diff --git a/.github/.release-please-manifest.json b/.github/.release-please-manifest.json
index d0f8c0ee4d..661ffa458c 100644
--- a/.github/.release-please-manifest.json
+++ b/.github/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "1.24.1"
+  ".": "1.25.1"
 }
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1efb2b2412..c486780192 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## [1.25.1](https://github.com/google/adk-python/compare/v1.25.0...v1.25.1) (2026-02-18)
+
+### Bug Fixes
+
+* Fix pickling lock errors in McpSessionManager ([4e2d615](https://github.com/google/adk-python/commit/4e2d6159ae3552954aaae295fef3e09118502898))
+
 ## [1.25.0](https://github.com/google/adk-python/compare/v1.24.1...v1.25.0) (2026-02-11)
 
 ### Features
diff --git a/src/google/adk/tools/mcp_tool/mcp_session_manager.py b/src/google/adk/tools/mcp_tool/mcp_session_manager.py
index 0e9b938609..f4339f8678 100644
--- a/src/google/adk/tools/mcp_tool/mcp_session_manager.py
+++ b/src/google/adk/tools/mcp_tool/mcp_session_manager.py
@@ -500,6 +500,30 @@ async def create_session(
             )
         raise ConnectionError(f'Failed to create MCP session: {e}') from e
 
+  def __getstate__(self):
+    """Custom pickling to exclude non-picklable runtime objects."""
+    state = self.__dict__.copy()
+    # Remove unpicklable entries or those that shouldn't persist across pickle
+    state['_sessions'] = {}
+    state['_session_lock_map'] = {}
+
+    # Locks and file-like objects cannot be pickled
+    state.pop('_lock_map_lock', None)
+    state.pop('_errlog', None)
+
+    return state
+
+  def __setstate__(self, state):
+    """Custom unpickling to restore state."""
+    self.__dict__.update(state)
+    # Re-initialize members that were not pickled
+    self._sessions = {}
+    self._session_lock_map = {}
+    self._lock_map_lock = threading.Lock()
+    # If _errlog was removed during pickling, default to sys.stderr
+    if not hasattr(self, '_errlog') or self._errlog is None:
+      self._errlog = sys.stderr
+
   async def close(self):
     """Closes all sessions and cleans up resources."""
     async with self._session_lock:
diff --git a/src/google/adk/version.py b/src/google/adk/version.py
index 8002d61987..1ce0bf5e6c 100644
--- a/src/google/adk/version.py
+++ b/src/google/adk/version.py
@@ -13,4 +13,4 @@
 # limitations under the License.
 
 # version: major.minor.patch
-__version__ = "1.25.0"
+__version__ = "1.25.1"
diff --git a/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py b/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py
index ba9a7e801c..327df114a8 100644
--- a/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py
+++ b/tests/unittests/tools/mcp_tool/test_mcp_session_manager.py
@@ -607,6 +607,39 @@ async def test_close_skips_aclose_for_different_loop_sessions(self):
     exit_stack2.aclose.assert_not_called()
     assert len(manager._sessions) == 0
 
+  @pytest.mark.asyncio
+  async def test_pickle_mcp_session_manager(self):
+    """Verify that MCPSessionManager can be pickled and unpickled."""
+    import pickle
+
+    manager = MCPSessionManager(self.mock_stdio_connection_params)
+
+    # Access the lock to ensure it's initialized
+    lock = manager._session_lock
+    assert isinstance(lock, asyncio.Lock)
+
+    # Add a mock session to verify it's cleared on pickling
+    manager._sessions["test"] = (Mock(), Mock(), asyncio.get_running_loop())
+
+    # Pickle and unpickle
+    pickled = pickle.dumps(manager)
+    unpickled = pickle.loads(pickled)
+
+    # Verify basics are restored
+    assert unpickled._connection_params == manager._connection_params
+
+    # Verify transient/unpicklable members are re-initialized or cleared
+    assert unpickled._sessions == {}
+    assert unpickled._session_lock_map == {}
+    assert isinstance(unpickled._lock_map_lock, type(manager._lock_map_lock))
+    assert unpickled._lock_map_lock is not manager._lock_map_lock
+    assert unpickled._errlog == sys.stderr
+
+    # Verify we can still get a lock in the new instance
+    new_lock = unpickled._session_lock
+    assert isinstance(new_lock, asyncio.Lock)
+    assert new_lock is not lock
+
 
 @pytest.mark.asyncio
 async def test_retry_on_errors_decorator():