diff --git a/frontend/dashboard.css b/frontend/dashboard.css
index fcc993d..00656fb 100644
--- a/frontend/dashboard.css
+++ b/frontend/dashboard.css
@@ -926,6 +926,10 @@ textarea { resize: vertical; min-height: 120px; }
 
 .mobile-today-hud { display:none; }
 .mobile-today-hud[hidden], .mobile-today-hud[data-overlay-hidden="true"] { display:none; }
+.today-session-handoff { position:fixed; left:12px; right:12px; bottom:calc(68px + env(safe-area-inset-bottom)); z-index:45; display:grid; grid-template-columns:minmax(0,1fr) auto; align-items:center; gap:10px; padding:12px; border:1px solid #60a5fa; border-radius:12px; background:rgba(16,38,65,.98); box-shadow:0 8px 28px rgba(0,0,0,.35); }
+.today-session-handoff[hidden] { display:none; }
+.today-session-handoff button { min-height:44px; }
+@media(max-width:420px) { .today-session-handoff { grid-template-columns:1fr; } .today-session-handoff button { width:100%; } }
 .mobile-task-action { min-width:0; min-height:44px; padding:6px 2px; border:0; border-radius:8px; background:transparent; display:grid; place-items:center; gap:2px; font-size:12px; }
 .mobile-task-action[hidden] { display:none; }
 .mobile-task-action[aria-current="page"] { color:#bfdbfe; background:#17365a; outline:1px solid #31577f; }
diff --git a/frontend/dashboard.js b/frontend/dashboard.js
index 4155af6..6c02429 100644
--- a/frontend/dashboard.js
+++ b/frontend/dashboard.js
@@ -1795,9 +1795,11 @@
         'Agenda progress could not be saved on this device. You can keep working.';
     },
   });
+  let todaySessionSync = null;
   const timer = createTodayTimer({
     storage: localStorage,
     getLogin: () => confirmedOwnerLogin,
+    onChange: snapshot => todaySessionSync?.publish(snapshot),
   });
   const timerView = createTodayTimerView({
     timer,
@@ -1814,6 +1816,15 @@
       return completeTodayItem(item);
     },
   });
+  todaySessionSync = attachTodaySessionHandoff({
+    fetchJson:fetchReviewJson, storage:localStorage, timer, qs,
+    items:() => [...todayMyWork, ...activeMyWork],
+    identity:item => todayWork.identity(item),
+    selectToday:() => selectTodayWork(),
+    startItem:item => workSession.start(item),
+    announce:message => { qs('#my-work-action-status').textContent = message; },
+    renderTimer:() => timerView.render(),
+  });
   renderAttentionInterruption();
   qs('#return-to-today').addEventListener('click', () => {
     const returned = timer.returnFromAttention();
diff --git a/frontend/index.html b/frontend/index.html
index 6d2c4f3..c49efc8 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -1515,6 +1515,14 @@
   </form>
 </dialog>
 
+<section class="today-session-handoff" id="today-session-handoff" aria-live="polite" hidden>
+  <div>
+    <strong>Today is active on another device</strong>
+    <div class="small" id="today-session-handoff-summary"></div>
+  </div>
+  <button id="continue-today-session" type="button">Continue here</button>
+</section>
+
 <section class="mobile-today-hud" data-mobile-today-hud aria-label="Active Today session" hidden>
   <button class="mobile-today-summary" data-mobile-today-open type="button" aria-label="Open current Today item"></button>
   <button data-mobile-today-complete type="button" hidden>Done &amp; next</button>
@@ -1616,6 +1624,7 @@
 <script src="static/work-selection.js"></script>
 <script src="static/today-work.js"></script>
 <script src="static/today-timer.js"></script>
+<script src="static/today-session-sync.js"></script>
 <script src="static/today-recap.js"></script>
 <script src="static/today-wrap-up.js"></script>
 <script src="static/today-handoff.js"></script>
diff --git a/frontend/service-worker.js b/frontend/service-worker.js
index 68a5ecb..1388fd9 100644
--- a/frontend/service-worker.js
+++ b/frontend/service-worker.js
@@ -51,6 +51,7 @@ const SHELL = [
   BASE + 'static/work-selection.js',
   BASE + 'static/today-work.js',
   BASE + 'static/today-timer.js',
+  BASE + 'static/today-session-sync.js',
   BASE + 'static/today-recap.js',
   BASE + 'static/today-wrap-up.js',
   BASE + 'static/today-handoff.js',
diff --git a/frontend/today-session-sync.js b/frontend/today-session-sync.js
new file mode 100644
index 0000000..b3e90ca
--- /dev/null
+++ b/frontend/today-session-sync.js
@@ -0,0 +1,149 @@
+function createTodaySessionSync({
+  fetchJson, getDeviceId, timer, onRemote = () => {}, onTransferred = () => {}, onStatus = () => {},
+  setInterval = globalThis.setInterval, clearInterval = globalThis.clearInterval,
+}) {
+  let current = null;
+  let ownedRevision = 0;
+  let pollTimer = null;
+  const endpoint = 'api/v1/today/session';
+  const deviceId = () => String(getDeviceId?.() || '').trim();
+
+  function adopt(session) {
+    if (!session || !Number.isInteger(session.revision)) return null;
+    const previousOwned = current?.device_id === deviceId() && current?.running;
+    current = session;
+    if (session.device_id === deviceId()) {
+      ownedRevision = session.revision;
+      onRemote(null);
+    } else if (session.running && session.identity) {
+      if (previousOwned) {
+        timer?.pause?.();
+        onTransferred(session);
+      }
+      onRemote(session);
+    } else {
+      onRemote(null);
+    }
+    return session;
+  }
+
+  async function refresh() {
+    try {
+      const session = await fetchJson(endpoint);
+      onStatus('online');
+      return adopt(session);
+    } catch (error) {
+      onStatus('offline', error);
+      return null;
+    }
+  }
+
+  async function claim() {
+    if (!current?.running || !current.identity || current.device_id === deviceId() || !deviceId()) return null;
+    try {
+      const session = await fetchJson(endpoint, {
+        method:'PATCH', headers:{'Content-Type':'application/json'},
+        body:JSON.stringify({
+          base_revision:current.revision, device_id:deviceId(), identity:current.identity,
+          elapsed_ms:current.elapsed_ms, running:true,
+        }),
+      });
+      adopt(session);
+      timer?.adopt?.(session.identity, session.elapsed_ms, session.running);
+      return session;
+    } catch (error) {
+      onStatus('conflict', error);
+      await refresh();
+      return null;
+    }
+  }
+
+  async function publish(snapshot = timer?.snapshot?.()) {
+    if (!snapshot?.identity || !deviceId()) return null;
+    try {
+      const session = await fetchJson(endpoint, {
+        method:'PATCH', headers:{'Content-Type':'application/json'},
+        body:JSON.stringify({
+          base_revision:ownedRevision, device_id:deviceId(), identity:snapshot.identity,
+          elapsed_ms:Math.max(0, Math.floor(Number(snapshot.elapsed_ms) || 0)),
+          running:Boolean(snapshot.running),
+        }),
+      });
+      adopt(session);
+      return session;
+    } catch (error) {
+      onStatus('offline', error);
+      return null;
+    }
+  }
+
+  const pulse = () => current?.device_id === deviceId() && current?.running
+    ? publish()
+    : refresh();
+
+  return {
+    refresh, claim, publish,
+    session:() => current,
+    start(intervalMs = 15000) {
+      if (pollTimer !== null) return false;
+      pulse();
+      pollTimer = setInterval?.(pulse, intervalMs);
+      pollTimer?.unref?.();
+      return true;
+    },
+    stop() {
+      if (pollTimer === null) return false;
+      clearInterval?.(pollTimer);
+      pollTimer = null;
+      return true;
+    },
+  };
+}
+
+function attachTodaySessionHandoff({
+  fetchJson, storage, timer, qs, items, identity, selectToday, startItem, announce, renderTimer,
+}) {
+  const deviceKey = 'stackchain.today-session-device.v1';
+  const getDeviceId = () => {
+    let value = storage.getItem(deviceKey);
+    if (!value) {
+      value = globalThis.crypto?.randomUUID?.() || Math.random().toString(36).slice(2);
+      storage.setItem(deviceKey, value);
+    }
+    return value;
+  };
+  let offered = null;
+  const sync = createTodaySessionSync({
+    fetchJson, getDeviceId, timer,
+    onRemote:session => {
+      offered = session;
+      const handoff = qs('#today-session-handoff');
+      handoff.hidden = !session;
+      if (!session) return;
+      const item = items().find(entry => identity(entry) === session.identity);
+      const minutes = Math.max(0, Math.floor(Number(session.elapsed_ms || 0) / 60000));
+      qs('#today-session-handoff-summary').textContent =
+        `${item?.title || 'Current Today item'} · ${minutes} min elapsed`;
+    },
+    onTransferred:() => {
+      announce('Today continued on another device. Timer paused here.');
+      renderTimer();
+    },
+  });
+  qs('#continue-today-session').addEventListener('click', async () => {
+    const target = offered;
+    const claimed = await sync.claim();
+    if (!claimed || !target) return;
+    selectToday();
+    const item = items().find(entry => identity(entry) === claimed.identity);
+    if (item) startItem(item);
+    else announce('Today session moved here; refresh work to open its item.');
+    renderTimer();
+  });
+  sync.start(5000);
+  return sync;
+}
+
+if (typeof module !== 'undefined' && module.exports) module.exports = {
+  createTodaySessionSync, attachTodaySessionHandoff,
+};
diff --git a/frontend/today-timer.js b/frontend/today-timer.js
index 0f5f1bd..de307c4 100644
--- a/frontend/today-timer.js
+++ b/frontend/today-timer.js
@@ -1,4 +1,4 @@
-function createTodayTimer({ storage, getLogin, now = () => Date.now() }) {
+function createTodayTimer({ storage, getLogin, now = () => Date.now(), onChange = () => {} }) {
   const key = () => {
     const login = String(getLogin?.() || '').trim().toLowerCase();
     return login ? 'stackchain.today-timer.v1.' + encodeURIComponent(login) : '';
@@ -24,6 +24,7 @@ function createTodayTimer({ storage, getLogin, now = () => Date.now() }) {
     if (!ownerKey || !storage) return false;
     try {
       storage.setItem(ownerKey, JSON.stringify(state));
+      onChange(snapshot());
       return true;
     } catch (_error) {
       return false;
@@ -60,6 +61,22 @@ function createTodayTimer({ storage, getLogin, now = () => Date.now() }) {
     return { identity:selected, elapsed_ms:elapsed, running:Boolean(entry.running) };
   };
   return {
+    adopt(identity, elapsedMs, running) {
+      if (!key() || typeof identity !== 'string' || !identity ||
+          !Number.isFinite(Number(elapsedMs)) || Number(elapsedMs) < 0) return false;
+      const state = read();
+      settle(state);
+      state.active_identity = identity;
+      state.entries[identity] = {
+        elapsed_ms:Math.floor(Number(elapsedMs)),
+        started_at:running ? now() : null,
+        running:Boolean(running),
+      };
+      state.away_at = null;
+      state.pending_interruption = null;
+      state.attention_interruption = null;
+      return write(state);
+    },
     activate(identity) {
       if (!key() || typeof identity !== 'string' || !identity) return false;
       const state = read();
diff --git a/src/frontend_bundle.py b/src/frontend_bundle.py
index 2a90f85..076638b 100644
--- a/src/frontend_bundle.py
+++ b/src/frontend_bundle.py
@@ -34,7 +34,7 @@ FEATURE_SOURCES = {
     "security-center": ("static/security-center.js",),
     "today-timer": (
         "static/conversation.js", "static/voice-transcript-store.js", "static/voice-conversation-capture.js", "static/mobile-launch.js", "static/mobile-insights.js", "static/mobile-app-shortcuts.js", "static/mobile-plan-today-nav.js", "static/mobile-find-work-nav.js", "static/mobile-pull-refresh.js", "static/live-data-status.js",
-        "static/today-completion.js", "static/card-planning.js", "static/work-detail-position.js", "static/work-route.js", "static/commands.js", "static/saved-searches.js", "static/task-overlay-history.js", "static/search-preview.js", "static/mobile-search-preview-nav.js", "static/search-reply-draft-store.js", "static/conversation-reply-draft-store.js", "static/conversation-photo-drafts.js", "static/search-defer.js", "static/mobile-search-viewport.js", "static/my-work.js", "static/protect-today.js", "static/mobile-task-dock.js", "static/mobile-work-entry.js", "static/mobile-queue-launcher.js", "static/mobile-delivery-recovery.js", "static/mobile-start-day.js", "static/update-triage-session.js", "static/update-review-handoff.js", "static/update-triage-launcher.js", "static/update-triage-gesture.js", "static/notification-undo.js", "static/today-timer.js", "static/today-recap.js", "static/today-wrap-up.js", "static/today-handoff.js",
+        "static/today-completion.js", "static/card-planning.js", "static/work-detail-position.js", "static/work-route.js", "static/commands.js", "static/saved-searches.js", "static/task-overlay-history.js", "static/search-preview.js", "static/mobile-search-preview-nav.js", "static/search-reply-draft-store.js", "static/conversation-reply-draft-store.js", "static/conversation-photo-drafts.js", "static/search-defer.js", "static/mobile-search-viewport.js", "static/agenda-replan.js", "static/my-work.js", "static/protect-today.js", "static/mobile-task-dock.js", "static/mobile-work-entry.js", "static/mobile-queue-launcher.js", "static/mobile-delivery-recovery.js", "static/mobile-start-day.js", "static/update-triage-session.js", "static/update-review-handoff.js", "static/update-triage-launcher.js", "static/update-triage-gesture.js", "static/notification-undo.js", "static/today-timer.js", "static/today-session-sync.js", "static/today-recap.js", "static/today-wrap-up.js", "static/today-handoff.js",
         "static/today-rollover.js", "static/later-work.js", "static/later-picker.js", "static/drafts.js", "static/unfiled-captures.js", "static/unfiled-draft-sync.js",
         "static/assign-and-start.js", "static/filed-claim.js", "static/queue-today.js", "static/create-and-start.js",
         "static/draft-filing-session.js", "static/draft-capacity-dialog.js", "static/work-selection.js",
diff --git a/src/main.py b/src/main.py
index 8115b70..fc101e1 100644
--- a/src/main.py
+++ b/src/main.py
@@ -64,7 +64,7 @@ from src.unfiled_draft_store import UnfiledDraftConflict, UnfiledDraftStore
 from src.security_event_store import SecurityEventStore, SecurityEventStoreError
 from src.suggestion_engine import compute
 from src.later_store import LaterStore
-from src.today_store import TodayPlanFull, TodayStore
+from src.today_store import TodayPlanFull, TodaySessionConflict, TodayStore
 from src.views import FRONTEND_BUILD, router as frontend_router
 
 
@@ -588,6 +588,14 @@ class TodayRecap(BaseModel):
     items: list[TodayRecapItem] = Field(min_length=1, max_length=20)
 
 
+class TodaySessionUpdate(BaseModel):
+    base_revision: int = Field(ge=0)
+    device_id: str = Field(min_length=1, max_length=100)
+    identity: str = Field(max_length=500)
+    elapsed_ms: int = Field(ge=0, le=7 * 24 * 60 * 60 * 1000)
+    running: bool
+
+
 class TodayRecapTimeLog(TodayRecap):
     log_identities: list[str] = Field(min_length=1, max_length=20)
 
@@ -1322,7 +1330,7 @@ async def require_operator_session(request: Request, call_next):
 async def prevent_live_api_caching(request, call_next):
     response = await call_next(request)
     path = dashboard_auth.application_path(request)
-    if path in {"/api/v1/context", "/api/v1/background-identity", "/api/v1/events", "/api/v1/live", "/api/v1/available-issues", "/api/v1/search", "/api/v1/work-route", "/api/v1/today", "/api/v1/later", "/api/v1/saved-searches", "/api/v1/completed-filed-reviews", "/api/v1/security-events", "/api/v1/push-subscription"} or path.startswith("/api/v1/work/") or (
+    if path in {"/api/v1/context", "/api/v1/background-identity", "/api/v1/events", "/api/v1/live", "/api/v1/available-issues", "/api/v1/search", "/api/v1/work-route", "/api/v1/today", "/api/v1/today/session", "/api/v1/later", "/api/v1/saved-searches", "/api/v1/completed-filed-reviews", "/api/v1/security-events", "/api/v1/push-subscription"} or path.startswith("/api/v1/work/") or (
         path.startswith("/api/v1/repos/")
         and path.endswith("/review")
     ) or path.startswith("/api/v1/notifications") or (
@@ -2456,6 +2464,37 @@ async def update_today_plan(payload: TodayOperation | TodayOperationBatch):
         )
 
 
+@app.get("/api/v1/today/session")
+async def get_today_session():
+    login = await _confirmed_login()
+    try:
+        return await asyncio.to_thread(_today_store().get_session, login)
+    except (OSError, sqlite3.Error):
+        raise HTTPException(
+            status_code=503, detail="Today session synchronization is unavailable",
+            headers={"Retry-After": "1"},
+        )
+
+
+@app.patch("/api/v1/today/session")
+async def update_today_session(payload: TodaySessionUpdate):
+    login = await _confirmed_login()
+    try:
+        return await asyncio.to_thread(
+            _today_store().update_session, login, **payload.model_dump()
+        )
+    except TodaySessionConflict as error:
+        raise HTTPException(
+            status_code=409,
+            detail={"code": "session_changed", "session": error.session},
+        )
+    except (OSError, sqlite3.Error):
+        raise HTTPException(
+            status_code=503, detail="Today session synchronization is unavailable",
+            headers={"Retry-After": "1"},
+        )
+
+
 @app.get("/api/v1/today/recaps")
 async def get_today_recaps(response: Response):
     login = await _confirmed_login()
diff --git a/src/today_store.py b/src/today_store.py
index 527b19a..e668837 100644
--- a/src/today_store.py
+++ b/src/today_store.py
@@ -13,6 +13,14 @@ class TodayPlanFull(ValueError):
     """Raised when an add would exceed the bounded Today plan."""
 
 
+class TodaySessionConflict(ValueError):
+    """Raised when a device updates an obsolete active-session revision."""
+
+    def __init__(self, session: dict):
+        super().__init__("active Today session changed on another device")
+        self.session = session
+
+
 class TodayStore:
     def __init__(
         self,
@@ -36,7 +44,7 @@ class TodayStore:
 
     def _initialize(self) -> None:
         connection = connect_private_sqlite(self.path, timeout=self.timeout)
-        if connection.execute("PRAGMA user_version").fetchone()[0] >= 1:
+        if connection.execute("PRAGMA user_version").fetchone()[0] >= 2:
             connection.close()
             return
         connection.execute("PRAGMA journal_mode=WAL")
@@ -106,7 +114,20 @@ class TodayStore:
             )
             """
         )
-        connection.execute("PRAGMA user_version = 1")
+        connection.execute(
+            """
+            CREATE TABLE IF NOT EXISTS today_sessions (
+                login TEXT PRIMARY KEY,
+                revision INTEGER NOT NULL,
+                device_id TEXT NOT NULL,
+                identity TEXT NOT NULL,
+                elapsed_ms INTEGER NOT NULL,
+                running INTEGER NOT NULL,
+                updated_at REAL NOT NULL
+            )
+            """
+        )
+        connection.execute("PRAGMA user_version = 2")
         connection.commit()
         connection.close()
 
@@ -163,6 +184,57 @@ class TodayStore:
             ).fetchone()
         return self._snapshot(row)
 
+    @staticmethod
+    def _empty_session() -> dict:
+        return {
+            "revision": 0, "device_id": "", "identity": "",
+            "elapsed_ms": 0, "running": False, "updated_at": None,
+        }
+
+    def get_session(self, login: str) -> dict:
+        with self._connect() as connection:
+            row = connection.execute(
+                "SELECT revision, device_id, identity, elapsed_ms, running, updated_at "
+                "FROM today_sessions WHERE login = ?",
+                (self._normalize_login(login),),
+            ).fetchone()
+        if row is None:
+            return self._empty_session()
+        return {
+            "revision": int(row[0]), "device_id": row[1], "identity": row[2],
+            "elapsed_ms": int(row[3]), "running": bool(row[4]), "updated_at": row[5],
+        }
+
+    def update_session(
+        self, login: str, *, base_revision: int, device_id: str,
+        identity: str, elapsed_ms: int, running: bool,
+    ) -> dict:
+        login = self._normalize_login(login)
+        updated_at = self.clock()
+        with self._connect() as connection:
+            connection.execute("BEGIN IMMEDIATE")
+            current = connection.execute(
+                "SELECT revision, device_id, identity, elapsed_ms, running, updated_at "
+                "FROM today_sessions WHERE login = ?", (login,)
+            ).fetchone()
+            current_revision = int(current[0]) if current else 0
+            if base_revision != current_revision:
+                session = self._empty_session() if current is None else {
+                    "revision": current_revision, "device_id": current[1], "identity": current[2],
+                    "elapsed_ms": int(current[3]), "running": bool(current[4]), "updated_at": current[5],
+                }
+                raise TodaySessionConflict(session)
+            revision = current_revision + 1
+            connection.execute(
+                "INSERT INTO today_sessions(login, revision, device_id, identity, elapsed_ms, running, updated_at) "
+                "VALUES (?, ?, ?, ?, ?, ?, ?) "
+                "ON CONFLICT(login) DO UPDATE SET revision=excluded.revision, device_id=excluded.device_id, "
+                "identity=excluded.identity, elapsed_ms=excluded.elapsed_ms, running=excluded.running, "
+                "updated_at=excluded.updated_at",
+                (login, revision, device_id, identity, elapsed_ms, int(running), updated_at),
+            )
+        return self.get_session(login)
+
     @staticmethod
     def _normalize_recap_items(items: list[dict]) -> list[dict]:
         if not isinstance(items, list) or not items:
diff --git a/tests/test_service_worker.py b/tests/test_service_worker.py
index 1207b18..f3dfe2c 100644
--- a/tests/test_service_worker.py
+++ b/tests/test_service_worker.py
@@ -988,6 +988,7 @@ def test_install_precaches_complete_subpath_scoped_app_shell():
         "/dashboard/static/work-selection.js",
         "/dashboard/static/today-work.js",
         "/dashboard/static/today-timer.js",
+        "/dashboard/static/today-session-sync.js",
         "/dashboard/static/today-recap.js",
         "/dashboard/static/today-wrap-up.js",
         "/dashboard/static/today-handoff.js",
diff --git a/tests/test_today_session_handoff_ui.py b/tests/test_today_session_handoff_ui.py
new file mode 100644
index 0000000..cd001bb
--- /dev/null
+++ b/tests/test_today_session_handoff_ui.py
@@ -0,0 +1,23 @@
+from pathlib import Path
+
+from src.frontend_bundle import build_frontend
+
+
+ROOT = Path(__file__).parents[1]
+
+
+def test_dashboard_packages_a_mobile_today_session_handoff():
+    build = build_frontend(ROOT / "frontend")
+    html = build.dashboard_html
+    dashboard = (ROOT / "frontend" / "dashboard.js").read_text()
+    css = (ROOT / "frontend" / "dashboard.css").read_text()
+    today_bundle = build.feature_bundles["today-timer"].runtime_bytes.decode()
+
+    assert 'id="today-session-handoff"' in html
+    assert 'id="continue-today-session"' in html
+    assert 'aria-live="polite"' in html
+    assert "createTodaySessionSync" in today_bundle
+    assert "attachTodaySessionHandoff" in today_bundle
+    assert "todaySessionSync = attachTodaySessionHandoff" in dashboard
+    assert "todaySessionSync?.publish(snapshot)" in dashboard
+    assert "min-height:44px" in css
diff --git a/tests/test_today_session_sync.py b/tests/test_today_session_sync.py
new file mode 100644
index 0000000..5095a75
--- /dev/null
+++ b/tests/test_today_session_sync.py
@@ -0,0 +1,121 @@
+import json
+import subprocess
+from pathlib import Path
+
+
+SOURCE = Path(__file__).parents[1] / "frontend" / "today-session-sync.js"
+
+
+def run_node(script: str) -> dict:
+    completed = subprocess.run(
+        ["node", "-e", SOURCE.read_text() + "\n" + script],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return json.loads(completed.stdout)
+
+
+def test_second_device_sees_and_claims_the_running_session():
+    result = run_node(
+        r"""
+const calls = [];
+const remote = {revision:3, device_id:'desktop-a', identity:'issue:r:42:', elapsed_ms:90000, running:true, updated_at:10};
+let adopted = null;
+const offers = [];
+const sync = createTodaySessionSync({
+  getDeviceId:()=>'phone-b',
+  fetchJson:async (url, options={}) => {
+    calls.push({url, body:options.body ? JSON.parse(options.body) : null});
+    return options.method === 'PATCH' ? {...remote, revision:4, device_id:'phone-b'} : remote;
+  },
+  timer:{adopt:(identity, elapsed, running)=>{adopted={identity, elapsed, running};}},
+  onRemote:session=>{offers.push(session);},
+});
+(async()=>{
+  await sync.refresh();
+  const claimed = await sync.claim();
+  process.stdout.write(JSON.stringify({offers, claimed, adopted, calls}));
+})().catch(error=>{console.error(error);process.exit(1);});
+"""
+    )
+
+    assert result["offers"][0]["device_id"] == "desktop-a"
+    assert result["claimed"]["device_id"] == "phone-b"
+    assert result["adopted"] == {
+        "identity": "issue:r:42:", "elapsed": 90000, "running": True,
+    }
+    assert result["calls"][1] == {
+        "url": "api/v1/today/session",
+        "body": {
+            "base_revision": 3,
+            "device_id": "phone-b",
+            "identity": "issue:r:42:",
+            "elapsed_ms": 90000,
+            "running": True,
+        },
+    }
+
+
+def test_previous_owner_pauses_after_another_device_claims():
+    result = run_node(
+        r"""
+let response = {revision:1, device_id:'desktop-a', identity:'issue:r:42:', elapsed_ms:1000, running:true};
+let pauses = 0;
+let transferred = null;
+const sync = createTodaySessionSync({
+  getDeviceId:()=> 'desktop-a',
+  fetchJson:async()=>response,
+  timer:{pause:()=>{pauses += 1;}},
+  onTransferred:session=>{transferred=session;},
+});
+(async()=>{
+  await sync.refresh();
+  response = {...response, revision:2, device_id:'phone-b', elapsed_ms:2500};
+  await sync.refresh();
+  await sync.refresh();
+  process.stdout.write(JSON.stringify({pauses, transferred}));
+})().catch(error=>{console.error(error);process.exit(1);});
+"""
+    )
+
+    assert result["pauses"] == 1
+    assert result["transferred"]["device_id"] == "phone-b"
+
+
+def test_owner_poll_publishes_current_elapsed_time():
+    result = run_node(
+        r"""
+const calls = [];
+let tick = null;
+let elapsed = 1000;
+const owned = {revision:1, device_id:'desktop-a', identity:'issue:r:42:', elapsed_ms:1000, running:true};
+const sync = createTodaySessionSync({
+  getDeviceId:()=> 'desktop-a',
+  fetchJson:async (url, options={}) => {
+    calls.push({method:options.method || 'GET', body:options.body ? JSON.parse(options.body) : null});
+    return options.method === 'PATCH' ? {...owned, revision:2, elapsed_ms:elapsed} : owned;
+  },
+  timer:{snapshot:()=>({identity:owned.identity, elapsed_ms:elapsed, running:true})},
+  setInterval:callback=>{tick=callback; return 7;},
+});
+(async()=>{
+  await sync.refresh();
+  sync.start();
+  elapsed = 6500;
+  await tick();
+  process.stdout.write(JSON.stringify(calls));
+})().catch(error=>{console.error(error);process.exit(1);});
+"""
+    )
+
+    assert result[-1] == {
+        "method": "PATCH",
+        "body": {
+            "base_revision": 1,
+            "device_id": "desktop-a",
+            "identity": "issue:r:42:",
+            "elapsed_ms": 6500,
+            "running": True,
+        },
+    }
diff --git a/tests/test_today_store.py b/tests/test_today_store.py
index e958dbc..3ddf0a0 100644
--- a/tests/test_today_store.py
+++ b/tests/test_today_store.py
@@ -4,7 +4,80 @@ import pytest
 import httpx
 
 from src import main
-from src.today_store import TodayPlanFull, TodayStore
+from src.today_store import TodayPlanFull, TodaySessionConflict, TodayStore
+
+
+def test_active_session_is_durable_and_account_scoped(tmp_path):
+    path = tmp_path / "today.sqlite3"
+    store = TodayStore(path, clock=lambda: 1234.5)
+
+    saved = store.update_session(
+        "Timmy", base_revision=0, device_id="phone-a",
+        identity="issue:stackchain/dashboard:42:", elapsed_ms=90_000, running=True,
+    )
+
+    assert saved == {
+        "revision": 1,
+        "device_id": "phone-a",
+        "identity": "issue:stackchain/dashboard:42:",
+        "elapsed_ms": 90_000,
+        "running": True,
+        "updated_at": 1234.5,
+    }
+    assert TodayStore(path).get_session("timmy") == saved
+    assert store.get_session("alexander") == {
+        "revision": 0,
+        "device_id": "",
+        "identity": "",
+        "elapsed_ms": 0,
+        "running": False,
+        "updated_at": None,
+    }
+
+
+def test_active_session_claim_rejects_a_stale_revision(tmp_path):
+    store = TodayStore(tmp_path / "today.sqlite3")
+    first = store.update_session(
+        "timmy", base_revision=0, device_id="phone-a", identity="issue:r:1:",
+        elapsed_ms=1_000, running=True,
+    )
+    claimed = store.update_session(
+        "timmy", base_revision=first["revision"], device_id="phone-b",
+        identity=first["identity"], elapsed_ms=first["elapsed_ms"], running=True,
+    )
+
+    with pytest.raises(TodaySessionConflict) as raised:
+        store.update_session(
+            "timmy", base_revision=first["revision"], device_id="phone-a",
+            identity=first["identity"], elapsed_ms=5_000, running=True,
+        )
+
+    assert raised.value.session == claimed
+    assert store.get_session("timmy") == claimed
+
+
+@pytest.mark.anyio
+async def test_active_session_api_returns_current_state_on_claim_conflict(monkeypatch, tmp_path):
+    async def user():
+        return {"login": "timmy"}
+
+    store = TodayStore(tmp_path / "today.sqlite3")
+    current = store.update_session(
+        "timmy", base_revision=0, device_id="phone-a", identity="issue:r:1:",
+        elapsed_ms=5_000, running=True,
+    )
+    monkeypatch.setattr(main, "current_user", user)
+    monkeypatch.setattr(main, "_today_store", lambda: store)
+
+    payload = main.TodaySessionUpdate(
+        base_revision=0, device_id="phone-b", identity="issue:r:1:",
+        elapsed_ms=5_000, running=True,
+    )
+    with pytest.raises(main.HTTPException) as raised:
+        await main.update_today_session(payload)
+
+    assert raised.value.status_code == 409
+    assert raised.value.detail == {"code": "session_changed", "session": current}
 
 
 @pytest.mark.anyio
@@ -342,6 +415,21 @@ async def test_authenticated_today_api_uses_confirmed_account_and_csrf(monkeypat
             },
         )
         fetched = await client.get("/api/v1/today")
+        changed_session = await client.patch(
+            "/api/v1/today/session",
+            json={
+                "base_revision": 0,
+                "device_id": "phone-a",
+                "identity": "issue:stackchain/dashboard:357:",
+                "elapsed_ms": 12_000,
+                "running": True,
+            },
+            headers={
+                "Origin": "https://test",
+                "X-CSRF-Token": client.cookies["stackchain_csrf"],
+            },
+        )
+        fetched_session = await client.get("/api/v1/today/session")
 
     assert forbidden.status_code == 403
     assert changed.status_code == 200
@@ -361,3 +449,6 @@ async def test_authenticated_today_api_uses_confirmed_account_and_csrf(monkeypat
         "estimates": {},
     }
     assert fetched.headers["cache-control"] == "no-store"
+    assert changed_session.status_code == 200
+    assert fetched_session.json() == changed_session.json()
+    assert fetched_session.headers["cache-control"] == "no-store"
diff --git a/tests/test_today_timer_handoff.py b/tests/test_today_timer_handoff.py
new file mode 100644
index 0000000..a5e2985
--- /dev/null
+++ b/tests/test_today_timer_handoff.py
@@ -0,0 +1,51 @@
+import json
+import subprocess
+from pathlib import Path
+
+
+SOURCE = Path(__file__).parents[1] / "frontend" / "today-timer.js"
+
+
+def test_timer_adopts_claimed_elapsed_time():
+    script = SOURCE.read_text() + r"""
+const values = new Map();
+const storage = {
+  getItem:key=>values.get(key)||null,
+  setItem:(key,value)=>values.set(key,value),
+};
+let now = 100000;
+const timer = createTodayTimer({storage, getLogin:()=> 'timmy', now:()=>now});
+const adopted = timer.adopt('issue:r:42:', 90000, true);
+now = 105000;
+process.stdout.write(JSON.stringify({adopted, snapshot:timer.snapshot()}));
+"""
+    completed = subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)
+    result = json.loads(completed.stdout)
+
+    assert result == {
+        "adopted": True,
+        "snapshot": {"identity": "issue:r:42:", "elapsed_ms": 95000, "running": True},
+    }
+
+
+def test_timer_reports_durable_changes_for_session_sync():
+    script = SOURCE.read_text() + r"""
+const values = new Map();
+const storage = {getItem:key=>values.get(key)||null,setItem:(key,value)=>values.set(key,value)};
+let now = 0;
+const changes = [];
+const timer = createTodayTimer({
+  storage, getLogin:()=> 'timmy', now:()=>now,
+  onChange:snapshot=>changes.push(snapshot),
+});
+timer.activate('issue:r:42:');
+now = 5000;
+timer.pause();
+process.stdout.write(JSON.stringify(changes));
+"""
+    completed = subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)
+
+    assert json.loads(completed.stdout) == [
+        {"identity": "issue:r:42:", "elapsed_ms": 0, "running": True},
+        {"identity": "issue:r:42:", "elapsed_ms": 5000, "running": False},
+    ]