diff --git a/nanobot/agent/loop.py b/nanobot/agent/loop.py
index 3c0f9eab8..598d6213b 100644
--- a/nanobot/agent/loop.py
+++ b/nanobot/agent/loop.py
@@ -9,6 +9,7 @@ import dataclasses
 import inspect
 import os
 import time
+import weakref
 from collections.abc import Coroutine, Iterable, Mapping
 from contextlib import AbstractContextManager, ExitStack, nullcontext, suppress
 from dataclasses import dataclass, field
@@ -394,7 +395,9 @@ class AgentLoop:
         self._runtime_context_providers: list[RuntimeContextProvider] = []
         self._active_tasks: dict[str, set[asyncio.Task[Any]]] = {}
         self._background_tasks: set[asyncio.Task[Any]] = set()
-        self._session_locks: dict[str, asyncio.Lock] = {}
+        self._session_locks: weakref.WeakValueDictionary[str, asyncio.Lock] = (
+            weakref.WeakValueDictionary()
+        )
         # Per-session pending queues for mid-turn message injection.
         # When a session has an active task, new messages for that session
         # are routed here instead of creating a new task.
@@ -1206,7 +1209,7 @@ class AgentLoop:
         session_key = self._effective_session_key(msg)
         if session_key != msg.session_key:
             msg = dataclasses.replace(msg, session_key_override=session_key)
-        lock = self._session_locks.setdefault(session_key, asyncio.Lock())
+        lock = self._get_session_lock(session_key)
         gate = self._concurrency_gate or nullcontext()
 
         delivery = self.turn_delivery_factory.unrouted(msg, session_key)
@@ -2108,7 +2111,7 @@ class AgentLoop:
             content=content, media=media or [], metadata=metadata,
         )
         # Share the dispatch lock so direct calls serialize with bus turns.
-        lock = self._session_locks.setdefault(session_key, asyncio.Lock())
+        lock = self._get_session_lock(session_key)
         try:
             async with lock:
                 kwargs: dict[str, Any] = {
@@ -2139,3 +2142,11 @@ class AgentLoop:
         finally:
             await self.runtime_event_publisher.run_status_changed(msg, session_key, "idle")
             self.runtime_event_publisher.clear_turn(session_key)
+
+    def _get_session_lock(self, session_key: str) -> asyncio.Lock:
+        """Return the shared lock while allowing idle session entries to expire."""
+        lock = self._session_locks.get(session_key)
+        if lock is None:
+            lock = asyncio.Lock()
+            self._session_locks[session_key] = lock
+        return lock
diff --git a/tests/agent/test_session_lock_lifecycle.py b/tests/agent/test_session_lock_lifecycle.py
new file mode 100644
index 000000000..43a862175
--- /dev/null
+++ b/tests/agent/test_session_lock_lifecycle.py
@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+import asyncio
+import gc
+from unittest.mock import MagicMock
+
+import pytest
+
+
+def _make_loop(loop_factory):
+    provider = MagicMock()
+    provider.get_default_model.return_value = "test-model"
+    return loop_factory(provider=provider)
+
+
+def test_idle_agent_session_locks_are_released(loop_factory):
+    loop = _make_loop(loop_factory)
+
+    for index in range(1000):
+        lock = loop._get_session_lock(f"api:temporary-{index}")
+
+    del lock
+    gc.collect()
+
+    assert len(loop._session_locks) == 0
+
+
+@pytest.mark.asyncio
+async def test_waiter_keeps_agent_session_lock_alive(loop_factory):
+    loop = _make_loop(loop_factory)
+    owner_lock = loop._get_session_lock("api:shared")
+    await owner_lock.acquire()
+    waiter_started = asyncio.Event()
+    waiter_entered = asyncio.Event()
+
+    async def wait_for_lock() -> None:
+        lock = loop._get_session_lock("api:shared")
+        waiter_started.set()
+        async with lock:
+            waiter_entered.set()
+
+    waiter = asyncio.create_task(wait_for_lock())
+    await waiter_started.wait()
+
+    assert loop._get_session_lock("api:shared") is owner_lock
+    assert not waiter_entered.is_set()
+
+    owner_lock.release()
+    await waiter
+    del owner_lock
+    gc.collect()
+
+    assert "api:shared" not in loop._session_locks