+/// created as needed — this is always under the caller's chosen data dir,
+/// never inside a repository.
+pub fn write_script(path: &Path) -> Result<(), HooksError> {
+    if let Some(dir) = path.parent() {
+        std::fs::create_dir_all(dir)?;
+    }
+    std::fs::write(path, HOOK_SCRIPT)?;
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::PermissionsExt;
+        let mut perms = std::fs::metadata(path)?.permissions();
+        perms.set_mode(0o755);
+        std::fs::set_permissions(path, perms)?;
+    }
+    Ok(())
+}
+
+/// Install the full hook setup for `home`: write the script under
+/// `.local/share/maestro/` and register it in `.claude/settings.json`.
+/// Idempotent; returns whether anything changed on disk.
+pub fn install_for_home(home: &Path) -> Result<bool, HooksError> {
+    let script = script_path(home);
+    let settings = settings_path(home);
+    let before = std::fs::read_to_string(&settings).unwrap_or_default();
+    write_script(&script)?;
+    install(&settings, &script)?;
+    let after = std::fs::read_to_string(&settings).unwrap_or_default();
+    Ok(before != after)
+}
+
+/// Register the hook command for every event in [`HOOK_EVENTS`] in the
+/// settings file at `settings_path`. Idempotent: existing maestro entries are
+/// updated in place wherever they sit, stale ones removed, duplicates folded;
+/// every non-maestro entry and every unrelated top-level field survives.
+///
+/// Never inspects or writes anywhere but `settings_path` — callers pass the
+/// user's real home or a temp one in tests, never a repository.
+pub fn install(settings_path: &Path, script_path: &Path) -> Result<(), HooksError> {
+    let mut root = settings_json(settings_path)?;
+
+    // Strip every maestro entry wherever it currently sits, so an older
+    // install under different events is replaced rather than duplicated.
+    strip_maestro_entries(&mut root);
+
+    let hooks = ensure_object(&mut root, "hooks");
+    for (event, state) in HOOK_EVENTS {
+        let groups = ensure_array(hooks, event);
+        let mut group = serde_json::Map::new();
+        group.insert(
+            "hooks".into(),
+            serde_json::json!([{
+                "type": "command",
+                "command": format!("{} {state}", script_path.display()),
+            }]),
+        );
+        groups.push(serde_json::Value::Object(group));
+    }
+
+    write_atomic(settings_path, &root)
+}
+
+fn settings_json(path: &Path) -> Result<serde_json::Value, HooksError> {
+    match std::fs::read_to_string(path) {
+        Ok(s) => Ok(serde_json::from_str(&s)?),
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+            Ok(serde_json::Value::Object(Default::default()))
+        }
+        Err(e) => Err(e.into()),
+    }
+}
+
+/// Every command string in the file belonging to a maestro hook.
+fn find_maestro_commands(root: &serde_json::Value) -> impl Iterator<Item = &str> {
+    root.get("hooks")
+        .and_then(|h| h.as_object())
+        .into_iter()
+        .flat_map(|events| events.values())
+        .filter_map(|groups| groups.as_array())
+        .flatten()
+        .filter_map(|group| group.get("hooks"))
+        .filter_map(|inner| inner.as_array())
+        .flatten()
+        .filter_map(|hook| hook.get("command"))
+        .filter_map(|cmd| cmd.as_str())
+        .filter(|cmd| cmd.contains(HOOK_MARKER))
+}
+
+/// Remove all maestro inner-hook entries from every event group, dropping
+/// groups left empty. Non-maestro siblings survive untouched.
+fn strip_maestro_entries(root: &mut serde_json::Value) {
+    let Some(events) = root.get_mut("hooks").and_then(|h| h.as_object_mut()) else {
+        return;
+    };
+    for (_, groups) in events.iter_mut() {
+        let Some(list) = groups.as_array_mut() else {
+            continue;
+        };
+        for group in list.iter_mut() {
+            let Some(inner) = group.get_mut("hooks").and_then(|h| h.as_array_mut()) else {
+                continue;
+            };
+            inner.retain(|hook| {
+                !hook
+                    .get("command")
+                    .and_then(|c| c.as_str())
+                    .is_some_and(|c| c.contains(HOOK_MARKER))
+            });
+        }
+        list.retain(|group| {
+            group
+                .get("hooks")
+                .and_then(|h| h.as_array())
+                .is_some_and(|h| !h.is_empty())
+        });
+    }
+    // An event left with no groups at all (ours emptied it, or the user's
+    // file carried an empty list) reads better gone.
+    events.retain(|_, groups| groups.as_array().is_some_and(|g| !g.is_empty()));
+}
+
+fn ensure_object<'a>(
+    root: &'a mut serde_json::Value,
+    key: &str,
+) -> &'a mut serde_json::Map<String, serde_json::Value> {
+    if !root.get(key).map(|v| v.is_object()).unwrap_or(false) {
+        root[key] = serde_json::json!({});
+    }
+    root.get_mut(key).unwrap().as_object_mut().unwrap()
+}
+
+fn ensure_array<'a>(
+    parent: &'a mut serde_json::Map<String, serde_json::Value>,
+    key: &str,
+) -> &'a mut Vec<serde_json::Value> {
+    let v = parent
+        .entry(key.to_string())
+        .or_insert_with(|| serde_json::json!([]));
+    if !v.is_array() {
+        *v = serde_json::json!([]);
+    }
+    v.as_array_mut().unwrap()
+}
+
+fn write_atomic(path: &Path, value: &serde_json::Value) -> Result<(), HooksError> {
+    if let Some(dir) = path.parent() {
+        std::fs::create_dir_all(dir)?;
+    }
+    let tmp = path.with_extension("json.maestro-tmp");
+    std::fs::write(&tmp, serde_json::to_string_pretty(value)? + "\n")?;
+    std::fs::rename(&tmp, path)?;
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests;

crates/maestro-hooks/src/tests.rs

@@ -0,0 +1,340 @@
+//! Tests for the shipped hook script (run as a real `/bin/sh` subprocess)
+//! and the settings-file installer. Everything happens under a temp `$HOME`;
+//! the developer's own `~/.claude/settings.json` is never touched.
+
+use std::io::{Read, Write};
+use std::process::{Command, Stdio};
+
+use super::*;
+
+fn sh(script: &str, args: &[&str], envs: &[(&str, &str)], payload: &[u8]) -> std::process::Output {
+    let mut cmd = Command::new("/bin/sh");
+    // `sh -c CMD NAME ARGS…`: NAME fills $0, so the real arguments start at $1.
+    let mut argv = vec!["-c", script, HOOK_MARKER];
+    argv.extend(args.iter().copied());
+    cmd.args(&argv);
+    // A clean env: only what the test passes, plus PATH so curl can spawn.
+    cmd.env_clear();
+    cmd.env("PATH", std::env::var("PATH").unwrap_or_default());
+    for (k, v) in envs {
+        cmd.env(k, v);
+    }
+    let mut child = cmd
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .spawn()
+        .expect("spawn /bin/sh");
+    // Feed a Claude-Code-style payload on stdin; the script must drain it.
+    child.stdin.take().unwrap().write_all(payload).ok();
+    child.wait_with_output().expect("wait")
+}
+
+const SESSION: &str = "01234567-89ab-cdef-0123-456789abcdef";
+
+/// Accept exactly one HTTP request on a throwaway listener, answer 204, and
+/// hand back the raw request text. Runs `run` with the base URL once the
+/// acceptor thread is in place.
+fn capture_request(run: impl FnOnce(String) -> std::process::Output) -> String {
+    let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+    let addr = listener.local_addr().unwrap();
+    let handle = std::thread::spawn(move || {
+        let (mut sock, _) = listener.accept().unwrap();
+        let mut buf = [0u8; 4096];
+        let n = sock.read(&mut buf).unwrap();
+        sock.write_all(b"HTTP/1.1 204 No Content\r\nContent-Length: 0\r\n\r\n")
+            .unwrap();
+        String::from_utf8_lossy(&buf[..n]).to_string()
+    });
+    let out = run(format!("http://{addr}"));
+    assert!(out.status.success(), "{:?}", out.stderr);
+    handle.join().unwrap()
+}
+
+fn session_envs(url: &str) -> Vec<(&'static str, String)> {
+    vec![
+        ("MAESTRO_URL", url.to_string()),
+        ("MAESTRO_SESSION_ID", SESSION.to_string()),
+    ]
+}
+
+fn owned_envs<'a>(envs: &'a [(&'static str, String)]) -> Vec<(&'static str, &'a str)> {
+    envs.iter().map(|(k, v)| (*k, v.as_str())).collect()
+}
+
+/// The hook must be invisible outside maestro: no env vars (#103 unset) ⇒
+/// exit 0, no output, stdin drained.
+#[test]
+fn script_exits_silently_without_maestro_env() {
+    for (event, _) in HOOK_EVENTS {
+        let out = sh(HOOK_SCRIPT, &[event], &[], br#"{"session_id":"x"}"#);
+        assert!(out.status.success(), "{event:?}: {:?}", out.stderr);
+        assert!(out.stdout.is_empty(), "{event:?}: {:?}", out.stdout);
+        assert!(out.stderr.is_empty(), "{event:?}: {:?}", out.stderr);
+    }
+}
+
+/// With both env vars set the script POSTs the mapped state to the daemon's
+/// activity endpoint and still exits 0.
+#[test]
+fn script_posts_mapped_state_to_the_activity_endpoint() {
+    // The installer bakes the mapped state into the command's argv; the
+    // script relays whatever it is given.
+    let request = capture_request(|url| {
+        sh(
+            HOOK_SCRIPT,
+            &["Active"],
+            &owned_envs(&session_envs(&url)),
+            br#"{"session_id":"x"}"#,
+        )
+    });
+    assert!(request.starts_with("POST "), "{request}");
+    assert!(
+        request.contains(&format!("/api/sessions/{SESSION}/activity")),
+        "{request}"
+    );
+    assert!(request.contains(r#""state":"Active""#), "{request}");
+}
+
+/// Every event maps to the state the contract specifies.
+#[test]
+fn event_state_mapping_matches_the_contract() {
+    let map: std::collections::HashMap<&str, &str> = HOOK_EVENTS.into_iter().collect();
+    assert_eq!(map["PreToolUse"], "Active");
+    assert_eq!(map["PostToolUse"], "Active");
+    assert_eq!(map["Notification"], "AwaitingInput");
+    assert_eq!(map["Stop"], "AwaitingInput");
+    assert_eq!(map["StopFailure"], "Error");
+}
+
+/// A `StopFailure` payload names the API error that ended the turn; the
+/// script reports its `error` spelling as the report's `detail` so a card
+/// can show why (#114).
+#[test]
+fn script_reports_the_error_spelling_as_detail() {
+    let request = capture_request(|url| {
+        sh(
+            HOOK_SCRIPT,
+            &["Error"],
+            &owned_envs(&session_envs(&url)),
+            br#"{"session_id":"abc","hook_event_name":"StopFailure","error":"rate_limit","error_details":"429 Too Many Requests"}"#,
+        )
+    });
+    assert!(request.contains(r#""state":"Error""#), "{request}");
+    assert!(request.contains(r#""detail":"rate_limit""#), "{request}");
+}
+
+/// An `error` value that is not a plain enum spelling (or a payload without
+/// one at all) must never produce a broken body — no detail is posted.
+#[test]
+fn script_posts_no_detail_for_an_unexpected_error_value() {
+    let request = capture_request(|url| {
+        sh(
+            HOOK_SCRIPT,
+            &["Error"],
+            &owned_envs(&session_envs(&url)),
+            br#"{"hook_event_name":"StopFailure","error":"429 \"Too Many\" \\ requests"}"#,
+        )
+    });
+    assert!(request.contains(r#""state":"Error""#), "{request}");
+    assert!(!request.contains("detail"), "{request}");
+}
+
+/// Events whose payload carries no error fields post a bare body — never an
+/// empty or stale detail.
+#[test]
+fn script_posts_no_detail_for_events_without_an_error() {
+    let request = capture_request(|url| {
+        sh(
+            HOOK_SCRIPT,
+            &["AwaitingInput"],
+            &owned_envs(&session_envs(&url)),
+            br#"{"hook_event_name":"Stop","stop_hook_active":false}"#,
+        )
+    });
+    assert!(request.contains(r#""state":"AwaitingInput""#), "{request}");
+    assert!(!request.contains("detail"), "{request}");
+}
+
+/// A dead or unreachable daemon must never fail — or slow down — a session.
+#[test]
+fn script_ignores_daemon_failure() {
+    // Port 1 on localhost is closed in practice; curl fails fast.
+    let out = sh(
+        HOOK_SCRIPT,
+        &["Stop"],
+        &[
+            ("MAESTRO_URL", "http://127.0.0.1:1"),
+            ("MAESTRO_SESSION_ID", "00000000-0000-0000-0000-000000000000"),
+        ],
+        br#"{"session_id":"x"}"#,
+    );
+    assert!(out.status.success());
+}
+
+// --- installer -------------------------------------------------------------
+
+fn installed_commands(settings: &Path) -> Vec<String> {
+    serde_json::from_str::<serde_json::Value>(&std::fs::read_to_string(settings).unwrap())
+        .unwrap()
+        .get("hooks")
+        .and_then(|h| h.as_object())
+        .into_iter()
+        .flat_map(|events| events.values())
+        .filter_map(|g| g.as_array())
+        .flatten()
+        .filter_map(|g| g.get("hooks"))
+        .filter_map(|h| h.as_array())
+        .flatten()
+        .filter_map(|h| h.get("command"))
+        .filter_map(|c| c.as_str())
+        .map(str::to_string)
+        .collect()
+}
+
+/// Fresh install registers exactly one command per event, all pointing at the
+/// script with the right state argument.
+#[test]
+fn fresh_install_registers_every_event() {
+    let dir = tempfile::tempdir().unwrap();
+    let home = dir.path();
+    write_script(&script_path(home)).unwrap();
+    install(&settings_path(home), &script_path(home)).unwrap();
+
+    let cmds = installed_commands(&settings_path(home));
+    assert_eq!(cmds.len(), HOOK_EVENTS.len());
+    let joined = cmds.join("\n");
+    assert_eq!(joined.matches("Active").count(), 2);
+    assert!(joined.contains("Error"));
+    assert_eq!(joined.matches("AwaitingInput").count(), 2);
+    for cmd in &cmds {
+        assert!(cmd.contains(SCRIPT_FILE_NAME));
+    }
+}
+
+/// Running install twice changes nothing further — no duplicates.
+#[test]
+fn install_is_idempotent() {
+    let dir = tempfile::tempdir().unwrap();
+    let home = dir.path();
+    install(&settings_path(home), &script_path(home)).unwrap();
+    let once = std::fs::read_to_string(settings_path(home)).unwrap();
+
+    install(&settings_path(home), &script_path(home)).unwrap();
+    let twice = std::fs::read_to_string(settings_path(home)).unwrap();
+    assert_eq!(once, twice);
+    assert_eq!(
+        installed_commands(&settings_path(home)).len(),
+        HOOK_EVENTS.len()
+    );
+}
+
+/// An older maestro entry under the same events is updated in place, not
+/// duplicated next to the new one.
+#[test]
+fn install_updates_an_older_maestro_hook_in_place() {
+    let dir = tempfile::tempdir().unwrap();
+    let settings = dir.path().join(".claude/settings.json");
+    std::fs::create_dir_all(settings.parent().unwrap()).unwrap();
+    let old_script = "/old/location/maestro-claude-hook.sh Active";
+    std::fs::write(
+        &settings,
+        serde_json::json!({
+            "model": "sonnet",
+            "hooks": {
+                "Stop": [{"hooks": [{"type": "command", "command": old_script}]}],
+            },
+        })
+        .to_string(),
+    )
+    .unwrap();
+
+    let new_script = dir
+        .path()
+        .join(".local/share/maestro")
+        .join(SCRIPT_FILE_NAME);
+    install(&settings, &new_script).unwrap();
+
+    let cmds = installed_commands(&settings);
+    assert_eq!(
+        cmds.len(),
+        HOOK_EVENTS.len(),
+        "old entry replaced, not kept: {cmds:?}"
+    );
+    assert!(!cmds.iter().any(|c| c == old_script));
+    let root: serde_json::Value =
+        serde_json::from_str(&std::fs::read_to_string(&settings).unwrap()).unwrap();
+    assert_eq!(root["model"], "sonnet", "unrelated fields survive");
+}
+
+/// A stale maestro entry under an event we no longer use is cleaned up too;
+/// the user's own hooks are never touched.
+#[test]
+fn install_preserves_foreign_hooks_and_removes_stale_maestro_ones() {
+    let dir = tempfile::tempdir().unwrap();
+    let settings = dir.path().join(".claude/settings.json");
+    std::fs::create_dir_all(settings.parent().unwrap()).unwrap();
+    std::fs::write(
+        &settings,
+        serde_json::json!({
+            "hooks": {
+                "PreToolUse": [{"matcher": "Bash", "hooks": [
+                    {"type": "command", "command": "my-own-linter.sh"},
+                ]}],
+                "UserPromptSubmit": [{"hooks": [
+                    {"type": "command", "command": "/old/maestro-claude-hook.sh Active"},
+                ]}],
+            },
+        })
+        .to_string(),
+    )
+    .unwrap();
+
+    let new_script = dir
+        .path()
+        .join(".local/share/maestro")
+        .join(SCRIPT_FILE_NAME);
+    install(&settings, &new_script).unwrap();
+
+    let root: serde_json::Value =
+        serde_json::from_str(&std::fs::read_to_string(&settings).unwrap()).unwrap();
+    // Foreign hook untouched.
+    let pre = root["hooks"]["PreToolUse"].as_array().unwrap();
+    assert_eq!(pre[0]["hooks"][0]["command"], "my-own-linter.sh");
+    // Stale maestro event gone.
+    assert!(root["hooks"].get("UserPromptSubmit").is_none());
+    // Our events present, nothing else maestro-related anywhere.
+    let cmds = installed_commands(&settings);
+    assert_eq!(
+        cmds.iter().filter(|c| c.contains(HOOK_MARKER)).count(),
+        HOOK_EVENTS.len(),
+        "{cmds:?}"
+    );
+}
+
+/// `is_installed` flips only once a maestro entry exists.
+#[test]
+fn is_installed_tracks_reality() {
+    let dir = tempfile::tempdir().unwrap();
+    let home = dir.path();
+    assert!(!is_installed(&settings_path(home)));
+    install_for_home(home).unwrap();
+    assert!(is_installed(&settings_path(home)));
+}
+
+/// The full home-level install leaves the script executable.
+#[test]
+fn install_for_home_writes_an_executable_script() {
+    let dir = tempfile::tempdir().unwrap();
+    let home = dir.path();
+    install_for_home(home).unwrap();
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::PermissionsExt;
+        let mode = std::fs::metadata(script_path(home))
+            .unwrap()
+            .permissions()
+            .mode();
+        assert_eq!(mode & 0o111, 0o111);
+    }
+}

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.
@@ -117,7 +123,12 @@ pub enum SessionStatus {
 /// Inferred session activity, orthogonal to [`SessionStatus`] (NAP-340). The
 /// daemon derives this from tmux pane state, PTY quiescence, and `/proc`
 /// inspection — never by parsing the terminal byte stream.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+///
+/// Deserializes leniently (#110, SIDEBAR.md Precedence): an unknown variant
+/// from a newer daemon becomes [`ActivityState::Idle`] instead of failing the
+/// whole `Session`. Non-string values still error — only variant names are
+/// forgiven.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Default)]
 pub enum ActivityState {
     /// Producing output, or the foreground process is busy.
     Active,
@@ -131,6 +142,61 @@ pub enum ActivityState {
     Error,
 }
 
+impl<'de> Deserialize<'de> for ActivityState {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let s = String::deserialize(deserializer)?;
+        Ok(match s.as_str() {
+            "Active" => Self::Active,
+            "AwaitingInput" => Self::AwaitingInput,
+            "Error" => Self::Error,
+            // Known or not, any other spelling degrades to the neutral default
+            _ => Self::Idle,
+        })
+    }
+}
+
+/// 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 {
+    /// Parsed strictly, unlike [`ActivityState`]'s own lenient `Deserialize`
+    /// (#110). Leniency exists so a client reading a *newer* daemon's snapshot
+    /// degrades an unrecognised state to `Idle` instead of dropping the whole
+    /// session. A report travelling the other way has no such excuse: an agent
+    /// posting a state this daemon does not know is a bug in the agent, and
+    /// silently recording it as `Idle` would hide that behind a 204.
+    #[serde(deserialize_with = "strict_activity_state")]
+    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>,
+}
+
+/// Reject an unknown [`ActivityState`] spelling instead of degrading it to
+/// `Idle`. Used only by [`ReportActivityRequest`]; see the note there.
+fn strict_activity_state<'de, D>(deserializer: D) -> Result<ActivityState, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    use serde::de::Error as _;
+    let s = String::deserialize(deserializer)?;
+    match s.as_str() {
+        "Active" => Ok(ActivityState::Active),
+        "Idle" => Ok(ActivityState::Idle),
+        "AwaitingInput" => Ok(ActivityState::AwaitingInput),
+        "Error" => Ok(ActivityState::Error),
+        other => Err(D::Error::unknown_variant(
+            other,
+            &["Active", "Idle", "AwaitingInput", "Error"],
+        )),
+    }
+}
+
 /// Optional worktree request attached to a `CreateSessionRequest`.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct WorktreeSpec {
@@ -544,6 +610,7 @@ 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 +706,78 @@ mod tests {
         assert_eq!(ActivityState::default(), ActivityState::Idle);
     }
 
+    // #110 / SIDEBAR.md Precedence: a future daemon may send an
+    // `ActivityState` variant this client does not know; it must degrade to
+    // the neutral default instead of failing the whole `Session`.
+    #[test]
+    fn unknown_activity_state_variant_deserializes_to_idle() {
+        assert_eq!(
+            serde_json::from_value::<ActivityState>(serde_json::json!("Teleported")).unwrap(),
+            ActivityState::Idle
+        );
+
+        let mut future = serde_json::to_value(sample_session()).unwrap();
+        future["activity"] = serde_json::json!("QuantumEntangled");
+        let s: Session = serde_json::from_value(future).unwrap();
+        assert_eq!(s.activity, ActivityState::Idle);
+    }
+
+    #[test]
+    fn non_string_activity_payload_still_fails_deserialization() {
+        let json = serde_json::json!({
+            "id": Uuid::from_u128(1),
+            "cwd": "/tmp",
+            "command": "bash",
+            "status": "Running",
+            "created_at": "2026-05-24T12:00:00Z",
+            "activity": 42,
+        });
+        assert!(serde_json::from_value::<Session>(json).is_err());
+    }
+
+    #[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

@@ -160,12 +160,26 @@ impl Sidebar {
             spans.push(Span::styled(" ", base));
             spans.push(Span::styled(format_state_duration(d), dot_style));
         }
+        // The trailing fragments are measured up front so a long detail
+        // truncates ahead of them instead of pushing them off the line
+        // (docs/SIDEBAR.md).
+        let mut tail: Vec<Span<'static>> = Vec::new();
         if let Some(b) = blocked {
-            spans.push(Span::styled(format!(" ⚠ {b}"), muted));
+            tail.push(Span::styled(format!(" ⚠ {b}"), muted));
         }
         if dead {
-            spans.push(Span::styled(" exited", muted));
+            tail.push(Span::styled(" exited", muted));
         }
+        if let Some(detail) = s.activity_detail.as_deref().filter(|d| !d.is_empty()) {
+            let head_w: usize = spans.iter().map(|sp| sp.width()).sum();
+            let tail_w: usize = tail.iter().map(|sp| sp.width()).sum();
+            let budget = width.saturating_sub(head_w + tail_w);
+            spans.extend(fit_to_width(
+                vec![Span::styled(format!(" {detail}"), muted)],
+                budget,
+            ));
+        }
+        spans.extend(tail);
         Some(fit_to_width(spans, width))
     }
 }
@@ -271,6 +285,7 @@ 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,
@@ -530,6 +545,93 @@ mod tests {
         );
     }
 
+    /// #112: the agent-reported detail sits right after the duration on the
+    /// meta line, in the muted style (docs/SIDEBAR.md).
+    #[test]
+    fn errored_session_shows_its_reported_detail_on_the_meta_line() {
+        let mut s = session_named(1, "claude");
+        s.activity = ActivityState::Error;
+        s.activity_since = Utc::now() - Duration::seconds(90);
+        s.activity_detail = Some("cargo build failed".into());
+        let sb = Sidebar {
+            sessions: vec![s],
+            highlight: None,
+            attached: Default::default(),
+        };
+        let all = rows(&draw(&sb, 40, 8));
+        let idx = all.iter().position(|l| l.contains("claude")).unwrap();
+        let meta = &all[idx + 1];
+        assert!(meta.contains('●'), "meta line carries the dot: {meta:?}");
+        assert!(
+            meta.contains("1m"),
+            "duration precedes the detail: {meta:?}"
+        );
+        assert!(
+            meta.contains("cargo build failed"),
+            "reported detail must render: {meta:?}"
+        );
+    }
+
+    /// A fresh Error report earns line 2 on its own (contract rule 4), with
+    /// the detail even though no duration has accrued.
+    #[test]
+    fn fresh_errored_session_earns_a_meta_line_for_its_detail() {
+        let mut s = session_named(1, "claude");
+        s.activity = ActivityState::Error;
+        s.activity_since = Utc::now() - Duration::seconds(5);
+        s.activity_detail = Some("waiting for approval".into());
+        let sb = Sidebar {
+            sessions: vec![s],
+            highlight: None,
+            attached: Default::default(),
+        };
+        let all = rows(&draw(&sb, 40, 8));
+        let idx = all.iter().position(|l| l.contains("claude")).unwrap();
+        assert!(
+            all[idx + 1].contains("waiting for approval"),
+            "detail must render on a fresh report's own meta line: {:?}",
+            all[idx + 1]
+        );
+    }
+
+    /// #112 via docs/SIDEBAR.md: the detail truncates with `…` ahead of the
+    /// trailing fragments — a long detail never pushes blocked work or
+    /// `exited` off the line, and never wraps.
+    #[test]
+    fn long_detail_yields_to_blocked_work_and_exited() {
+        let mut s = session_named(1, "claude");
+        s.status = SessionStatus::Dead;
+        s.activity_since = Utc::now() - Duration::seconds(5);
+        s.activity_detail =
+            Some("an absurdly long error message that cannot possibly fit ".repeat(3));
+        s.blocked_work = Some(InTheWay {
+            unpushed_commits: 2,
+            ..InTheWay::default()
+        });
+        let sb = Sidebar {
+            sessions: vec![s],
+            highlight: None,
+            attached: Default::default(),
+        };
+        let all = rows(&draw(&sb, 40, 8));
+        let idx = all.iter().position(|l| l.contains("claude")).unwrap();
+        let meta = content(&all[idx + 1]);
+        assert!(meta.contains('…'), "truncated detail ends with …: {meta:?}");
+        assert!(
+            meta.contains("2 unpushed commits"),
+            "blocked work stays on the line: {meta:?}"
+        );
+        assert!(
+            meta.contains("exited"),
+            "exited fragment stays on the line: {meta:?}"
+        );
+        assert!(
+            content(&all[idx + 2]).trim().is_empty(),
+            "meta line must not wrap: {:?}",
+            all[idx + 2]
+        );
+    }
+
     /// docs/SIDEBAR.md: a payload predating #102 carries an epoch
     /// `activity_since` and so contributes no duration — a card must not claim
     /// the session has been idle since 1970. The dot still renders when some
@@ -579,6 +681,7 @@ 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,

crates/maestro/src/main.rs

@@ -55,6 +55,9 @@ enum Command {
     /// Kill a session by ID.
     #[cfg(feature = "cli")]
     Kill(maestro_cli::cli::KillArgs),
+    /// Install or update the agent activity hook (#106).
+    #[cfg(feature = "cli")]
+    Hooks(maestro_cli::cli::HooksArgs),
     /// Per-host daemon: tmux wrapper + session API.
     #[cfg(feature = "daemon")]
     Daemon {
@@ -123,6 +126,15 @@ async fn dispatch(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
             )
             .await?
         }
+        #[cfg(feature = "cli")]
+        Command::Hooks(args) => {
+            maestro_cli::dispatch(
+                maestro_cli::cli::Command::Hooks(args),
+                cli.config,
+                cli.daemon_url,
+            )
+            .await?
+        }
         #[cfg(feature = "daemon")]
         Command::Daemon { cmd } => maestro_daemon::cli::dispatch(cli.config, cmd).await?,
         #[cfg(feature = "tui")]
@@ -157,7 +169,11 @@ fn init_tracing(cli: &Cli) {
 fn is_cli_subcommand(command: &Command) -> bool {
     matches!(
         command,
-        Command::List(_) | Command::Spawn(_) | Command::Attach(_) | Command::Kill(_)
+        Command::List(_)
+            | Command::Spawn(_)
+            | Command::Attach(_)
+            | Command::Kill(_)
+            | Command::Hooks(_)
     )
 }
 

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 +52,14 @@ 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. The detail
+truncates with `…` ahead of the trailing fragments — a long detail never
+pushes `<blocked work>`, `exited`, or `<command>` off the line, and neither
+client ever lets it wrap.
+
 `<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).
@@ -94,11 +102,32 @@ 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`.
+
+Two rulings specific to a reported `Error` (#114):
+
+- **The five-minute TTL applies to `Error` like to any other state.** A
+  session that failed is still failed after five minutes, but a card pinned
+  red forever lies once the session moves on; when the report lapses the card
+  returns to the daemon's own inference — usually `Idle` for a session that
+  has gone quiet.
+- **Only a new report clears an `Error` before its TTL.** The next
+  `ActivityState` of any kind replaces it (with #106's hook that is typically
+  `PreToolUse` → `Active` when the user prompts again). PTY output and
+  process changes do not clear it — only the agent itself may withdraw what
+  the agent itself reported. Re-reporting `Error` extends the TTL rather than
+  stacking reports.
 
 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)
 until the daemon sends a variant it does know. Rust clients deserialize
 activity leniently for the same reason.
+
+Leniency runs one way only. It covers a client *reading* a newer daemon, where
+dropping the whole session would be worse than dimming one card. A report
+arriving at `POST /api/sessions/:id/activity` is parsed strictly and an unknown
+state is a 4xx: an agent posting a state the daemon does not know is a bug in
+the agent, and answering 204 would hide it.

web/src/catalog.test.ts

@@ -105,6 +105,38 @@ describe("blocked-work indicator (#83)", () => {
   });
 });
 
+describe("agent-reported detail (#112)", () => {
+  const ago = (ms: number): string => new Date(Date.now() - ms).toISOString();
+
+  it("renders the reported detail right after the duration on line 2", () => {
+    const s = session({
+      id: "aaaaaaaa-0000-0000-0000-000000000020",
+      status: "Dead",
+      activity: "Error",
+      activity_since: ago(90_000),
+      activity_detail: "cargo build failed",
+    });
+    const el = renderSidebar(storeWith([s])).el;
+    const line2 = rowFor(el, s.id)?.querySelector<HTMLElement>(".card-line2");
+    expect(line2?.querySelector(".activity-detail")?.textContent).toBe("cargo build failed");
+    const order = [...line2!.children].map((c) => c.className);
+    expect(order.indexOf("activity-detail")).toBeGreaterThan(order.indexOf("activity-duration"));
+    expect(order.indexOf("activity-detail")).toBeLessThan(
+      order.findIndex((c) => c.startsWith("card-flag")),
+    );
+  });
+
+  it("omits the detail when no report is in force", () => {
+    const s = session({
+      id: "aaaaaaaa-0000-0000-0000-000000000021",
+      activity: "Active",
+      activity_since: ago(90_000),
+    });
+    const el = renderSidebar(storeWith([s])).el;
+    expect(rowFor(el, s.id)?.querySelector(".activity-detail")).toBeNull();
+  });
+});
+
 describe("session cards (#107)", () => {
   const ago = (ms: number): string => new Date(Date.now() - ms).toISOString();
 

web/src/catalog.ts

@@ -42,6 +42,9 @@ function metaLine(s: Session, now: number): string {
   const fragments = [
     `<span class="activity-dot" role="img" title="${escapeHtml(stateWord)}" aria-label="${escapeHtml(stateWord)}"></span>`,
     duration ? `<span class="activity-duration">${duration}</span>` : "",
+    s.activity_detail
+      ? `<span class="activity-detail">${escapeHtml(s.activity_detail)}</span>`
+      : "",
     blockers
       ? `<span class="card-flag leaf-blocked" title="Kill will be refused unless forced">⚠ ${escapeHtml(formatInTheWay(blockers))}</span>`
       : "",

web/src/styles.css

@@ -851,6 +851,17 @@ textarea {
   flex: 0 0 auto;
 }
 
+/* Agent-reported detail (#112): sits after the duration, shrinks with an
+   ellipsis so it never pushes the trailing fragments off the line. */
+.activity-detail {
+  color: var(--faint);
+  flex: 0 1 auto;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
 /* Blocked-work / exited / command fragments recede: the ⚠ glyph already
    signals severity and "exited" dims with the row. */
 .card-flag {

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