diff --git a/gateway/run.py b/gateway/run.py
index 0304da3ddd..60a67d88b6 100644
--- a/gateway/run.py
+++ b/gateway/run.py
@@ -2655,15 +2655,27 @@ def _watch_gateway_turn_inactivity(
     *, agent_holder, task_id: str, process_baseline, timeout: float, worker_done: threading.Event,
     timeout_fired: threading.Event, cleanup_lock: threading.Lock, poll_interval: float = 5.0,
     is_still_current: Optional[Callable[[], bool]] = None) -> None:
-    """Thread watchdog that remains runnable when gateway asyncio is starved."""
+    """Thread watchdog that remains runnable when gateway asyncio is starved.
+
+    Until an agent publishes a usable activity snapshot, elapsed worker time is the
+    liveness clock.  Otherwise a provider hang before activity initialization can
+    retain the session turn lease forever because every watchdog poll just skips it.
+    """
+    activity_origin = time.monotonic()
     while not worker_done.wait(max(0.01, poll_interval)):
+        now = time.monotonic()
+        idle_seconds = now - activity_origin
         agent = agent_holder[0] if agent_holder else None
-        if agent is None or not hasattr(agent, "get_activity_summary"):
-            continue
-        try:
-            idle_seconds = float(agent.get_activity_summary().get("seconds_since_activity", 0.0))
-        except Exception:
-            continue
+        if agent is not None and hasattr(agent, "get_activity_summary"):
+            try:
+                reported_idle = agent.get_activity_summary().get("seconds_since_activity")
+                if reported_idle is not None:
+                    idle_seconds = max(0.0, float(reported_idle))
+                    # Preserve the most recent usable activity clock as the fallback if
+                    # a later provider-side diagnostic read raises or returns None.
+                    activity_origin = now - idle_seconds
+            except Exception:
+                pass
         if idle_seconds < timeout:
             continue
         _abandon_timed_out_gateway_turn(
diff --git a/tests/gateway/test_abandoned_turn_process_cleanup.py b/tests/gateway/test_abandoned_turn_process_cleanup.py
index da0fcdaa0c..3f37453090 100644
--- a/tests/gateway/test_abandoned_turn_process_cleanup.py
+++ b/tests/gateway/test_abandoned_turn_process_cleanup.py
@@ -22,10 +22,65 @@ class _IdleAgent:
         self.interrupts.append(reason)
 
 
+class _UnavailableActivityAgent:
+    def __init__(self):
+        self.interrupts = []
+
+    def get_activity_summary(self):
+        return {"seconds_since_activity": None}
+
+    def interrupt(self, reason):
+        self.interrupts.append(reason)
+
+
 def _state():
     return threading.Event(), threading.Event(), threading.Lock()
 
 
+def test_thread_watchdog_times_out_when_agent_activity_is_unavailable(monkeypatch):
+    agent = _UnavailableActivityAgent()
+    worker_done, timeout_fired, cleanup_lock = _state()
+    calls = []
+    monkeypatch.setattr(
+        process_registry,
+        "kill_started_since",
+        lambda task_id, baseline, *, source: calls.append(
+            (task_id, baseline, source)
+        )
+        or 0,
+    )
+
+    watchdog = threading.Thread(
+        target=_watch_gateway_turn_inactivity,
+        kwargs={
+            "agent_holder": [agent],
+            "task_id": "session-without-agent",
+            "process_baseline": frozenset(),
+            "timeout": 0.03,
+            "worker_done": worker_done,
+            "timeout_fired": timeout_fired,
+            "cleanup_lock": cleanup_lock,
+            "poll_interval": 0.01,
+        },
+    )
+    watchdog.start()
+    watchdog.join(timeout=1)
+    if watchdog.is_alive():
+        worker_done.set()
+        watchdog.join(timeout=1)
+
+    assert not watchdog.is_alive()
+    assert timeout_fired.is_set()
+    assert agent.interrupts == ["Execution timed out (inactivity)"]
+    assert calls == [
+        (
+            "session-without-agent",
+            frozenset(),
+            "gateway_turn_timeout",
+        )
+    ]
+
+
 def test_thread_watchdog_reaps_only_processes_created_by_timed_out_turn(monkeypatch):
     agent = _IdleAgent()
     worker_done, timeout_fired, cleanup_lock = _state()