diff --git a/crates/maestro-daemon/src/local_api.rs b/crates/maestro-daemon/src/local_api.rs
index 6ae1c3a..06c3100 100644
--- a/crates/maestro-daemon/src/local_api.rs
+++ b/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()
 }
diff --git a/crates/maestro-daemon/tests/local_api.rs b/crates/maestro-daemon/tests/local_api.rs
index c5d1ab7..88a712b 100644
--- a/crates/maestro-daemon/tests/local_api.rs
+++ b/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]