diff --git a/api/routes.py b/api/routes.py
index 64c5c6d9f..122ea3137 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -14139,8 +14139,9 @@ def _active_run_stream_for_session(session_id: str | None) -> str | None:
     now = time.time()
     try:
         from api import config as _live_config
+        stale_stream_ids = []
         with _live_config.ACTIVE_RUNS_LOCK:
-            for run_stream_id, raw in (_live_config.ACTIVE_RUNS or {}).items():
+            for run_stream_id, raw in list((_live_config.ACTIVE_RUNS or {}).items()):
                 stream_id = str((raw or {}).get("stream_id") or run_stream_id or "").strip()
                 run_sid = str((raw or {}).get("session_id") or "").strip()
                 if run_sid != sid or not stream_id:
@@ -14149,11 +14150,14 @@ def _active_run_stream_for_session(session_id: str | None) -> str | None:
                     started_at = float((raw or {}).get("started_at") or 0)
                 except (TypeError, ValueError):
                     started_at = 0.0
-                # Ignore a stale/wedged entry past the unwind ceiling so it can't
-                # block the session permanently.
+                # Reconcile stale/wedged entries past the unwind ceiling so they
+                # cannot keep health/recovery polling in a half-alive state.
                 if started_at and (now - started_at) > ceiling:
+                    stale_stream_ids.append(stream_id)
                     continue
                 return stream_id
+            for stale_stream_id in stale_stream_ids:
+                (_live_config.ACTIVE_RUNS or {}).pop(stale_stream_id, None)
     except Exception:
         return None
     return None
diff --git a/api/streaming.py b/api/streaming.py
index 7a2486bf6..adddf30ab 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -8903,6 +8903,11 @@ def cancel_stream(stream_id: str) -> bool:
         if active_run_entry and not active_run_session_id:
             active_run_session_id = str(active_run_entry.get("session_id") or "").strip() or None
 
+    # Mark the worker lifecycle registry immediately. The SSE maps may be popped
+    # below while the worker is still unwinding; ACTIVE_RUNS is what recovery /
+    # health polling sees during that detached window.
+    update_active_run(stream_id, phase="cancelling")
+
     # Set WebUI layer cancel flag. Prefer the snapshot captured under the lock;
     # fall back to a fresh lookup for the ACTIVE_RUNS-only path (stream absent).
     flag = _snap_flag if _snap_flag is not None else cancel_flags.get(stream_id)
diff --git a/tests/test_cancel_interrupt.py b/tests/test_cancel_interrupt.py
index c2d35aac1..375d0ef52 100644
--- a/tests/test_cancel_interrupt.py
+++ b/tests/test_cancel_interrupt.py
@@ -127,6 +127,7 @@ class TestCancelInterrupt:
             result = cancel_stream(stream_id)
 
         assert result is True
+        assert ACTIVE_RUNS[stream_id]["phase"] == "cancelling"
         mock_agent.interrupt.assert_called_once_with("Cancelled by user")
         assert mock_session.active_stream_id is None
         assert mock_session.pending_user_message is None
diff --git a/tests/test_stale_stream_cleanup.py b/tests/test_stale_stream_cleanup.py
index 8d5756a96..2d9e929fe 100644
--- a/tests/test_stale_stream_cleanup.py
+++ b/tests/test_stale_stream_cleanup.py
@@ -319,6 +319,9 @@ def test_chat_start_not_permanently_blocked_by_stale_active_run(monkeypatch, tmp
 
     # The bounded guard should treat it as stale and NOT report it as blocking.
     assert routes._active_run_stream_for_session(session.session_id) is None
+    # It must also reconcile the zombie registry entry immediately so health /
+    # recovery polling does not keep advertising a half-alive run forever.
+    assert stale_stream_id not in config.ACTIVE_RUNS
 
     class NoopThread:
         def __init__(self, *args, **kwargs):