diff --git a/apps/extension/PRIVACY.md b/apps/extension/PRIVACY.md
index 29527a4..9517a3c 100644
--- a/apps/extension/PRIVACY.md
+++ b/apps/extension/PRIVACY.md
@@ -48,6 +48,7 @@ The Extension requests the following Chrome permissions. Each is used solely for
 - **`tabs`** — Inspect, create, and close tabs in the Agent Window; query tab metadata.
 - **`windows`** — Create and manage the dedicated Agent Window that isolates agent activity from the user's normal browsing.
 - **`alarms`** — Periodically wake the service worker to keep the local WebSocket connection alive.
+- **`idle`** — Detect when the device returns from idle/locked so the Extension can promptly re-establish the local WebSocket connection after the machine wakes. No idle data is stored or transmitted.
 - **`notifications`** — Show a system notification to obtain user approval before the agent borrows a user-owned tab.
 - **`storage`** — Persist a random instance ID and optional label in `chrome.storage.local`.
 - **Host permission `<all_urls>`** — Inject a small status overlay (showing "Agent Active") on pages controlled by the agent, and enable automation across whatever sites the user directs the agent to. The Extension does **not** read or transmit page content from sites the agent is not actively driving.
diff --git a/apps/extension/package.json b/apps/extension/package.json
index d651876..87a92db 100644
--- a/apps/extension/package.json
+++ b/apps/extension/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@browser-skill/extension",
-  "version": "0.1.4",
+  "version": "0.1.5",
   "private": true,
   "type": "module",
   "scripts": {
diff --git a/apps/extension/src/entrypoints/background.ts b/apps/extension/src/entrypoints/background.ts
index 26fd7b0..6192811 100644
--- a/apps/extension/src/entrypoints/background.ts
+++ b/apps/extension/src/entrypoints/background.ts
@@ -1,6 +1,7 @@
 import { i18n } from "@browser-skill/i18n";
 import { ChromiumCdp } from "@/browser-driver/chromium-cdp";
 import { ConnectionController } from "@/lib/connection-controller";
+import { startHeartbeat } from "@/lib/heartbeat";
 import {
   getConnectionEnabled,
   setConnectionEnabled as persistConnectionEnabled,
@@ -139,6 +140,45 @@ export default defineBackground(() => {
     shouldConnect: () => controller.isConnectionEnabled,
   });
 
+  // Application-level heartbeat (Chrome 116+): while the post-handshake
+  // link is live, beat every 20s so WebSocket activity keeps the service
+  // worker — and thus the daemon connection — alive during use, rather
+  // than depending on the boundary-hugging 30s keepalive alarm. The
+  // daemon also uses these beats to reap a silently-dead browser.
+  startHeartbeat({
+    send: (frame) => transport.send(frame),
+    onActiveChange: (cb) =>
+      controller.subscribe((snap) =>
+        cb(snap.state === "connected" || snap.state === "version_skew"),
+      ),
+  });
+
+  // Wake-driven reconnect (best effort). An MV3 service worker is killed
+  // across OS sleep regardless of any keepalive, and the setTimeout-based
+  // transport backoff dies with it. These Chrome lifecycle events revive
+  // the worker and let us reconnect immediately instead of waiting for
+  // the next 30s alarm tick. They only help when the daemon is actually
+  // running; a cold daemon is (re)spawned by the next `bsk` command.
+  const reconnectIfNeeded = () => {
+    if (!controller.isConnectionEnabled) return;
+    if (transport.state === "connected") return;
+    void transport.connect().catch((err) => {
+      console.debug("[browser-skill] wake reconnect attempt failed", err);
+    });
+  };
+  if (typeof chrome.runtime?.onStartup?.addListener === "function") {
+    chrome.runtime.onStartup.addListener(reconnectIfNeeded);
+  }
+  if (typeof chrome.idle?.onStateChanged?.addListener === "function") {
+    chrome.idle.onStateChanged.addListener((state) => {
+      // "active" fires when the user returns from idle/locked — the most
+      // reliable "machine just woke" signal we get.
+      if (state === "active") reconnectIfNeeded();
+    });
+    // Treat >60s of no input as idle so the active transition is timely.
+    chrome.idle.setDetectionInterval?.(60);
+  }
+
   void (async () => {
     const connectionEnabled = await getConnectionEnabled();
     await controller.attach(transport, detectBrowserMeta(), connectionEnabled);
diff --git a/apps/extension/src/lib/__tests__/heartbeat.test.ts b/apps/extension/src/lib/__tests__/heartbeat.test.ts
new file mode 100644
index 0000000..12ddf74
--- /dev/null
+++ b/apps/extension/src/lib/__tests__/heartbeat.test.ts
@@ -0,0 +1,130 @@
+import { describe, expect, it, vi } from "vitest";
+import type { ProtocolFrame } from "@/transport/types";
+import { HEARTBEAT_EVENT, HEARTBEAT_INTERVAL_MS, startHeartbeat } from "../heartbeat";
+
+function makeTimers() {
+  let nextId = 1;
+  const intervals = new Map<number, { cb: () => void; ms: number }>();
+  return {
+    intervals,
+    api: {
+      setInterval: (cb: () => void, ms: number) => {
+        const id = nextId++;
+        intervals.set(id, { cb, ms });
+        return id;
+      },
+      clearInterval: (id: number) => {
+        intervals.delete(id);
+      },
+    },
+    tick: (id: number) => intervals.get(id)?.cb(),
+  };
+}
+
+/** Drives `onActiveChange` subscribers so a test can flip the link state. */
+function makeActiveSource() {
+  const listeners = new Set<(active: boolean) => void>();
+  return {
+    subscribe: (cb: (active: boolean) => void) => {
+      // Mirror the controller contract: fire immediately with the current
+      // value (starts inactive), then on every change.
+      cb(false);
+      listeners.add(cb);
+      return () => listeners.delete(cb);
+    },
+    set: (active: boolean) => {
+      for (const cb of listeners) cb(active);
+    },
+    listenerCount: () => listeners.size,
+  };
+}
+
+describe("startHeartbeat", () => {
+  it("does not beat while the link is inactive", () => {
+    const timers = makeTimers();
+    const active = makeActiveSource();
+    const send = vi.fn();
+    startHeartbeat({ send, onActiveChange: active.subscribe, timers: timers.api });
+
+    expect(timers.intervals.size).toBe(0);
+    expect(send).not.toHaveBeenCalled();
+  });
+
+  it("starts a 20s interval on activation and sends heartbeat events", () => {
+    const timers = makeTimers();
+    const active = makeActiveSource();
+    const send = vi.fn();
+    startHeartbeat({ send, onActiveChange: active.subscribe, timers: timers.api });
+
+    active.set(true);
+    expect(timers.intervals.size).toBe(1);
+    const [id, info] = [...timers.intervals.entries()][0];
+    expect(info.ms).toBe(HEARTBEAT_INTERVAL_MS);
+
+    timers.tick(id);
+    expect(send).toHaveBeenCalledWith({ event: HEARTBEAT_EVENT, payload: {} });
+  });
+
+  it("stops beating when the link goes inactive", () => {
+    const timers = makeTimers();
+    const active = makeActiveSource();
+    const send = vi.fn();
+    startHeartbeat({ send, onActiveChange: active.subscribe, timers: timers.api });
+
+    active.set(true);
+    expect(timers.intervals.size).toBe(1);
+    active.set(false);
+    expect(timers.intervals.size).toBe(0);
+  });
+
+  it("does not stack intervals on repeated activation", () => {
+    const timers = makeTimers();
+    const active = makeActiveSource();
+    startHeartbeat({ send: vi.fn(), onActiveChange: active.subscribe, timers: timers.api });
+
+    active.set(true);
+    active.set(true);
+    expect(timers.intervals.size).toBe(1);
+  });
+
+  it("swallows send errors so the interval keeps running", () => {
+    const timers = makeTimers();
+    const active = makeActiveSource();
+    const send = vi.fn(() => {
+      throw new Error("socket closed");
+    });
+    const handle = startHeartbeat({ send, onActiveChange: active.subscribe, timers: timers.api });
+    expect(() => handle.beatForTest()).not.toThrow();
+  });
+
+  it("dispose clears the interval and unsubscribes", () => {
+    const timers = makeTimers();
+    const active = makeActiveSource();
+    const handle = startHeartbeat({
+      send: vi.fn(),
+      onActiveChange: active.subscribe,
+      timers: timers.api,
+    });
+    active.set(true);
+    expect(timers.intervals.size).toBe(1);
+
+    handle.dispose();
+    expect(timers.intervals.size).toBe(0);
+    expect(active.listenerCount()).toBe(0);
+  });
+
+  it("sends a well-formed protocol frame", () => {
+    const timers = makeTimers();
+    const active = makeActiveSource();
+    let captured: ProtocolFrame | null = null;
+    const handle = startHeartbeat({
+      send: (f) => {
+        captured = f;
+      },
+      onActiveChange: active.subscribe,
+      timers: timers.api,
+    });
+    handle.beatForTest();
+    expect(captured).toEqual({ event: HEARTBEAT_EVENT, payload: {} });
+  });
+});
diff --git a/apps/extension/src/lib/heartbeat.ts b/apps/extension/src/lib/heartbeat.ts
new file mode 100644
index 0000000..7869291
--- /dev/null
+++ b/apps/extension/src/lib/heartbeat.ts
@@ -0,0 +1,110 @@
+/**
+ * Application-level WebSocket heartbeat.
+ *
+ * Since Chrome 116, sending or receiving a message over a WebSocket
+ * resets the MV3 service-worker idle timer (see
+ * https://developer.chrome.com/docs/extensions/how-to/web-platform/websockets).
+ * By emitting a tiny `system.heartbeat` event every 20s — comfortably
+ * inside the 30s idle window — we keep the worker (and therefore the
+ * daemon link) alive for as long as the connection is up, instead of
+ * relying solely on the 30s keepalive alarm that sits right on the
+ * eviction boundary.
+ *
+ * The daemon also treats the heartbeat as a liveness signal, so a
+ * silently-dead browser can be reaped on its side.
+ *
+ * The heartbeat only runs while the *post-handshake* link is live: the
+ * daemon rejects any first frame that is not `system.handshake`, so
+ * beating before the handshake completes would get the connection
+ * kicked. Callers wire {@link HeartbeatOptions.onActiveChange} to the
+ * connection controller, which only reports `connected` / `version_skew`
+ * after the handshake has landed.
+ */
+
+import type { ProtocolFrame } from "@/transport/types";
+
+export const HEARTBEAT_EVENT = "system.heartbeat";
+/** 20s: the interval Chrome's own WebSocket keepalive sample recommends. */
+export const HEARTBEAT_INTERVAL_MS = 20_000;
+
+/** Subset of the timer API we use; lets tests inject fakes. */
+export interface HeartbeatTimers {
+  setInterval(cb: () => void, ms: number): number;
+  clearInterval(handle: number): void;
+}
+
+export interface HeartbeatOptions {
+  /** Send a frame over the live transport. */
+  send: (frame: ProtocolFrame) => void;
+  /**
+   * Subscribe to "is the post-handshake link live" changes. Must invoke
+   * the callback with the current value immediately and again on every
+   * transition, and return an unsubscribe function.
+   */
+  onActiveChange: (cb: (active: boolean) => void) => () => void;
+  /** Override the beat interval (tests). */
+  intervalMs?: number;
+  /** Inject fake timers (tests). */
+  timers?: HeartbeatTimers;
+}
+
+export interface HeartbeatHandle {
+  /** Stop beating and detach the active-state listener. */
+  dispose(): void;
+  /** Test hook: emit one heartbeat frame right now. */
+  beatForTest(): void;
+}
+
+function defaultTimers(): HeartbeatTimers {
+  return {
+    setInterval: (cb, ms) => setInterval(cb, ms) as unknown as number,
+    clearInterval: (handle) => clearInterval(handle),
+  };
+}
+
+/**
+ * Start a heartbeat that beats every `intervalMs` while the link is
+ * active and pauses entirely while it is not.
+ */
+export function startHeartbeat(options: HeartbeatOptions): HeartbeatHandle {
+  const intervalMs = options.intervalMs ?? HEARTBEAT_INTERVAL_MS;
+  const timers = options.timers ?? defaultTimers();
+  let timer: number | null = null;
+
+  const beat = () => {
+    try {
+      options.send({ event: HEARTBEAT_EVENT, payload: {} });
+    } catch (err) {
+      // The socket raced a close between the state change and this
+      // tick. The transport's own reconnect path recovers; swallow so
+      // the interval keeps running (it is cleared on the disconnect
+      // notification anyway).
+      console.debug("[bsk heartbeat] send failed", err);
+    }
+  };
+
+  const stop = () => {
+    if (timer !== null) {
+      timers.clearInterval(timer);
+      timer = null;
+    }
+  };
+
+  const start = () => {
+    if (timer !== null) return;
+    timer = timers.setInterval(beat, intervalMs);
+  };
+
+  const unsubscribe = options.onActiveChange((active) => {
+    if (active) start();
+    else stop();
+  });
+
+  return {
+    dispose: () => {
+      unsubscribe();
+      stop();
+    },
+    beatForTest: beat,
+  };
+}
diff --git a/apps/extension/wxt.config.ts b/apps/extension/wxt.config.ts
index 4fd0ce8..34c72b6 100644
--- a/apps/extension/wxt.config.ts
+++ b/apps/extension/wxt.config.ts
@@ -25,9 +25,14 @@ export default defineConfig({
     name: "BrowserSkill",
     description:
       "Let AI agents use your logged-in browser in a separate Agent Window—without interrupting your work. Powered by the bsk CLI.",
+    // Chrome 116 is the first version where WebSocket activity resets the
+    // service-worker idle timer, which the 20s heartbeat relies on to
+    // keep the daemon link alive.
+    minimum_chrome_version: "116",
     permissions: [
       "alarms",
       "debugger",
+      "idle",
       "notifications",
       "tabs",
       "storage",
diff --git a/crates/bsk-cli/src/cli/browser_wait.rs b/crates/bsk-cli/src/cli/browser_wait.rs
index 56ef246..1ac4767 100644
--- a/crates/bsk-cli/src/cli/browser_wait.rs
+++ b/crates/bsk-cli/src/cli/browser_wait.rs
@@ -2,10 +2,16 @@
 
 use std::time::Duration;
 
-use crate::daemon::browsers::EXTENSION_CONNECT_WAIT;
-
 /// Default wait passed to daemon status/list RPCs when the registry is empty.
-pub const DEFAULT_BROWSER_CONNECT_WAIT: Duration = EXTENSION_CONNECT_WAIT;
+///
+/// Deliberately shorter than the daemon's [`EXTENSION_CONNECT_WAIT`]
+/// (used by `session.start`): `bsk status` / `bsk browsers` are quick
+/// informational snapshots, so blocking them for a full keepalive-alarm
+/// period would hurt more than it helps. The long wait is reserved for
+/// the command path that actually needs the extension online.
+///
+/// [`EXTENSION_CONNECT_WAIT`]: crate::daemon::browsers::EXTENSION_CONNECT_WAIT
+pub const DEFAULT_BROWSER_CONNECT_WAIT: Duration = Duration::from_secs(5);
 /// Keep CLI-side waits aligned with the daemon's IPC clamp so a bad env value
 /// cannot inflate read timeouts after the daemon has already returned.
 const MAX_BROWSER_CONNECT_WAIT: Duration = Duration::from_secs(60);
diff --git a/crates/bsk-cli/src/cli/session.rs b/crates/bsk-cli/src/cli/session.rs
index 1fb1d36..198f568 100644
--- a/crates/bsk-cli/src/cli/session.rs
+++ b/crates/bsk-cli/src/cli/session.rs
@@ -6,6 +6,8 @@
 //! structured JSON instead.
 
 use std::path::PathBuf;
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::time::Duration;
 
 use anyhow::Context;
@@ -17,9 +19,18 @@ use serde::{Deserialize, Serialize};
 
 use crate::cli::ensure_daemon::ensure_daemon;
 use crate::cli::error::{self, CliError, Format, RenderExtras};
+use crate::daemon::browsers::EXTENSION_CONNECT_WAIT;
 
 const SESSION_STOP_IPC_TIMEOUT: Duration = Duration::from_secs(60 * 60);
 
+/// IPC read budget for `session.start`. The daemon holds this RPC open
+/// while it polls for the extension to (re)connect for up to
+/// [`EXTENSION_CONNECT_WAIT`], so the CLI-side timeout must exceed that
+/// window plus scheduling slack — otherwise the CLI would give up and
+/// surface a spurious timeout before the daemon ever answers.
+const SESSION_START_IPC_TIMEOUT: Duration =
+    EXTENSION_CONNECT_WAIT.saturating_add(Duration::from_secs(10));
+
 /// `bsk session …` subcommand tree.
 #[derive(Debug, Clone, Args)]
 pub struct SessionCmd {
@@ -113,7 +124,24 @@ pub fn dispatch(cmd: SessionCmd, format: Format) -> Result<(), CliError> {
 }
 
 fn run_start(sock: PathBuf, args: SessionStartArgs, format: Format) -> Result<(), CliError> {
+    // The daemon may hold `session.start` open for up to
+    // `EXTENSION_CONNECT_WAIT` while a just-woken service worker
+    // reconnects. Let the user know we are waiting rather than hung —
+    // but only if it actually takes a moment, so the common (already
+    // connected) fast path stays silent. Human output only; the hint
+    // goes to stderr so it never contaminates the session id on stdout.
+    let waited = Arc::new(AtomicBool::new(false));
+    if matches!(format, Format::Human) {
+        let waited = Arc::clone(&waited);
+        std::thread::spawn(move || {
+            std::thread::sleep(Duration::from_millis(750));
+            if !waited.swap(true, Ordering::SeqCst) {
+                eprintln!("waiting for browser extension to connect…");
+            }
+        });
+    }
     let result = start_session(sock, args.browser);
+    waited.store(true, Ordering::SeqCst);
     match result {
         Ok(reply) => match format {
             Format::Json => {
@@ -144,7 +172,7 @@ pub fn start_session(sock: PathBuf, browser: Option<String>) -> Result<StartRepl
         Some(StartParams {
             browser_instance_id: browser,
         }),
-        Duration::from_secs(30),
+        SESSION_START_IPC_TIMEOUT,
     )
 }
 
diff --git a/crates/bsk-cli/src/daemon/browsers.rs b/crates/bsk-cli/src/daemon/browsers.rs
index c25f3de..b1dad44 100644
--- a/crates/bsk-cli/src/daemon/browsers.rs
+++ b/crates/bsk-cli/src/daemon/browsers.rs
@@ -8,6 +8,7 @@
 use std::collections::HashMap;
 use std::fmt;
 use std::sync::Mutex;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::time::{Duration, Instant};
 
 use anyhow::Result;
@@ -22,10 +23,29 @@ use bsk_protocol::system::BrowserStatusEntry;
 /// seconds to wake, discover the WS port, and finish `system.handshake`.
 /// `session.start` polls the registry for up to this long before
 /// returning `no_browser_connected`.
-pub const EXTENSION_CONNECT_WAIT: Duration = Duration::from_secs(5);
+///
+/// Sized to comfortably cover one MV3 keepalive-alarm period (30s, the
+/// smallest Chrome allows) plus handshake slack: after a long idle the
+/// service worker is only revived on its next alarm tick, so a shorter
+/// window would make the first command after idle fail with
+/// `no_browser_connected` even though the extension is about to
+/// reconnect. Callers that want a snapshot (`bsk status`/`browsers`)
+/// pass their own, shorter wait instead of this constant.
+pub const EXTENSION_CONNECT_WAIT: Duration = Duration::from_secs(35);
 
 const EXTENSION_CONNECT_POLL: Duration = Duration::from_millis(50);
 
+/// A connected browser is considered dead if no frame (including the
+/// ~20s `system.heartbeat`) has arrived within this window. Set to three
+/// missed heartbeats so a briefly-busy service worker (GC, heavy CDP) is
+/// not reaped on a single late beat. Used by the liveness reaper to drop
+/// half-open connections that never delivered a socket close — e.g. after
+/// the OS resumed from sleep and killed the worker.
+pub const BROWSER_LIVENESS_TIMEOUT: Duration = Duration::from_secs(60);
+
+/// How often the liveness reaper scans the registry for stale browsers.
+pub const BROWSER_LIVENESS_TICK: Duration = Duration::from_secs(15);
+
 /// Process-wide monotonic counter for [`BrowserClient::generation`]. Used
 /// by the reconnect-race guard: when an old WS task tears down it only
 /// removes itself from the registry if the registry's current
@@ -125,9 +145,47 @@ pub struct BrowserClient {
     /// `true` when the peer's `protocol_version` differs from ours
     /// (same major, minor drift). Set during WS handshake (M10.4).
     pub version_skew: bool,
+    /// Monotonic timestamp of the most recent inbound frame from this
+    /// browser (any response/event, including `system.heartbeat`).
+    /// Bumped by [`BrowserClient::touch`] from the WS read loop and read
+    /// by the liveness reaper to detect a silently-dead connection.
+    pub last_seen: Mutex<Instant>,
+    /// `true` once this browser has sent at least one `system.heartbeat`.
+    /// The liveness reaper only acts on heartbeat-capable browsers: a
+    /// pre-heartbeat extension never sets this, so it is never reaped on
+    /// silence and instead relies on socket-close detection exactly as
+    /// before — otherwise a new daemon paired with an old extension would
+    /// wrongly drop a live-but-idle connection.
+    pub heartbeat_seen: AtomicBool,
 }
 
 impl BrowserClient {
+    /// Record that a frame just arrived from this browser.
+    pub fn touch(&self) {
+        if let Ok(mut last) = self.last_seen.lock() {
+            *last = Instant::now();
+        }
+    }
+
+    /// Note that a `system.heartbeat` arrived, opting this browser in to
+    /// liveness reaping.
+    pub fn mark_heartbeat_seen(&self) {
+        self.heartbeat_seen.store(true, Ordering::Relaxed);
+    }
+
+    /// Whether this browser has ever sent a heartbeat.
+    pub fn has_heartbeat(&self) -> bool {
+        self.heartbeat_seen.load(Ordering::Relaxed)
+    }
+
+    /// How long since the last inbound frame from this browser.
+    pub fn idle_for(&self) -> Duration {
+        match self.last_seen.lock() {
+            Ok(last) => last.elapsed(),
+            Err(poisoned) => poisoned.into_inner().elapsed(),
+        }
+    }
+
     pub fn status_entry(&self, session_count: u32) -> BrowserStatusEntry {
         BrowserStatusEntry {
             instance_id: self.id.0.clone(),
@@ -206,6 +264,23 @@ impl BrowserRegistry {
         guard.values().cloned().collect()
     }
 
+    /// Return every heartbeat-capable browser whose last inbound frame is
+    /// older than `threshold`. Browsers that have never sent a heartbeat
+    /// (legacy extensions) are excluded so the reaper never drops a
+    /// live-but-idle connection it cannot probe. The caller is
+    /// responsible for dropping the returned entries (via
+    /// [`Self::remove_if_generation_matches`]) and purging their sessions
+    /// — done outside the lock so session teardown never blocks the
+    /// registry.
+    pub fn stale_browsers(&self, threshold: Duration) -> Vec<std::sync::Arc<BrowserClient>> {
+        let guard = self.inner.lock().expect("browser registry poisoned");
+        guard
+            .values()
+            .filter(|c| c.has_heartbeat() && c.idle_for() >= threshold)
+            .cloned()
+            .collect()
+    }
+
     pub fn len(&self) -> usize {
         self.inner.lock().expect("browser registry poisoned").len()
     }
@@ -348,6 +423,8 @@ mod tests {
             generation: next_browser_generation(),
             connected_at_ms: 0,
             version_skew: false,
+            last_seen: Mutex::new(Instant::now()),
+            heartbeat_seen: AtomicBool::new(false),
         })
     }
 
@@ -543,6 +620,51 @@ mod tests {
         assert_eq!(reg.len(), 1);
     }
 
+    #[test]
+    fn touch_resets_idle_for() {
+        let client = fake_client("a", "");
+        // Force the last_seen well into the past, then touch.
+        *client.last_seen.lock().unwrap() = Instant::now() - Duration::from_secs(120);
+        assert!(client.idle_for() >= Duration::from_secs(120));
+        client.touch();
+        assert!(client.idle_for() < Duration::from_secs(1));
+    }
+
+    #[test]
+    fn stale_browsers_reports_only_silent_heartbeat_capable_connections() {
+        let reg = BrowserRegistry::new();
+        let fresh = fake_client("fresh", "");
+        fresh.mark_heartbeat_seen();
+        let stale = fake_client("stale", "");
+        stale.mark_heartbeat_seen();
+        *stale.last_seen.lock().unwrap() = Instant::now() - Duration::from_secs(90);
+        reg.insert(fresh);
+        reg.insert(stale);
+
+        let stale_ids: Vec<String> = reg
+            .stale_browsers(Duration::from_secs(60))
+            .into_iter()
+            .map(|c| c.id.0.clone())
+            .collect();
+        assert_eq!(stale_ids, vec!["stale".to_string()]);
+    }
+
+    #[test]
+    fn stale_browsers_never_reaps_a_browser_that_never_sent_a_heartbeat() {
+        // A legacy extension that predates the heartbeat must fall back
+        // to socket-close detection, never liveness reaping — even when
+        // it has been silent well past the threshold.
+        let reg = BrowserRegistry::new();
+        let legacy = fake_client("legacy", "");
+        *legacy.last_seen.lock().unwrap() = Instant::now() - Duration::from_secs(600);
+        reg.insert(legacy);
+
+        assert!(
+            reg.stale_browsers(Duration::from_secs(60)).is_empty(),
+            "a browser that never heartbeated must not be reaped"
+        );
+    }
+
     #[tokio::test]
     async fn wait_for_any_connected_skips_when_already_populated() {
         let reg = BrowserRegistry::new();
diff --git a/crates/bsk-cli/src/daemon/mod.rs b/crates/bsk-cli/src/daemon/mod.rs
index 99ee7e6..c639f73 100644
--- a/crates/bsk-cli/src/daemon/mod.rs
+++ b/crates/bsk-cli/src/daemon/mod.rs
@@ -45,6 +45,7 @@ pub async fn run(
         None => None,
     };
     let session_idle_task = start::spawn_session_idle_reaper(Arc::clone(&state));
+    let browser_liveness_task = start::spawn_browser_liveness_reaper(Arc::clone(&state));
     // Ensure WS/IPC accept loops have polled `shutdown.notified()` before any
     // caller can invoke `DaemonHandle::shutdown()` — `Notify::notify_waiters()`
     // drops wakeups when nothing is registered yet, which otherwise hangs
@@ -55,5 +56,6 @@ pub async fn run(
         ws_handle,
         ipc_handle,
         session_idle_task,
+        browser_liveness_task,
     ))
 }
diff --git a/crates/bsk-cli/src/daemon/start.rs b/crates/bsk-cli/src/daemon/start.rs
index 1d72e63..33327a1 100644
--- a/crates/bsk-cli/src/daemon/start.rs
+++ b/crates/bsk-cli/src/daemon/start.rs
@@ -23,7 +23,7 @@ use tracing::{debug, info, warn};
 
 use crate::cli::daemon::StartArgs;
 use crate::daemon::{
-    browsers::EXTENSION_CONNECT_WAIT,
+    browsers::{BROWSER_LIVENESS_TICK, BROWSER_LIVENESS_TIMEOUT, EXTENSION_CONNECT_WAIT},
     info as daemon_info, ipc, lockfile, paths,
     sessions::{StopSessionError, forget_session, stop_session},
     state::DaemonState,
@@ -45,6 +45,12 @@ pub struct DaemonConfig {
     /// How long `session.start` polls for an extension handshake before
     /// returning `no_browser_connected`.
     pub extension_connect_wait: Duration,
+    /// A heartbeat-capable browser silent for at least this long is
+    /// reaped by the liveness task. Defaults to [`BROWSER_LIVENESS_TIMEOUT`].
+    pub browser_liveness_timeout: Duration,
+    /// How often the liveness task scans the registry. Defaults to
+    /// [`BROWSER_LIVENESS_TICK`].
+    pub browser_liveness_tick: Duration,
 }
 
 impl DaemonConfig {
@@ -58,6 +64,8 @@ impl DaemonConfig {
             daemon_idle: Duration::from_secs(60 * 30),
             allow_any_origin: false,
             extension_connect_wait: EXTENSION_CONNECT_WAIT,
+            browser_liveness_timeout: BROWSER_LIVENESS_TIMEOUT,
+            browser_liveness_tick: BROWSER_LIVENESS_TICK,
         }
     }
 
@@ -65,6 +73,15 @@ impl DaemonConfig {
         self.extension_connect_wait = wait;
         self
     }
+
+    /// Override the liveness reaper's silence threshold and scan cadence.
+    /// Primarily for tests that need the reaper to act within
+    /// sub-second windows instead of the production 60s/15s defaults.
+    pub fn with_browser_liveness(mut self, timeout: Duration, tick: Duration) -> Self {
+        self.browser_liveness_timeout = timeout;
+        self.browser_liveness_tick = tick;
+        self
+    }
 }
 
 impl From<&StartArgs> for DaemonConfig {
@@ -75,6 +92,8 @@ impl From<&StartArgs> for DaemonConfig {
             daemon_idle: args.resolved_daemon_idle(),
             allow_any_origin: false,
             extension_connect_wait: EXTENSION_CONNECT_WAIT,
+            browser_liveness_timeout: BROWSER_LIVENESS_TIMEOUT,
+            browser_liveness_tick: BROWSER_LIVENESS_TICK,
         }
     }
 }
@@ -200,6 +219,7 @@ pub fn run_foreground(cfg: DaemonConfig) -> Result<()> {
 
         let state = Arc::new(DaemonState::new(cfg.clone()));
         let session_idle_task = spawn_session_idle_reaper(Arc::clone(&state));
+        let browser_liveness_task = spawn_browser_liveness_reaper(Arc::clone(&state));
         let ws_addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), cfg.ws_port);
         let ws_handle = ws::WsServer::new(Arc::clone(&state))
             .bind(ws_addr)
@@ -361,6 +381,8 @@ pub fn run_foreground(cfg: DaemonConfig) -> Result<()> {
         let _ = ipc_task.await;
         session_idle_task.abort();
         let _ = session_idle_task.await;
+        browser_liveness_task.abort();
+        let _ = browser_liveness_task.await;
         ws_handle.shutdown.notify_waiters();
         let _ = ws_handle.task.await;
 
@@ -373,6 +395,53 @@ pub fn run_foreground(cfg: DaemonConfig) -> Result<()> {
     Ok(())
 }
 
+/// Spawn the browser-liveness reaper shared by the production foreground
+/// daemon and the test/embed entry point.
+///
+/// A normal disconnect closes the WebSocket, which the per-connection
+/// read loop observes and cleans up. But if the OS resumed from sleep
+/// and killed the MV3 service worker, the socket can be left half-open
+/// with no close frame ever delivered. Without the ~20s `system.heartbeat`
+/// arriving, such a connection would otherwise sit in the registry
+/// forever — pinning a phantom "connected" browser and preventing the
+/// daemon from ever idle-exiting. This reaper drops any browser that has
+/// gone silent past [`BROWSER_LIVENESS_TIMEOUT`] and purges its sessions,
+/// mirroring the WS disconnect cleanup.
+pub(crate) fn spawn_browser_liveness_reaper(
+    state: Arc<DaemonState>,
+) -> tokio::task::JoinHandle<()> {
+    let timeout = state.config.browser_liveness_timeout;
+    let tick = state.config.browser_liveness_tick;
+    tokio::spawn(async move {
+        let mut ticker = tokio::time::interval(tick);
+        ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+        // The first tick is immediate; skip it so a freshly connected
+        // browser always gets at least one full window before a scan.
+        ticker.tick().await;
+        loop {
+            ticker.tick().await;
+            for client in state.browsers.stale_browsers(timeout) {
+                if state
+                    .browsers
+                    .remove_if_generation_matches(&client.id, client.generation)
+                    .is_some()
+                {
+                    warn!(
+                        id = %client.id,
+                        idle_secs = client.idle_for().as_secs(),
+                        "reaping unresponsive browser (no heartbeat within liveness window)"
+                    );
+                    for s in state.sessions.purge_browser(&client.id) {
+                        state.tool_queues.remove(&s.id);
+                        state.session_interrupts.drop_session(&s.id);
+                        debug!(session = %s.id, "purged session on browser liveness timeout");
+                    }
+                }
+            }
+        }
+    })
+}
+
 /// Spawn the cooperative session-idle reaper shared by the production
 /// foreground daemon and the test/embed daemon entry point.
 pub(crate) fn spawn_session_idle_reaper(state: Arc<DaemonState>) -> tokio::task::JoinHandle<()> {
diff --git a/crates/bsk-cli/src/daemon/state.rs b/crates/bsk-cli/src/daemon/state.rs
index 203b3ac..d6632da 100644
--- a/crates/bsk-cli/src/daemon/state.rs
+++ b/crates/bsk-cli/src/daemon/state.rs
@@ -82,6 +82,7 @@ pub struct DaemonHandle {
     ws: WsHandle,
     ipc: Option<IpcHandle>,
     session_idle_task: JoinHandle<()>,
+    browser_liveness_task: JoinHandle<()>,
 }
 
 impl DaemonHandle {
@@ -90,12 +91,14 @@ impl DaemonHandle {
         ws: WsHandle,
         ipc: Option<IpcHandle>,
         session_idle_task: JoinHandle<()>,
+        browser_liveness_task: JoinHandle<()>,
     ) -> Self {
         Self {
             state,
             ws,
             ipc,
             session_idle_task,
+            browser_liveness_task,
         }
     }
 
@@ -116,6 +119,8 @@ impl DaemonHandle {
     pub async fn shutdown(self) {
         self.session_idle_task.abort();
         let _ = await_join(self.session_idle_task).await;
+        self.browser_liveness_task.abort();
+        let _ = await_join(self.browser_liveness_task).await;
         self.ws.shutdown.notify_waiters();
         let _ = await_join(self.ws.task).await;
         if let Some(ipc) = self.ipc {
diff --git a/crates/bsk-cli/src/daemon/ws.rs b/crates/bsk-cli/src/daemon/ws.rs
index 4073771..7d34cc1 100644
--- a/crates/bsk-cli/src/daemon/ws.rs
+++ b/crates/bsk-cli/src/daemon/ws.rs
@@ -304,6 +304,8 @@ async fn drive_connection(
         generation,
         connected_at_ms,
         version_skew,
+        last_seen: std::sync::Mutex::new(std::time::Instant::now()),
+        heartbeat_seen: std::sync::atomic::AtomicBool::new(false),
     });
     state.browsers.insert(Arc::clone(&client));
     info!(
@@ -352,15 +354,21 @@ async fn drive_connection(
                 msg = reader.next() => {
                     match msg {
                         Some(Ok(Message::Text(t))) => {
+                            // Any inbound frame — tool response, event, or the
+                            // ~20s heartbeat — counts as proof of life.
+                            pump_browser.touch();
                             handle_inbound_text(&pump_state, &pump_browser, &t).await;
                         }
                         Some(Ok(Message::Binary(_))) => {
                             warn!("ignoring binary message");
                         }
                         Some(Ok(Message::Ping(p))) => {
+                            pump_browser.touch();
                             let _ = writer.send(Message::Pong(p)).await;
                         }
-                        Some(Ok(Message::Pong(_))) => {}
+                        Some(Ok(Message::Pong(_))) => {
+                            pump_browser.touch();
+                        }
                         Some(Ok(Message::Frame(_))) => {}
                         Some(Ok(Message::Close(_))) | None => break,
                         Some(Err(err)) => {
@@ -416,6 +424,13 @@ async fn handle_inbound_text(state: &Arc<DaemonState>, client: &Arc<BrowserClien
             }
         }
         Frame::Event(ev) => match ev.event {
+            bsk_protocol::EventKind::SystemHeartbeat => {
+                // `touch()` already ran for this frame in the read loop;
+                // additionally opt this browser in to liveness reaping now
+                // that we know it speaks the heartbeat.
+                client.mark_heartbeat_seen();
+                debug!(id = %client.id, "heartbeat");
+            }
             bsk_protocol::EventKind::SessionWindowClosed => {
                 handle_session_window_closed(state, &client.id, &ev.payload);
             }
diff --git a/crates/bsk-cli/tests/browser_list_ordering.rs b/crates/bsk-cli/tests/browser_list_ordering.rs
index f19f320..86031d7 100644
--- a/crates/bsk-cli/tests/browser_list_ordering.rs
+++ b/crates/bsk-cli/tests/browser_list_ordering.rs
@@ -51,6 +51,8 @@ fn fake_client(id: &str, label: &str, connected_at_ms: i64) -> Arc<BrowserClient
         generation: next_browser_generation(),
         connected_at_ms,
         version_skew: false,
+        last_seen: Mutex::new(std::time::Instant::now()),
+        heartbeat_seen: std::sync::atomic::AtomicBool::new(false),
     })
 }
 
diff --git a/crates/bsk-cli/tests/browser_liveness.rs b/crates/bsk-cli/tests/browser_liveness.rs
new file mode 100644
index 0000000..444b7ab
--- /dev/null
+++ b/crates/bsk-cli/tests/browser_liveness.rs
@@ -0,0 +1,111 @@
+//! Integration coverage for the daemon's browser-liveness reaper
+//! (`spawn_browser_liveness_reaper`).
+//!
+//! These drive the *task* — not just the `stale_browsers` predicate —
+//! by spinning up a real daemon with a sub-second liveness window and
+//! observing that a silent, heartbeat-capable browser (and its sessions)
+//! get dropped, while a legacy browser that never heartbeated is left
+//! alone (backward-compat guard).
+
+use std::sync::Mutex;
+use std::sync::atomic::AtomicBool;
+use std::time::{Duration, Instant};
+
+use bsk::daemon::browsers::{
+    BrowserClient, BrowserId, BrowserSink, Pending, next_browser_generation,
+};
+use bsk::daemon::sessions::{Session, SessionId};
+use bsk::daemon::{self, DaemonConfig};
+use tokio::sync::mpsc;
+
+/// Build a registry client. `heartbeat_seen` opts it in to reaping and
+/// `idle_secs` back-dates its `last_seen` so it looks silent immediately.
+fn fake_client(id: &str, heartbeat_seen: bool, idle_secs: u64) -> std::sync::Arc<BrowserClient> {
+    let (tx, _rx) = mpsc::unbounded_channel::<bsk_protocol::Frame>();
+    std::sync::Arc::new(BrowserClient {
+        id: BrowserId(id.into()),
+        browser_name: "chrome".into(),
+        browser_version: "131.0".into(),
+        extension_version: "0.1.4".into(),
+        extension_protocol_version: "1.0".into(),
+        label: String::new(),
+        sink: BrowserSink { tx },
+        pending: Mutex::new(Pending::default()),
+        generation: next_browser_generation(),
+        connected_at_ms: 0,
+        version_skew: false,
+        last_seen: Mutex::new(Instant::now() - Duration::from_secs(idle_secs)),
+        heartbeat_seen: AtomicBool::new(heartbeat_seen),
+    })
+}
+
+fn fake_session(session_id: &str, browser_id: &str) -> Session {
+    Session {
+        id: SessionId(session_id.into()),
+        browser_id: BrowserId(browser_id.into()),
+        agent_window_id: None,
+        created_at_ms: 0,
+    }
+}
+
+async fn spawn_reaping_daemon() -> daemon::DaemonHandle {
+    // Tiny window so the reaper acts within the test's lifetime.
+    let config = DaemonConfig::new(0)
+        .with_browser_liveness(Duration::from_millis(150), Duration::from_millis(25));
+    daemon::run(config, None).await.unwrap()
+}
+
+async fn wait_until<F: Fn() -> bool>(pred: F, within: Duration) -> bool {
+    let deadline = Instant::now() + within;
+    while Instant::now() < deadline {
+        if pred() {
+            return true;
+        }
+        tokio::time::sleep(Duration::from_millis(10)).await;
+    }
+    pred()
+}
+
+#[tokio::test]
+async fn reaper_drops_silent_heartbeat_browser_and_purges_its_sessions() {
+    let handle = spawn_reaping_daemon().await;
+    let state = handle.state();
+
+    state.browsers.insert(fake_client("hb", true, 10));
+    state.sessions.insert(fake_session("sess", "hb"));
+    assert_eq!(state.browsers.len(), 1);
+    assert_eq!(state.sessions.count_for_browser(&BrowserId("hb".into())), 1);
+
+    let dropped = wait_until(
+        || state.browsers.get(&BrowserId("hb".into())).is_none(),
+        Duration::from_secs(2),
+    )
+    .await;
+    assert!(dropped, "silent heartbeat-capable browser should be reaped");
+
+    // The reaper also tears down the browser's sessions.
+    assert!(
+        wait_until(|| state.sessions.is_empty(), Duration::from_secs(1)).await,
+        "reaping a browser must purge its sessions"
+    );
+
+    handle.shutdown().await;
+}
+
+#[tokio::test]
+async fn reaper_leaves_legacy_browser_without_heartbeat_alone() {
+    let handle = spawn_reaping_daemon().await;
+    let state = handle.state();
+
+    // Silent far past the window, but never heartbeated → must survive.
+    state.browsers.insert(fake_client("legacy", false, 600));
+
+    // Give the reaper several scan cycles to (wrongly) act.
+    tokio::time::sleep(Duration::from_millis(300)).await;
+    assert!(
+        state.browsers.get(&BrowserId("legacy".into())).is_some(),
+        "a browser that never heartbeated must not be reaped on silence"
+    );
+
+    handle.shutdown().await;
+}
diff --git a/crates/bsk-cli/tests/handshake_compat.rs b/crates/bsk-cli/tests/handshake_compat.rs
index 596d35e..a5e91b0 100644
--- a/crates/bsk-cli/tests/handshake_compat.rs
+++ b/crates/bsk-cli/tests/handshake_compat.rs
@@ -242,6 +242,8 @@ async fn status_surfaces_version_skew_for_skewed_browser() {
         generation: bsk::daemon::browsers::next_browser_generation(),
         connected_at_ms: 0,
         version_skew: true,
+        last_seen: Mutex::new(std::time::Instant::now()),
+        heartbeat_seen: std::sync::atomic::AtomicBool::new(false),
     });
     state.browsers.insert(client);
 
diff --git a/crates/bsk-protocol/src/frame.rs b/crates/bsk-protocol/src/frame.rs
index 019e58a..996d487 100644
--- a/crates/bsk-protocol/src/frame.rs
+++ b/crates/bsk-protocol/src/frame.rs
@@ -40,6 +40,14 @@ pub struct EventFrame {
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum EventKind {
+    /// Application-level keepalive emitted by the extension roughly
+    /// every 20s while the WS link is up. Two purposes: (1) the
+    /// send/receive activity resets the MV3 service-worker idle timer
+    /// (Chrome 116+), keeping the worker — and therefore the socket —
+    /// alive during use; (2) the daemon treats it as a liveness signal
+    /// so a silently-dead browser can be reaped. Carries no payload.
+    #[serde(rename = "system.heartbeat")]
+    SystemHeartbeat,
     #[serde(rename = "session.activity")]
     SessionActivity,
     #[serde(rename = "session.window_closed")]
@@ -230,6 +238,24 @@ impl<'de> Visitor<'de> for FrameVisitor {
 mod tests {
     use super::*;
 
+    #[test]
+    fn system_heartbeat_serialises_as_dotted_name() {
+        // The extension hardcodes the literal string "system.heartbeat"
+        // when it emits the keepalive event; this locks the daemon-side
+        // serde name to the same wire value so a rename cannot silently
+        // break liveness/keepalive.
+        let v = serde_json::to_value(EventKind::SystemHeartbeat).unwrap();
+        assert_eq!(v, serde_json::json!("system.heartbeat"));
+    }
+
+    #[test]
+    fn system_heartbeat_event_frame_round_trips_from_extension_shape() {
+        // Mirrors exactly what the extension sends: { event, payload: {} }.
+        let wire = serde_json::json!({ "event": "system.heartbeat", "payload": {} });
+        let frame: EventFrame = serde_json::from_value(wire).unwrap();
+        assert_eq!(frame.event, EventKind::SystemHeartbeat);
+    }
+
     #[test]
     fn session_user_interrupt_serialises_as_snake_case() {
         let v = serde_json::to_value(EventKind::SessionUserInterrupt).unwrap();