feat(maestro): add POST /api/sessions/:id/resume

A dormant session comes back under the same uuid and sidecar, so its
name, created_at, env and worktree ownership survive; only the tmux
session and the PTY bridge are new. The command is the matching preset's
resume_command with {agent_session} filled in, falling back to the
command the session was spawned with.

Refused with 409 for a session that is already running or whose cwd is
gone. Attaching to a session with no bridge now answers 409
not_attachable instead of 404 unknown_session, so a client can tell a
resumable session from one that does not exist.

CLAUDE.md

@@ -14,6 +14,7 @@ maestro daemon            tmux wrapper + session API + SPA on 127.0.0.1:7070
   ├── DELETE   /api/sessions/:id    kill
   ├── POST     /api/sessions/:id/input  type text into a session
   ├── POST     /api/sessions/:id/prompt an agent reports its first prompt (#131)
+  ├── POST     /api/sessions/:id/resume respawn a dormant session, same id (#166)
   ├── GET      /api/info            hostname, version, home dir, spawn presets
   ├── GET      /api/dirs            directory listing (spawn autocomplete)
   ├── GET      /api/recent-dirs     spawn history

crates/maestro-cli/tests/against_mock_daemon.rs

@@ -28,6 +28,7 @@ async fn info_carries_version_and_presets() {
             id: "claude".into(),
             label: "Claude".into(),
             command: "claude".into(),
+            resume_command: None,
         }],
         ..Default::default()
     });

crates/maestro-config/src/lib.rs

@@ -313,16 +313,19 @@ pub fn builtin_presets() -> Vec<SpawnPreset> {
             id: "claude".into(),
             label: "Claude".into(),
             command: "claude".into(),
+            resume_command: Some("claude --resume {agent_session}".into()),
         },
         SpawnPreset {
             id: "opencode".into(),
             label: "opencode".into(),
             command: "opencode".into(),
+            resume_command: None,
         },
         SpawnPreset {
             id: "zsh".into(),
             label: "Shell".into(),
             command: "zsh".into(),
+            resume_command: None,
         },
     ]
 }
@@ -334,6 +337,7 @@ pub fn custom_preset() -> SpawnPreset {
         id: CUSTOM_PRESET_ID.into(),
         label: "Custom…".into(),
         command: String::new(),
+        resume_command: None,
     }
 }
 
@@ -507,6 +511,18 @@ auto_install = false
         assert!(custom.command.is_empty());
     }
 
+    #[test]
+    fn builtin_claude_preset_resumes_by_agent_session() {
+        let claude = builtin_presets()
+            .into_iter()
+            .find(|preset| preset.id == "claude")
+            .expect("Claude preset");
+        assert_eq!(
+            claude.resume_command.as_deref(),
+            Some("claude --resume {agent_session}")
+        );
+    }
+
     #[test]
     fn spawn_presets_override_replaces_builtins() {
         // Present list replaces the built-ins entirely; custom still appended.
@@ -515,6 +531,7 @@ auto_install = false
                 id: "fish".into(),
                 label: "fish".into(),
                 command: "fish".into(),
+                resume_command: None,
             }]),
             ..Default::default()
         };
@@ -551,6 +568,7 @@ auto_install = false
                 id: "  ".into(),
                 label: "x".into(),
                 command: "x".into(),
+                resume_command: None,
             }]),
             ..Default::default()
         };
@@ -561,6 +579,7 @@ auto_install = false
                 id: "x".into(),
                 label: "x".into(),
                 command: "".into(),
+                resume_command: None,
             }]),
             ..Default::default()
         };
@@ -578,11 +597,13 @@ auto_install = false
                     id: "a".into(),
                     label: "a".into(),
                     command: "a".into(),
+                    resume_command: None,
                 },
                 SpawnPreset {
                     id: "a".into(),
                     label: "a2".into(),
                     command: "a2".into(),
+                    resume_command: None,
                 },
             ]),
             ..Default::default()
@@ -597,6 +618,7 @@ auto_install = false
                 id: "custom".into(),
                 label: "Custom".into(),
                 command: "whatever".into(),
+                resume_command: None,
             }]),
             ..Default::default()
         };
@@ -632,6 +654,27 @@ presets = [
         assert_eq!(ids, ["claude", "zsh", "custom"]);
     }
 
+    #[test]
+    fn spawn_preset_parses_resume_command_from_issue_example() {
+        let cfg = toml::from_str::<MaestroConfigFile>(
+            r#"
+[spawn]
+presets = [
+  { id = "claude", command = "claude --dangerously-skip-permissions", resume_command = "claude --dangerously-skip-permissions --resume {agent_session}" }
+]
+"#,
+        )
+        .expect("issue #165 example parses");
+
+        let preset = &cfg.spawn.presets.expect("configured presets")[0];
+        assert_eq!(preset.id, "claude");
+        assert_eq!(preset.command, "claude --dangerously-skip-permissions");
+        assert_eq!(
+            preset.resume_command.as_deref(),
+            Some("claude --dangerously-skip-permissions --resume {agent_session}")
+        );
+    }
+
     #[test]
     fn spawn_project_scan_fields_default_to_none() {
         // Absent [spawn].project_* → None; the daemon applies its own defaults.

crates/maestro-daemon/src/lib.rs

@@ -25,6 +25,7 @@ pub mod proc;
 pub mod projects;
 pub mod pty;
 pub mod release;
+pub mod resume;
 pub mod session;
 pub mod sidecar;
 pub mod spawn_history;

crates/maestro-daemon/src/local_api.rs

@@ -52,6 +52,7 @@ pub fn router(state: LocalApiState) -> Router {
         .route(routes::patterns::SESSION_ACTIVITY, post(report_activity))
         .route(routes::patterns::SESSION_INPUT, post(send_input))
         .route(routes::patterns::SESSION_FOCUS, post(set_focus))
+        .route(routes::patterns::SESSION_RESUME, post(resume_session))
         .route(
             routes::patterns::SESSION,
             delete(kill_session).patch(rename_session),
@@ -170,6 +171,9 @@ fn session_error_status(code: &str) -> StatusCode {
         // The session outlived its pane: nothing to type into, and nothing
         // the daemon can do about it.
         "not_running" => StatusCode::CONFLICT,
+        // Resume refusals (#166): the session exists and the request was
+        // well-formed, it just cannot be brought back in this state.
+        "already_running" | "cwd_missing" => StatusCode::CONFLICT,
         _ => StatusCode::BAD_GATEWAY,
     }
 }
@@ -368,6 +372,27 @@ async fn set_focus(
     }
 }
 
+/// `POST /api/sessions/:id/resume` (#166): respawn a session that has no pane
+/// under its own id and sidecar, so its name, `created_at`, env and worktree
+/// ownership survive. Answers with the updated session, which also goes out as
+/// `SessionUpdated` to every events subscriber.
+///
+/// Refused with `409` when the session is already running or its `cwd` is
+/// gone; `404` when there is no such session.
+async fn resume_session(State(s): State<LocalApiState>, Path(id): Path<Uuid>) -> Response {
+    match s.manager.resume(id, &s.presets).await {
+        Ok(session) => (StatusCode::OK, Json(session)).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()
 }
@@ -410,6 +435,16 @@ async fn attach_ws(
     ws: axum::extract::WebSocketUpgrade,
 ) -> Result<Response, Response> {
     let Some((replay, rx)) = s.manager.subscribe_pty_with_replay(id).await else {
+        // #166: a session the daemon knows but has no bridge for — a dormant
+        // one — is not missing, it is not running. Reporting it as unknown
+        // left the client unable to tell "gone" from "resumable".
+        if s.manager.has(id).await {
+            return Err(error_response(
+                StatusCode::CONFLICT,
+                "not_attachable",
+                "session has no live pane; resume it first",
+            ));
+        }
         return Err(error_response(
             StatusCode::NOT_FOUND,
             "unknown_session",

crates/maestro-daemon/src/resume.rs

@@ -0,0 +1,99 @@
+//! Choosing the command a resumed session comes back on (#166).
+//!
+//! A session records the command it was spawned with. Bringing it back means
+//! running that preset's `resume_command` (#165) instead, so an agent picks up
+//! its old conversation rather than starting a fresh one. Everything else
+//! about the session — id, sidecar, name, `created_at`, env, worktree — is
+//! untouched, so only the command is decided here.
+
+use maestro_protocol::{Session, SpawnPreset};
+
+/// The command to respawn `session` with.
+///
+/// The preset is matched on the command the session was spawned with, which is
+/// what the sidecar holds. A session with no matching preset, a preset with no
+/// `resume_command`, or a `resume_command` needing an agent session id the
+/// session never reported all fall back to the original command: respawning
+/// what the operator asked for is always honest, where a half-filled resume
+/// template is not.
+pub fn resume_command(session: &Session, presets: &[SpawnPreset]) -> String {
+    presets
+        .iter()
+        .find(|p| p.command == session.command)
+        .and_then(|p| p.resume_command_for(session.agent_session_id.as_deref()))
+        .unwrap_or_else(|| session.command.clone())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::collections::HashMap;
+
+    fn presets() -> Vec<SpawnPreset> {
+        vec![
+            SpawnPreset {
+                id: "claude".into(),
+                label: "Claude".into(),
+                command: "claude".into(),
+                resume_command: Some("claude --resume {agent_session}".into()),
+            },
+            SpawnPreset {
+                id: "zsh".into(),
+                label: "Shell".into(),
+                command: "zsh".into(),
+                resume_command: None,
+            },
+        ]
+    }
+
+    fn session(command: &str, agent_session_id: Option<&str>) -> Session {
+        Session {
+            id: uuid::Uuid::new_v4(),
+            cwd: "/tmp".into(),
+            command: command.into(),
+            env: HashMap::new(),
+            metadata: HashMap::new(),
+            worktree: None,
+            status: maestro_protocol::SessionStatus::Dormant,
+            created_at: chrono::Utc::now(),
+            foreground: None,
+            activity: maestro_protocol::ActivityState::Idle,
+            activity_detail: None,
+            last_activity_at: chrono::Utc::now(),
+            activity_since: chrono::Utc::now(),
+            current_cwd: None,
+            blocked_work: None,
+            last_focused_at: None,
+            agent_session_id: agent_session_id.map(str::to_string),
+        }
+    }
+
+    #[test]
+    fn uses_the_matching_presets_resume_command() {
+        assert_eq!(
+            resume_command(&session("claude", Some("conv-7")), &presets()),
+            "claude --resume conv-7"
+        );
+    }
+
+    #[test]
+    fn falls_back_when_the_agent_session_id_is_unknown() {
+        assert_eq!(
+            resume_command(&session("claude", None), &presets()),
+            "claude"
+        );
+    }
+
+    #[test]
+    fn falls_back_for_a_preset_without_a_resume_command() {
+        assert_eq!(resume_command(&session("zsh", None), &presets()), "zsh");
+    }
+
+    #[test]
+    fn falls_back_when_no_preset_matches() {
+        assert_eq!(
+            resume_command(&session("bash --noprofile", Some("conv-7")), &presets()),
+            "bash --noprofile"
+        );
+    }
+}

crates/maestro-daemon/src/session.rs

@@ -112,6 +112,15 @@ pub enum SessionError {
     /// a stray keystroke on an attach socket, which is dropped.
     #[error("session is no longer running: {0}")]
     NotRunning(Uuid),
+    /// A resume (#166) was asked for a session that is already running. There
+    /// is nothing to bring back, and respawning would strand the pane that is
+    /// already there.
+    #[error("session is already running: {0}")]
+    AlreadyRunning(Uuid),
+    /// A resume (#166) was asked for a session whose working directory is
+    /// gone. Nothing can be resumed in a directory that no longer exists.
+    #[error("session working directory no longer exists: {0}")]
+    CwdMissing(PathBuf),
     /// The kill was refused (#79): the session's worktree holds unsaved work.
     /// Nothing happened — the session keeps running and the tree is untouched,
     /// so it stays the operator's handle on their own unfinished work.
@@ -130,6 +139,8 @@ impl SessionError {
             SessionError::InvalidEnv(_) => "invalid_env",
             SessionError::NotRunning(_) => "not_running",
             SessionError::KillGuarded(_) => "kill_guarded",
+            SessionError::AlreadyRunning(_) => "already_running",
+            SessionError::CwdMissing(_) => "cwd_missing",
         };
         let unsaved_work = match self {
             SessionError::KillGuarded(b) => Some(*b),
@@ -586,6 +597,112 @@ impl SessionManager {
         Ok(session)
     }
 
+    /// Bring a session that has no pane back to life (#166).
+    ///
+    /// The session comes back under the **same** uuid and the same sidecar, so
+    /// `metadata.name`, `created_at`, `env` and worktree ownership all survive
+    /// — only the tmux session and the PTY bridge are new. That is the whole
+    /// point: spawning a fresh session at the same path instead produces a
+    /// stranger with no worktree to clean up on kill.
+    ///
+    /// The command is [`crate::resume::resume_command`]: the matching preset's
+    /// `resume_command` (#165) with the session's own agent conversation id
+    /// substituted in, falling back to the command the session was spawned
+    /// with. `Session::command` keeps the original either way, so a second
+    /// resume still resolves the same preset.
+    ///
+    /// Refused for a session that is already running, and for one whose `cwd`
+    /// has disappeared — neither is something a respawn could fix.
+    pub async fn resume(
+        &self,
+        id: Uuid,
+        presets: &[maestro_protocol::SpawnPreset],
+    ) -> Result<Session, SessionError> {
+        let mut session = {
+            let sessions = self.inner.sessions.lock().await;
+            let rec = sessions.get(&id).ok_or(SessionError::NotFound(id))?;
+            if rec.session.status == SessionStatus::Running {
+                return Err(SessionError::AlreadyRunning(id));
+            }
+            rec.session.clone()
+        };
+        if !session.cwd.exists() {
+            return Err(SessionError::CwdMissing(session.cwd.clone()));
+        }
+        let command = crate::resume::resume_command(&session, presets);
+
+        // Same fifo path as the original spawn: it is derived from the id, and
+        // `mkfifo` tolerates the stale node the previous run left behind.
+        let fifo_path = self.inner.fifo_dir.join(format!("{id}.fifo"));
+        let bridge =
+            PtyBridge::start_with_replay(fifo_path.clone(), self.inner.replay_bytes).await?;
+        let mut session_env = sanitize_env(&session.env)?;
+        session_env.insert("MAESTRO_SESSION_ID".into(), id.to_string());
+        if let Some(url) = self
+            .inner
+            .daemon_url
+            .lock()
+            .expect("daemon_url mutex")
+            .clone()
+        {
+            session_env.insert("MAESTRO_URL".into(), url);
+        }
+        let spawned = {
+            let tmux = self.inner.tmux.clone();
+            let cwd = session.cwd.clone();
+            let command = command.clone();
+            let sink = pipe_pane_command(&fifo_path);
+            tmux_blocking(move || tmux.new_session_piped(id, &cwd, &command, &session_env, &sink))
+                .await
+        };
+        if let Err(e) = spawned {
+            // `bridge` drops here, closing the FIFO reader. Nothing else was
+            // touched, so the session stays exactly as dormant as it was.
+            return Err(SessionError::Tmux(e));
+        }
+
+        session.status = SessionStatus::Running;
+        // A resumed session is producing its initial output again; the
+        // watcher refines this on its next tick, exactly as after a spawn.
+        session.foreground = None;
+        session.activity = ActivityState::Active;
+        session.activity_detail = None;
+        session.last_activity_at = Utc::now();
+        session.activity_since = Utc::now();
+        session.current_cwd = None;
+        session.last_focused_at = None;
+
+        if let Err(e) = self.inner.sidecar.write(&session) {
+            // tmux already spawned; kill it rather than leave a pane the
+            // sidecar does not describe.
+            let tmux = self.inner.tmux.clone();
+            let _ = tmux_blocking(move || tmux.kill_session(id)).await;
+            return Err(SessionError::Sidecar(e));
+        }
+
+        {
+            let mut sessions = self.inner.sessions.lock().await;
+            let Some(rec) = sessions.get_mut(&id) else {
+                // Killed while we were spawning. Undo the pane instead of
+                // leaving one behind with no record of it.
+                let tmux = self.inner.tmux.clone();
+                let _ = tmux_blocking(move || tmux.kill_session(id)).await;
+                return Err(SessionError::NotFound(id));
+            };
+            rec.session = session.clone();
+            rec.fifo_path = fifo_path;
+            rec.bridge = Some(bridge);
+            rec.reported_activity = None;
+            rec.settled = false;
+            rec.focused_since = None;
+            rec.awaiting_suppressed = false;
+        }
+        let _ = self.inner.events.send(CatalogEvent::SessionUpdated {
+            session: session.clone(),
+        });
+        Ok(session)
+    }
+
     /// Best-effort removal of a worktree created earlier in `create` when a
     /// later step fails. No-op when the request had no worktree.
     fn rollback_worktree(&self, base: &Option<PathBuf>, path: &Option<PathBuf>) {

crates/maestro-daemon/tests/activity_report.rs

@@ -223,4 +223,22 @@ async fn a_reported_session_id_survives_recovery_from_the_sidecar() {
         .expect("sidecar read")
         .expect("sidecar present");
     assert_eq!(stored.agent_session_id.as_deref(), Some("conv-abc123"));
+
+    // The sidecar holding the id is only half the claim: a restarted daemon
+    // has to carry it back onto the recovered session, which is what a resume
+    // reads. Build a parallel manager over the same tmux server and state
+    // dirs and recover through it.
+    let revived = SessionManager::new(
+        Arc::new(fx._tmux.tmux.clone()),
+        fx.sidecar.clone(),
+        WorktreeManager::new(None),
+        fx._state_dir.path().join("fifos"),
+    );
+    revived.recover().await.expect("recover");
+    let recovered = observed(&revived, id).await;
+    assert_eq!(
+        recovered.agent_session_id.as_deref(),
+        Some("conv-abc123"),
+        "the recovered session must carry the agent session id, not just the sidecar"
+    );
 }

crates/maestro-daemon/tests/attach.rs

@@ -94,6 +94,74 @@ async fn rejects_unknown_session() {
     assert!(format!("{err}").contains("404"));
 }
 
+/// #166: a session the daemon knows but has no bridge for is not unknown, it
+/// is not running. `404 unknown_session` made "gone" and "resumable" look the
+/// same to a client that could have offered a resume.
+#[tokio::test]
+async fn dormant_session_is_not_attachable_rather_than_unknown() {
+    let h = spawn_daemon_ws().await;
+    let cwd = TempDir::new().unwrap();
+    let session = h
+        .manager
+        .create(CreateSessionRequest {
+            cwd: cwd.path().to_path_buf(),
+            command: "sleep 30".into(),
+            env: HashMap::new(),
+            metadata: HashMap::new(),
+            worktree: None,
+        })
+        .await
+        .unwrap();
+
+    // The host reboot: tmux is gone, the sidecar is not, and a fresh manager
+    // registers the session dormant.
+    h._fx.tmux.tmux.kill_server().unwrap();
+    let dormant = SessionManager::new(
+        std::sync::Arc::new(h._fx.tmux.tmux.clone()),
+        maestro_daemon::sidecar::SidecarStore::new(h._fx.sidecar_dir()),
+        maestro_daemon::worktree::WorktreeManager::new(None),
+        h._fx.fifo_dir(),
+    );
+    dormant.recover().await.unwrap();
+    let dormant_addr = serve_manager(dormant).await;
+
+    let err = connect_attach(dormant_addr, session.id).await.unwrap_err();
+    let tokio_tungstenite::tungstenite::Error::Http(resp) = err else {
+        panic!("want an HTTP rejection, got {err}");
+    };
+    assert_eq!(resp.status().as_u16(), 409);
+    let body: maestro_protocol::ApiError =
+        serde_json::from_slice(resp.body().as_deref().expect("error body")).unwrap();
+    assert_eq!(body.error.code, "not_attachable");
+
+    // A session that really is unknown still reads as unknown.
+    let err = connect_attach(dormant_addr, Uuid::new_v4())
+        .await
+        .unwrap_err();
+    assert!(format!("{err}").contains("404"));
+}
+
+/// Serve `manager` on its own listener, for tests that need a second daemon
+/// over the same tmux/sidecar state.
+async fn serve_manager(manager: SessionManager) -> SocketAddr {
+    let router = local_api::router(local_api::state_for(
+        manager,
+        "test-host",
+        "/home/test",
+        Vec::new(),
+        std::sync::Arc::new(maestro_daemon::projects::ProjectScanner::disabled()),
+        maestro_config::DEFAULT_LEADER,
+    ));
+    let listener = TcpListener::bind(SocketAddr::from((Ipv4Addr::LOCALHOST, 0)))
+        .await
+        .unwrap();
+    let addr = listener.local_addr().unwrap();
+    tokio::spawn(async move {
+        let _ = axum::serve(listener, router).await;
+    });
+    addr
+}
+
 #[tokio::test]
 async fn pty_round_trip_through_real_tmux() {
     let h = spawn_daemon_ws().await;

crates/maestro-daemon/tests/resume.rs

@@ -0,0 +1,359 @@
+//! End-to-end tests for `POST /api/sessions/:id/resume` (#166) and the
+//! manager call behind it, against real tmux.
+//!
+//! The point of resume is that the session comes back under the *same* uuid
+//! and sidecar — name, `created_at`, env and worktree ownership intact — with
+//! only the tmux session and the PTY bridge new.
+
+mod common;
+
+use std::collections::HashMap;
+use std::net::{Ipv4Addr, SocketAddr};
+use std::sync::Arc;
+use std::time::Duration;
+
+use common::{wait_for_async, DaemonFixture};
+use maestro_daemon::{local_api, SessionError, SessionManager};
+use maestro_protocol::{CreateSessionRequest, Session, SessionStatus, SpawnPreset};
+use tempfile::TempDir;
+use tokio::net::TcpListener;
+use tokio::sync::oneshot;
+use uuid::Uuid;
+
+/// A fresh manager over the same tmux server, sidecars and fifos — what the
+/// daemon builds after a restart.
+fn revive(fx: &DaemonFixture) -> SessionManager {
+    SessionManager::new(
+        Arc::new(fx.tmux.tmux.clone()),
+        maestro_daemon::sidecar::SidecarStore::new(fx.sidecar_dir()),
+        maestro_daemon::worktree::WorktreeManager::new(None),
+        fx.fifo_dir(),
+    )
+}
+
+/// Create a session, then simulate the host reboot that leaves it dormant:
+/// the tmux server dies, the sidecar stays, a new manager recovers.
+async fn dormant_session(
+    fx: &DaemonFixture,
+    cwd: &TempDir,
+    command: &str,
+    name: &str,
+) -> (SessionManager, Session) {
+    let mut metadata = HashMap::new();
+    metadata.insert(
+        maestro_protocol::NAME_KEY.to_string(),
+        serde_json::Value::String(name.into()),
+    );
+    let session = fx
+        .manager
+        .create(CreateSessionRequest {
+            cwd: cwd.path().to_path_buf(),
+            command: command.into(),
+            env: HashMap::from([("MAESTRO_TEST_ENV".to_string(), "kept".to_string())]),
+            metadata,
+            worktree: None,
+        })
+        .await
+        .expect("create");
+    fx.tmux.tmux.kill_server().expect("kill tmux server");
+    let revived = revive(fx);
+    revived.recover().await.expect("recover");
+    (revived, session)
+}
+
+fn marker_preset(command: &str, marker_dir: &TempDir) -> Vec<SpawnPreset> {
+    vec![SpawnPreset {
+        id: "agent".into(),
+        label: "Agent".into(),
+        command: command.into(),
+        resume_command: Some(format!(
+            "sh -c 'touch {}/resumed-{{agent_session}}; sleep 30'",
+            marker_dir.path().display()
+        )),
+    }]
+}
+
+/// The keystone: a dormant session comes back running under its own id, with
+/// its name, `created_at`, env and sidecar untouched.
+#[tokio::test]
+async fn resume_brings_a_dormant_session_back_under_the_same_id() {
+    let fx = DaemonFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let (manager, original) = dormant_session(&fx, &cwd, "sleep 30", "long-runner").await;
+    assert_eq!(
+        manager.status(original.id).await,
+        Some(SessionStatus::Dormant)
+    );
+
+    let resumed = manager.resume(original.id, &[]).await.expect("resume");
+
+    assert_eq!(resumed.id, original.id, "same uuid");
+    assert_eq!(resumed.status, SessionStatus::Running);
+    assert_eq!(resumed.created_at, original.created_at);
+    assert_eq!(resumed.cwd, original.cwd);
+    assert_eq!(resumed.command, original.command);
+    assert_eq!(resumed.env, original.env);
+    assert_eq!(
+        resumed
+            .metadata
+            .get(maestro_protocol::NAME_KEY)
+            .and_then(|v| v.as_str()),
+        Some("long-runner")
+    );
+
+    // tmux really holds it again, and the sidecar is the same file, updated.
+    let live = fx.tmux.tmux.list_sessions().expect("list");
+    assert_eq!(live, vec![original.id]);
+    let stored = maestro_daemon::sidecar::SidecarStore::new(fx.sidecar_dir())
+        .read(original.id)
+        .expect("read sidecar")
+        .expect("sidecar still there");
+    assert_eq!(stored.status, SessionStatus::Running);
+    assert_eq!(stored.created_at, original.created_at);
+
+    // And it is attachable again, which a dormant session never was.
+    assert!(manager
+        .subscribe_pty_with_replay(original.id)
+        .await
+        .is_some());
+}
+
+/// The worktree the session owns stays the session's, so killing the resumed
+/// session still cleans the tree up.
+#[tokio::test]
+async fn resume_keeps_worktree_ownership() {
+    let fx = DaemonFixture::new();
+    let repo = TempDir::new().unwrap();
+    common::init_repo(repo.path());
+
+    let session = fx
+        .manager
+        .create(CreateSessionRequest {
+            cwd: repo.path().to_path_buf(),
+            command: "sleep 30".into(),
+            env: HashMap::new(),
+            metadata: HashMap::new(),
+            worktree: Some(maestro_protocol::WorktreeSpec {
+                base_repo: repo.path().to_path_buf(),
+                branch: "feature".into(),
+                create_branch: true,
+            }),
+        })
+        .await
+        .expect("create");
+    let worktree = session.worktree.clone().expect("worktree");
+
+    fx.tmux.tmux.kill_server().expect("kill tmux server");
+    let manager = revive(&fx);
+    manager.recover().await.expect("recover");
+    let resumed = manager.resume(session.id, &[]).await.expect("resume");
+
+    assert_eq!(resumed.worktree, Some(worktree.clone()));
+    assert_eq!(
+        resumed.cwd, worktree.path,
+        "resumed inside its own worktree"
+    );
+}
+
+/// The command comes from the preset's `resume_command`, with the session's
+/// own agent conversation id (#161) substituted in.
+#[tokio::test]
+async fn resume_runs_the_presets_resume_command() {
+    let fx = DaemonFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let markers = TempDir::new().unwrap();
+    let presets = marker_preset("sleep 30", &markers);
+
+    let (manager, session) = dormant_session(&fx, &cwd, "sleep 30", "agentic").await;
+    // The agent reported its conversation id before the reboot, so it is on
+    // the sidecar and the resume template can be filled in.
+    fx.manager
+        .report_activity(
+            session.id,
+            maestro_protocol::ActivityState::Idle,
+            None,
+            Some("conv-42".into()),
+        )
+        .await
+        .ok();
+    // Re-read the sidecar the report updated.
+    let manager = {
+        drop(manager);
+        let m = revive(&fx);
+        m.recover().await.expect("recover");
+        m
+    };
+
+    manager.resume(session.id, &presets).await.expect("resume");
+
+    let marker = markers.path().join("resumed-conv-42");
+    wait_for_async(Duration::from_secs(5), || {
+        let marker = marker.clone();
+        async move { marker.exists() }
+    })
+    .await;
+}
+
+/// A session that is already running has nothing to bring back, and
+/// respawning would strand the pane that is already there.
+#[tokio::test]
+async fn resume_refuses_a_running_session() {
+    let fx = DaemonFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let session = fx
+        .manager
+        .create(CreateSessionRequest {
+            cwd: cwd.path().to_path_buf(),
+            command: "sleep 30".into(),
+            env: HashMap::new(),
+            metadata: HashMap::new(),
+            worktree: None,
+        })
+        .await
+        .expect("create");
+
+    let err = fx
+        .manager
+        .resume(session.id, &[])
+        .await
+        .expect_err("already running");
+    assert!(matches!(err, SessionError::AlreadyRunning(id) if id == session.id));
+    assert_eq!(err.to_error_info().code, "already_running");
+}
+
+/// Nothing can be resumed in a directory that no longer exists.
+#[tokio::test]
+async fn resume_refuses_when_the_cwd_is_gone() {
+    let fx = DaemonFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let (manager, session) = dormant_session(&fx, &cwd, "sleep 30", "homeless").await;
+
+    std::fs::remove_dir_all(cwd.path()).unwrap();
+
+    let err = manager
+        .resume(session.id, &[])
+        .await
+        .expect_err("cwd is gone");
+    assert_eq!(err.to_error_info().code, "cwd_missing");
+    assert_eq!(
+        manager.status(session.id).await,
+        Some(SessionStatus::Dormant),
+        "a refused resume leaves the session exactly as it was"
+    );
+}
+
+#[tokio::test]
+async fn resume_of_an_unknown_session_is_not_found() {
+    let fx = DaemonFixture::new();
+    let err = fx
+        .manager
+        .resume(Uuid::new_v4(), &[])
+        .await
+        .expect_err("unknown");
+    assert_eq!(err.to_error_info().code, "not_found");
+}
+
+// ---------------------------------------------------------------------------
+// HTTP surface
+// ---------------------------------------------------------------------------
+
+struct Harness {
+    addr: SocketAddr,
+    _shutdown: oneshot::Sender<()>,
+}
+
+async fn serve(manager: SessionManager, presets: Vec<SpawnPreset>) -> Harness {
+    let router = local_api::router(local_api::state_for(
+        manager,
+        "test-host",
+        "/home/test",
+        presets,
+        Arc::new(maestro_daemon::projects::ProjectScanner::disabled()),
+        maestro_config::DEFAULT_LEADER,
+    ));
+    let listener = TcpListener::bind(SocketAddr::from((Ipv4Addr::LOCALHOST, 0)))
+        .await
+        .unwrap();
+    let addr = listener.local_addr().unwrap();
+    let (tx, rx) = oneshot::channel::<()>();
+    tokio::spawn(async move {
+        let _ = axum::serve(listener, router)
+            .with_graceful_shutdown(async move {
+                let _ = rx.await;
+            })
+            .await;
+    });
+    Harness {
+        addr,
+        _shutdown: tx,
+    }
+}
+
+#[tokio::test]
+async fn post_resume_returns_the_revived_session() {
+    let fx = DaemonFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let (manager, session) = dormant_session(&fx, &cwd, "sleep 30", "web-resumed").await;
+    let h = serve(manager, Vec::new()).await;
+
+    let resp = reqwest::Client::new()
+        .post(format!(
+            "http://{}{}",
+            h.addr,
+            maestro_protocol::routes::session_resume(session.id)
+        ))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), reqwest::StatusCode::OK);
+    let body: Session = resp.json().await.unwrap();
+    assert_eq!(body.id, session.id);
+    assert_eq!(body.status, SessionStatus::Running);
+}
+
+#[tokio::test]
+async fn post_resume_on_a_running_session_is_a_conflict() {
+    let fx = DaemonFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let session = fx
+        .manager
+        .create(CreateSessionRequest {
+            cwd: cwd.path().to_path_buf(),
+            command: "sleep 30".into(),
+            env: HashMap::new(),
+            metadata: HashMap::new(),
+            worktree: None,
+        })
+        .await
+        .expect("create");
+    let h = serve(fx.manager.clone(), Vec::new()).await;
+
+    let resp = reqwest::Client::new()
+        .post(format!(
+            "http://{}{}",
+            h.addr,
+            maestro_protocol::routes::session_resume(session.id)
+        ))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), reqwest::StatusCode::CONFLICT);
+    let err: maestro_protocol::ApiError = resp.json().await.unwrap();
+    assert_eq!(err.error.code, "already_running");
+}
+
+#[tokio::test]
+async fn post_resume_on_an_unknown_session_is_404() {
+    let fx = DaemonFixture::new();
+    let h = serve(fx.manager.clone(), Vec::new()).await;
+    let resp = reqwest::Client::new()
+        .post(format!(
+            "http://{}{}",
+            h.addr,
+            maestro_protocol::routes::session_resume(Uuid::new_v4())
+        ))
+        .send()
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), reqwest::StatusCode::NOT_FOUND);
+}

crates/maestro-protocol/src/lib.rs

@@ -529,8 +529,34 @@ impl ApiError {
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct SpawnPreset {
     pub id: String,
+    #[serde(default)]
     pub label: String,
     pub command: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub resume_command: Option<String>,
+}
+
+/// Placeholder a preset's `resume_command` may carry for the agent's own
+/// conversation id (#165), substituted from [`Session::agent_session_id`].
+pub const AGENT_SESSION_PLACEHOLDER: &str = "{agent_session}";
+
+impl SpawnPreset {
+    /// The command that resumes a session spawned from this preset, with
+    /// [`AGENT_SESSION_PLACEHOLDER`] filled in from `agent_session`.
+    ///
+    /// `None` when the preset has no `resume_command` — a plain shell has
+    /// nothing to resume — and also when the template asks for an agent
+    /// session id the session never reported: `claude --resume` with an empty
+    /// id resumes nothing, so the caller is better off respawning the original
+    /// command than running a broken one.
+    pub fn resume_command_for(&self, agent_session: Option<&str>) -> Option<String> {
+        let template = self.resume_command.as_deref()?;
+        if !template.contains(AGENT_SESSION_PLACEHOLDER) {
+            return Some(template.to_string());
+        }
+        let id = agent_session.filter(|s| !s.is_empty())?;
+        Some(template.replace(AGENT_SESSION_PLACEHOLDER, id))
+    }
 }
 
 /// Build provenance of the running daemon: the commit it was compiled
@@ -782,6 +808,47 @@ pub enum ClientControl {
 mod tests {
     use super::*;
 
+    fn preset(resume: Option<&str>) -> SpawnPreset {
+        SpawnPreset {
+            id: "claude".into(),
+            label: "Claude".into(),
+            command: "claude".into(),
+            resume_command: resume.map(str::to_string),
+        }
+    }
+
+    #[test]
+    fn resume_command_substitutes_the_agent_session() {
+        let p = preset(Some("claude --resume {agent_session}"));
+        assert_eq!(
+            p.resume_command_for(Some("abc-123")).as_deref(),
+            Some("claude --resume abc-123")
+        );
+    }
+
+    #[test]
+    fn resume_command_without_a_placeholder_ignores_the_agent_session() {
+        let p = preset(Some("opencode --continue"));
+        assert_eq!(
+            p.resume_command_for(None).as_deref(),
+            Some("opencode --continue")
+        );
+    }
+
+    /// A template that wants an id the session never reported would run
+    /// `claude --resume` against nothing; the caller respawns instead.
+    #[test]
+    fn resume_command_is_none_without_the_id_it_asks_for() {
+        let p = preset(Some("claude --resume {agent_session}"));
+        assert!(p.resume_command_for(None).is_none());
+        assert!(p.resume_command_for(Some("")).is_none());
+    }
+
+    #[test]
+    fn a_preset_without_a_resume_command_has_none() {
+        assert!(preset(None).resume_command_for(Some("abc")).is_none());
+    }
+
     #[test]
     fn version_compatibility_is_major_only() {
         assert!(is_compatible(PROTOCOL_VERSION));

crates/maestro-protocol/src/routes.rs

@@ -66,6 +66,12 @@ pub fn session_focus(id: Uuid) -> String {
     format!("{SESSIONS}/{id}/focus")
 }
 
+/// `POST /api/sessions/:id/resume` (#166): respawn a dormant session under its
+/// own id, keeping its sidecar, name, worktree and `created_at`.
+pub fn session_resume(id: Uuid) -> String {
+    format!("{SESSIONS}/{id}/resume")
+}
+
 /// `WS /api/attach/:id` — raw PTY bytes.
 pub fn attach(id: Uuid) -> String {
     format!("{API_PREFIX}/attach/{id}")
@@ -79,6 +85,7 @@ pub mod patterns {
     pub const SESSION_INPUT: &str = "/api/sessions/:id/input";
     pub const SESSION_PROMPT: &str = "/api/sessions/:id/prompt";
     pub const SESSION_FOCUS: &str = "/api/sessions/:id/focus";
+    pub const SESSION_RESUME: &str = "/api/sessions/:id/resume";
     pub const ATTACH: &str = "/api/attach/:id";
 }
 
@@ -202,6 +209,10 @@ mod tests {
             session_focus(id()),
             "/api/sessions/6f1a7c0e-0f5e-4a2b-9a3d-6b1c2d3e4f50/focus"
         );
+        assert_eq!(
+            session_resume(id()),
+            "/api/sessions/6f1a7c0e-0f5e-4a2b-9a3d-6b1c2d3e4f50/resume"
+        );
         assert_eq!(
             attach(id()),
             "/api/attach/6f1a7c0e-0f5e-4a2b-9a3d-6b1c2d3e4f50"
@@ -236,6 +247,10 @@ mod tests {
             patterns::SESSION_FOCUS.trim_end_matches(":id/focus"),
             session(id()).trim_end_matches(&id().to_string())
         );
+        assert_eq!(
+            patterns::SESSION_RESUME.trim_end_matches(":id/resume"),
+            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-tui/src/draft.rs

@@ -1705,6 +1705,7 @@ mod tests {
             id: "aider".into(),
             label: "Aider".into(),
             command: "aider".into(),
+            resume_command: None,
         }]);
         assert_eq!(
             draft.command(),
@@ -1727,11 +1728,13 @@ mod tests {
                 id: "claude".into(),
                 label: "Claude".into(),
                 command: "claude".into(),
+                resume_command: None,
             },
             SpawnPreset {
                 id: "aider".into(),
                 label: "Aider".into(),
                 command: "aider".into(),
+                resume_command: None,
             },
         ]);
         assert_eq!(draft.command(), "aider");
@@ -1751,6 +1754,7 @@ mod tests {
             id: "claude".into(),
             label: "Claude".into(),
             command: "claude".into(),
+            resume_command: None,
         }]);
         assert!(draft.is_custom());
         assert_eq!(draft.command(), "bash -l");

crates/maestro-tui/src/fast_forward.rs

@@ -183,6 +183,7 @@ mod tests {
             id: id.into(),
             label: id.into(),
             command: command.into(),
+            resume_command: None,
         }
     }
 

docs/RESTART.md

@@ -79,13 +79,33 @@ to adopt while every session's sidecar is still on disk. Recovery makes a
 second pass over those sidecars and registers each as a `Dormant` session
 (#162) — id, name, cwd, command, worktree and `created_at` intact. A dormant
 session has no pane and no PTY bridge: it cannot be attached to, typed into or
-resized, and neither watcher pass touches it. It is a record of what was
-running, and the only thing left to do with one is dismiss it, which removes
-the sidecar so the next recovery does not bring it back.
+resized, and neither watcher pass touches it.
 
-`SessionStatus` is a wire type, so it deserializes leniently on both clients:
-a spelling this build does not know reads as `Dead` rather than failing the
-whole session. Clients render a dormant session as known-but-not-running —
+A dormant session is brought back with `POST /api/sessions/:id/resume` (#166),
+which respawns tmux under the **same** uuid and sidecar — `metadata.name`,
+`created_at`, `env` and worktree ownership all survive, and only the tmux
+session and the PTY bridge are new. The command is the matching spawn preset's
+`resume_command` with `{agent_session}` filled in from the session's own
+`agent_session_id`, so an agent comes back inside its old conversation; a
+session with no matching preset, no `resume_command`, or no reported
+conversation id is respawned on the command it was created with. `Session::command`
+always keeps that original command, so a second resume resolves the same preset.
+Resume is refused with `409` for a session that is already running
+(`already_running`) and for one whose `cwd` has disappeared (`cwd_missing`) —
+neither is something a respawn could fix. The other thing to do with a dormant
+session is dismiss it, which removes the sidecar so the next recovery does not
+bring it back.
+
+Attaching to a session that has no bridge answers `409 not_attachable`, not
+`404 unknown_session`: the session exists, and a client that cannot tell "gone"
+from "not running" cannot offer the resume.
+
+`SessionStatus` is a wire type, so from this build forward it deserializes
+leniently on both clients: a spelling the client does not know reads as `Dead`
+rather than failing the whole session. That leniency lives in the client's own
+binary, so it covers spellings added *after* this build, not this one — a CLI
+or TUI still running an older build fails to parse a `Dormant` session, and
+with it the whole session list, until it is upgraded. Clients render a dormant session as known-but-not-running —
 see `docs/SIDEBAR.md` for the card and `docs/COMMAND-BAR.md` for how it ranks.
 
 ## Timing