diff --git a/docs/turn-loop/README.md b/docs/turn-loop/README.md
index 03fa27ac..15338e7e 100644
--- a/docs/turn-loop/README.md
+++ b/docs/turn-loop/README.md
@@ -126,6 +126,7 @@ else a `TurnError`) drives the post-claude branch:
 | `Err(AuthFailed)` | emit `needs_login_idle` sentinel, requeue inflight, park in `wait_for_login` |
 | `Err(SessionNotFound)` | resume + create self-heal both missed ("shouldn't happen"); requeue inflight so the next turn creates fresh — no status park, message not dropped |
 | `Err(ApiStall)` | idle watchdog killed claude after `HIVE_TURN_IDLE_SECS` (default 600) of output silence; sleep `HIVE_STALL_SLEEP_SECS` (default 60), requeue inflight, status back to `online` |
+| `Err(AgentStall(note))` | the same, for an ACP agent: after `HIVE_TURN_IDLE_SECS` with no `session/update` the harness sent the agent `session/cancel`, and killed it if it ignored that; `note` says which |
 | `Err(Failed(err))` | route `[system] \`<qualified-label>\` claude turn failed:\n<err>` to `operator` via `send_to_operator` |
 
 `ApiStall` catches an Anthropic API stall — a multi-retry connection storm where
@@ -136,6 +137,10 @@ only complete output silence for the window trips it. The harness sets the
 window from `HIVE_TURN_IDLE_SECS` (`0` disables) and maps the driver's
 `Error::IdleTimeout` onto `TurnError::ApiStall`.
 
+`hive-runtime` applies the same window to an ACP agent's turns. That's
+also how a provider HTTP 429 ends an ACP turn when the agent retries it without
+reporting it: the turn goes silent until the watchdog stops it.
+
 After the outcome handler, the stats sink records a row. `handle_turn`
 reports the result to `serve_loop` via `TurnControl { auth_failed }` —
 on auth failure the loop parks in `wait_for_login`; otherwise it loops
diff --git a/docs/web-ui/agent.md b/docs/web-ui/agent.md
index e00ebf89..de4dc5d6 100644
--- a/docs/web-ui/agent.md
+++ b/docs/web-ui/agent.md
@@ -244,8 +244,10 @@ Slash commands today:
 - `/help` — list commands locally.
 - `/clear` — wipe the local terminal view (server history kept).
 - `/cancel` — `POST /api/cancel` → host shellouts `pkill -INT
-  claude`, emits a Note. Also surfaces as a `■ cancel turn`
-  button in the state row while state=thinking.
+  claude`, emits a Note. On an ACP agent it sends the agent
+  `session/cancel` instead, and kills it if the turn hasn't ended 10s
+  later. Also surfaces as a `■ cancel turn` button in the state row
+  while state=thinking.
 - `/compact` — `POST /api/compact` → sets the deferred
   `Bus::request_compact()` flag and returns immediately. The harness
   runs the `/compact` at the next turn boundary (end of the in-flight
diff --git a/hive-agent/src/events.rs b/hive-agent/src/events.rs
index 2ad82a8b..ef48be69 100644
--- a/hive-agent/src/events.rs
+++ b/hive-agent/src/events.rs
@@ -10,7 +10,7 @@
 
 use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, AtomicI64, AtomicU64, Ordering};
-use std::sync::{Arc, Mutex};
+use std::sync::{Arc, Mutex, OnceLock};
 
 use chrono::Utc;
 use hive_claude::TokenUsage;
@@ -331,6 +331,10 @@ pub struct Bus {
     /// set, the compact has already run and that flag has already been
     /// cleared by `take_compact`.
     post_compact_wake: Arc<Mutex<Option<String>>>,
+    /// Stops the turn in flight, for a runtime that offers it (ACP). Set once
+    /// by the serve loop when it builds the session; unset on claude, whose
+    /// turn `/api/cancel` stops by signalling the `claude` process.
+    turn_canceller: Arc<OnceLock<hive_runtime::Canceller>>,
     /// Current fresh-claude-session id (FK to `sessions.id`). Set by the
     /// bin loop after minting a session row on a fresh start; stamped onto
     /// every `turn_stats` row until the next fresh session. `None` before
@@ -422,6 +426,7 @@ impl Bus {
             session_reset_pending: Arc::new(AtomicBool::new(false)),
             compact_pending: Arc::new(Mutex::new(None)),
             post_compact_wake: Arc::new(Mutex::new(None)),
+            turn_canceller: Arc::default(),
             session_id: Arc::new(Mutex::new(None)),
             fresh_session: Arc::new(AtomicBool::new(false)),
             tool_calls: Arc::new(Mutex::new(std::collections::HashMap::new())),
@@ -445,6 +450,19 @@ impl Bus {
         self.event_seq.fetch_add(1, Ordering::SeqCst) + 1
     }
 
+    /// Record how the session's turn in flight is stopped. Only the first call
+    /// takes effect; there is one session per harness.
+    pub fn set_turn_canceller(&self, canceller: hive_runtime::Canceller) {
+        let _ = self.turn_canceller.set(canceller);
+    }
+
+    /// How to stop the turn in flight, if the runtime offers a way (see
+    /// [`Self::set_turn_canceller`]).
+    #[must_use]
+    pub fn turn_canceller(&self) -> Option<&hive_runtime::Canceller> {
+        self.turn_canceller.get()
+    }
+
     /// Request a session reset (operator `POST /api/new-session`). Deferred:
     /// the flag is consumed at the next turn boundary by `drive_turn`, which
     /// archives the current session so no claude process is mid-write when the
diff --git a/hive-agent/src/main.rs b/hive-agent/src/main.rs
index 3ed7743f..f65f884a 100644
--- a/hive-agent/src/main.rs
+++ b/hive-agent/src/main.rs
@@ -662,6 +662,9 @@ async fn serve_loop<S: Surface>(
     // The durable agent session, built once and reused for every turn +
     // idle compaction below (it's effectively stateless).
     let session = turn::make_session(&bus)?;
+    if let Some(canceller) = hive_runtime::Runtime::canceller(&session) {
+        bus.set_turn_canceller(canceller);
+    }
     // Tracks the last observed pause state so the transitions get logged
     // once each instead of twelve lines a minute while parked.
     let mut was_paused = false;
@@ -1040,9 +1043,13 @@ async fn handle_turn_error_recovery<S: Surface>(
         S::requeue_inflight(socket).await;
         bus.emit_status("online");
     }
-    if matches!(outcome, Err(turn::TurnError::ApiStall)) {
-        // Idle watchdog killed claude on a suspected API stall. Park briefly to
-        // let the API recover, then requeue — same shape as the rate-limit path.
+    if matches!(
+        outcome,
+        Err(turn::TurnError::ApiStall | turn::TurnError::AgentStall(_))
+    ) {
+        // Idle watchdog stopped the turn on a suspected API stall. Park briefly
+        // to let the API recover, then requeue — same shape as the rate-limit
+        // path.
         let secs = turn::stall_sleep_secs();
         bus.emit_status("api_stall");
         bus.emit(LiveEvent::Note {
diff --git a/hive-agent/src/serve_common.rs b/hive-agent/src/serve_common.rs
index 4beb839d..d56e3ccc 100644
--- a/hive-agent/src/serve_common.rs
+++ b/hive-agent/src/serve_common.rs
@@ -100,6 +100,7 @@ pub fn build_row(args: TurnRowArgs<'_>) -> TurnStatRow {
         Err(TurnError::AuthFailed) => ("auth_failed", None),
         Err(TurnError::SessionNotFound) => ("session_not_found", None),
         Err(TurnError::ApiStall) => ("api_stall", None),
+        Err(TurnError::AgentStall(note)) => ("api_stall", Some(note.clone())),
         Err(TurnError::Failed(e)) => ("failed", Some(format!("{e:#}"))),
     };
     TurnStatRow {
diff --git a/hive-agent/src/turn.rs b/hive-agent/src/turn.rs
index 8d05dfc2..6b007318 100644
--- a/hive-agent/src/turn.rs
+++ b/hive-agent/src/turn.rs
@@ -195,6 +195,12 @@ pub enum TurnError {
     /// The serve loop parks for `stall_sleep_secs()` and requeues, like the
     /// rate-limit path — NOT a crash.
     ApiStall,
+    /// The ACP backend's idle watchdog stopped a turn that sent nothing for
+    /// `turn_idle_secs()`: the agent was sent `session/cancel`, and killed if
+    /// it ignored that. Carries what happened. An ACP agent retrying a failing
+    /// provider (e.g. HTTP 429) without reporting it ends up here. Handled
+    /// like [`Self::ApiStall`].
+    AgentStall(String),
     /// A hard failure with no recovery — the serve loop escalates it to the
     /// operator (`send_to_operator`).
     Failed(anyhow::Error),
@@ -565,6 +571,13 @@ pub fn emit_turn_end(bus: &Bus, outcome: &TurnOutcome) {
             });
             tracing::warn!("turn killed: API stall (idle watchdog)");
         }
+        Err(TurnError::AgentStall(note)) => {
+            bus.emit(LiveEvent::TurnEnd {
+                ok: false,
+                note: Some(format!("{note} — parking + requeueing")),
+            });
+            tracing::warn!(note = %note, "ACP turn stopped by the idle watchdog");
+        }
         Err(TurnError::Failed(e)) => {
             let note = format!("{e:#}");
             bus.emit(LiveEvent::TurnEnd {
@@ -694,10 +707,15 @@ fn error_to_turn(err: hive_claude::Error) -> TurnOutcome {
 }
 
 /// [`error_to_turn`] for a runtime error: the claude backend's errors map as
-/// above, every other runtime's as `Failed`.
+/// above, the ACP idle watchdog's as `AgentStall`, everything else as
+/// `Failed`.
 fn runtime_error_to_turn(err: hive_runtime::Error) -> TurnOutcome {
+    use hive_runtime::{AcpError, Error};
     match err {
-        hive_runtime::Error::Claude(err) => error_to_turn(err),
+        Error::Claude(err) => error_to_turn(err),
+        Error::Acp(err @ (AcpError::IdleTimeout { .. } | AcpError::IdleKilled { .. })) => {
+            Err(TurnError::AgentStall(err.to_string()))
+        }
         other => Err(TurnError::Failed(other.into())),
     }
 }
@@ -809,7 +827,10 @@ fn archive_session(bus: &Bus, session: &AgentSession) {
 
 #[cfg(test)]
 mod tests {
-    use super::{PermissionAsk, acp_permits, nonzero_or, parse_u64};
+    use super::{
+        PermissionAsk, TurnError, acp_permits, nonzero_or, parse_u64, runtime_error_to_turn,
+    };
+    use hive_runtime::{AcpError, Error};
 
     #[test]
     fn an_absent_or_blank_value_is_not_a_number() {
@@ -909,4 +930,40 @@ mod tests {
         assert!(acp_permits(&ask("fetch"), true));
         assert!(!acp_permits(&ask("fetch"), false));
     }
+
+    #[test]
+    fn a_stalled_acp_turn_parks_and_requeues_with_what_happened() {
+        for err in [
+            AcpError::IdleTimeout { silent_secs: 600 },
+            AcpError::IdleKilled {
+                silent_secs: 600,
+                grace_secs: 10,
+            },
+        ] {
+            let expected = err.to_string();
+            match runtime_error_to_turn(Error::Acp(err)) {
+                Err(TurnError::AgentStall(note)) => assert_eq!(note, expected),
+                other => panic!("{expected}: {other:?}"),
+            }
+        }
+    }
+
+    #[test]
+    fn a_rate_limited_acp_turn_says_so_when_it_stalls() {
+        let Err(TurnError::AgentStall(note)) =
+            runtime_error_to_turn(Error::Acp(AcpError::IdleTimeout { silent_secs: 600 }))
+        else {
+            panic!("not a stall");
+        };
+        assert!(note.contains("600s"), "{note}");
+        assert!(note.contains("HTTP 429"), "{note}");
+    }
+
+    #[test]
+    fn a_cancel_the_agent_ignored_is_a_failure() {
+        assert!(matches!(
+            runtime_error_to_turn(Error::Acp(AcpError::CancelIgnored { grace_secs: 10 })),
+            Err(TurnError::Failed(_))
+        ));
+    }
 }
diff --git a/hive-agent/src/web_ui/actions.rs b/hive-agent/src/web_ui/actions.rs
index c5e7d115..f4ca0cd2 100644
--- a/hive-agent/src/web_ui/actions.rs
+++ b/hive-agent/src/web_ui/actions.rs
@@ -52,6 +52,22 @@ pub(super) async fn post_send(
 }
 
 pub(super) async fn post_cancel_turn(State(state): State<AppState>) -> Response {
+    if let Some(canceller) = state.bus.turn_canceller() {
+        let note = if canceller.cancel() {
+            // Same as a signalled claude turn: the next wake prompt says it
+            // was cut off.
+            state
+                .interrupted
+                .store(true, std::sync::atomic::Ordering::Relaxed);
+            "operator: /cancel — sent session/cancel to the ACP agent"
+        } else {
+            "operator: /cancel — no ACP turn to cancel"
+        };
+        state
+            .bus
+            .emit(crate::events::LiveEvent::Note { text: note.into() });
+        return (axum::http::StatusCode::OK, "ok").into_response();
+    }
     let out = super::sigint_claude().await;
     let note = match out {
         SigintOutcome::Signalled => {