diff --git a/control/chat_api.py b/control/chat_api.py
index 11ff4c3..36e2785 100644
--- a/control/chat_api.py
+++ b/control/chat_api.py
@@ -185,5 +185,14 @@ async def voice_cancel(room: str, rid: str, request: Request):
     return {'run': await request.app.state.chat_runs.cancel(account['id'], rid)}
 
 
+@router.post('/internal/chat/rooms/{room}/operations/{key}/cancel', dependencies=[Depends(internal_authorized)])
+async def voice_cancel_operation(room: str, key: str, request: Request):
+    account = await room_owner(request, room)
+    try:
+        return await request.app.state.chat_runs.cancel_operation(account['id'], account['conversation_id'], key)
+    except KeyError:
+        raise HTTPException(404, 'Chat resource not found') from None
+
+
 def create_chat_router():
     return router
diff --git a/control/chat_runs.py b/control/chat_runs.py
index a459cf1..bdc340d 100644
--- a/control/chat_runs.py
+++ b/control/chat_runs.py
@@ -108,6 +108,10 @@ class ChatRuns:
             accounts.db.execute('''CREATE TABLE IF NOT EXISTS chat_transport (
                 run_id TEXT PRIMARY KEY, owner TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
                 conversation_id TEXT NOT NULL, metadata TEXT NOT NULL)''')
+            accounts.db.execute('''CREATE TABLE IF NOT EXISTS chat_operation_cancellations (
+                owner TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+                conversation_id TEXT NOT NULL, operation_key TEXT NOT NULL,
+                PRIMARY KEY(owner,conversation_id,operation_key))''')
 
     def capabilities(self):
         enabled = bool(getattr(self.adapter, 'available', True))
@@ -132,6 +136,28 @@ class ChatRuns:
             row = self.accounts.db.execute('SELECT metadata FROM chat_transport WHERE owner=? AND run_id=?', (owner, rid)).fetchone()
             return json.loads(row[0]) if row else None
 
+    def operation_cancelled(self, owner, cid, key):
+        with self.accounts.lock:
+            return self.accounts.db.execute('SELECT 1 FROM chat_operation_cancellations WHERE owner=? AND conversation_id=? AND operation_key=?', (owner, cid, key)).fetchone() is not None
+
+    async def cancel_operation(self, owner, cid, key):
+        if not isinstance(key, str) or not 1 <= len(key) <= 200 or '\x00' in key:
+            raise HTTPException(400, 'Invalid operation key')
+        async with self.admission_locks[owner]:
+            await self.call('get_conversation', owner, cid)
+            def remember():
+                with self.accounts.lock, self.accounts.db:
+                    existing = self.accounts.db.execute('SELECT 1 FROM chat_operation_cancellations WHERE owner=? AND conversation_id=? AND operation_key=?', (owner, cid, key)).fetchone()
+                    if not existing and self.accounts.db.execute('SELECT count(*) FROM chat_operation_cancellations WHERE owner=?', (owner,)).fetchone()[0] >= 10000:
+                        raise HTTPException(429, 'Operation cancellation limit reached')
+                    self.accounts.db.execute('INSERT OR IGNORE INTO chat_operation_cancellations VALUES(?,?,?)', (owner, cid, key))
+                    row = self.accounts.db.execute('SELECT run_id FROM chat_idempotency WHERE owner=? AND conversation_id=? AND key=?', (owner, cid, key)).fetchone()
+                    return row[0] if row else None
+            rid = await asyncio.to_thread(remember)
+            if rid:
+                return {'ok': True, 'run': await self.cancel(owner, rid)}
+            return {'ok': True, 'run': None}
+
     def authorized(self, owner, metadata):
         with self.accounts.lock:
             if metadata.get('room'):
@@ -176,6 +202,10 @@ class ChatRuns:
                         raise HTTPException(429, 'Chat queue full')
                 operation = await self.call('send', owner, cid, content, idempotency_key, attachment_ids, mode=mode)
                 rid = operation['run']['id']
+                if await asyncio.to_thread(self.operation_cancelled, owner, cid, idempotency_key):
+                    if operation['run']['status'] not in TERMINAL:
+                        operation['run'] = await self.cancel(owner, rid)
+                    return operation
                 metadata = await asyncio.to_thread(self.metadata, owner, rid)
                 if metadata is None:
                     settings = await asyncio.to_thread(self.accounts.get_model_settings, owner)
diff --git a/tests/unit/test_chat_runs_limits.py b/tests/unit/test_chat_runs_limits.py
index bb55bb7..201a851 100644
--- a/tests/unit/test_chat_runs_limits.py
+++ b/tests/unit/test_chat_runs_limits.py
@@ -31,3 +31,26 @@ def test_event_capacity_failure_persists_stop_and_reserved_terminal(tmp_path):
         assert store.events(owner['id'],rid)[-1]['event'] == 'done'
         accounts.close()
     asyncio.run(scenario())
+
+
+def test_cancel_key_before_admission_never_starts_and_is_owner_scoped(tmp_path):
+    async def scenario():
+        accounts = Accounts(str(tmp_path/'cancel.db'))
+        user = accounts.create('test','Test','admin','offline-test-password')
+        other = accounts.create('other','Other','user','offline-test-password')
+        token, _ = accounts.issue_session(user['id'],3600,user['password_hash'])
+        store = ChatStore(accounts)
+        cid = store.create_conversation(user['id'])['id']
+        runs = ChatRuns(accounts,store,adapter=object())
+        assert await runs.cancel_operation(user['id'],cid,'uncertain-key') == {'ok':True,'run':None}
+        operation = await runs.send(user['id'],cid,token,content='Hi',idempotency_key='uncertain-key')
+        assert operation['run']['status'] == 'cancelled'
+        assert runs.tasks == {}
+        assert (await runs.send(user['id'],cid,token,content='Hi',idempotency_key='uncertain-key'))['run']['id'] == operation['run']['id']
+        try:
+            await runs.cancel_operation(other['id'],cid,'uncertain-key')
+            assert False, 'Foreign owner cancellation was accepted'
+        except KeyError:
+            pass
+        accounts.close()
+    asyncio.run(scenario())