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Add test reproducing MCP session invalidation on server restart and implement partial fix
Co-authored-by: g3force <779094+g3force@users.noreply.github.com>
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‎adk/agenticlayer/agent.py‎

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from agenticlayer.config import InteractionType, McpTool, SubAgent
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from agenticlayer.constants import EXTERNAL_TOKEN_SESSION_KEY
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from agenticlayer.mcp_session_patch import apply_mcp_session_patch
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logger = logging.getLogger(__name__)
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# Apply the MCP session manager patch on module import
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# This fixes the session invalidation issue when MCP servers restart
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apply_mcp_session_patch()
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def _get_mcp_headers_from_session(readonly_context: ReadonlyContext) -> dict[str, str]:
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"""Header provider function for MCP tools that retrieves token from ADK session.

‎adk/agenticlayer/mcp_session_patch.py‎

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"""
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Monkey-patch for Google ADK's MCPSessionManager to fix session invalidation on server restart.
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This module patches both the MCPSessionManager.create_session method and the retry_on_errors
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decorator to properly handle the case where an MCP server restarts and loses session state.
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Root Cause:
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-----------
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When an MCP server restarts:
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1. The server loses all session state
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2. Client-side session streams remain open (not disconnected)
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3. Cached session appears valid because _is_session_disconnected() only checks local streams
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4. Server returns 404 for requests with old session IDs
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5. Tool calls time out waiting for responses
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6. On retry, the same bad cached session is reused
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The Fix:
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--------
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We patch the retry_on_errors decorator to:
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1. Detect when an error occurs during MCP operations
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2. Force-close the streams of the cached session
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3. This makes _is_session_disconnected() return True
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4. On retry, create_session() sees the session is disconnected and creates a fresh one
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This is a temporary workaround until the fix is merged upstream in Google ADK.
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Issue: https://github.com/agentic-layer/sdk-python/issues/XXX
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"""
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import functools
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import logging
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from typing import Any, Callable
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from google.adk.tools.mcp_tool import mcp_toolset
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from google.adk.tools.mcp_tool.mcp_session_manager import MCPSessionManager
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logger = logging.getLogger(__name__)
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# Store the original decorator
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_original_retry_on_errors = None
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def _patched_retry_on_errors(func: Callable[..., Any]) -> Callable[..., Any]:
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"""Patched version of retry_on_errors that invalidates sessions on error.
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This wraps the original decorator and adds logic to close cached session streams
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when an error occurs, ensuring the session is marked as disconnected for retry.
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"""
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# First, apply the original decorator if it exists
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if _original_retry_on_errors:
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func = _original_retry_on_errors(func)
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@functools.wraps(func)
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async def wrapper(self: Any, *args: Any, **kwargs: Any) -> Any:
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try:
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return await func(self, *args, **kwargs)
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except Exception as e:
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# When an error occurs, try to invalidate any cached MCP sessions
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# by closing their streams, so retry gets a fresh session
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if hasattr(self, "_mcp_session_manager"):
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session_manager: MCPSessionManager = self._mcp_session_manager
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logger.info(
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f"[PATCH] Error in MCP operation ({func.__name__}), invalidating cached sessions: {type(e).__name__}"
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)
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# Access the session cache and close all sessions' streams
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if hasattr(session_manager, "_sessions"):
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try:
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# Use the lock to safely access sessions
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num_sessions = len(session_manager._sessions) # type: ignore
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logger.debug(f"[PATCH] Found {num_sessions} cached sessions to invalidate")
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# We can't use the lock here because we're already in an async context
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# and the lock might be held. Instead, just try to close streams.
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for session_key, (session, _, _) in list(session_manager._sessions.items()): # type: ignore
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try:
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logger.debug(f"[PATCH] Invalidating session: {session_key}")
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# Force-close the read stream
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if hasattr(session, "_read_stream"):
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stream = session._read_stream
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logger.debug(
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f"[PATCH] Read stream type: {type(stream).__name__}, has aclose: {hasattr(stream, 'aclose')}"
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)
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if hasattr(stream, "aclose"):
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await stream.aclose()
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logger.debug("[PATCH] Closed read stream via aclose()")
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elif hasattr(stream, "close"):
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stream.close()
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logger.debug("[PATCH] Closed read stream via close()")
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else:
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logger.debug("[PATCH] Session has no _read_stream")
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# Force-close the write stream
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if hasattr(session, "_write_stream"):
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stream = session._write_stream
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logger.debug(
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f"[PATCH] Write stream type: {type(stream).__name__}, has aclose: {hasattr(stream, 'aclose')}"
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)
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if hasattr(stream, "aclose"):
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await stream._write_stream.aclose()
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logger.debug("[PATCH] Closed write stream via aclose()")
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elif hasattr(stream, "close"):
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stream.close()
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logger.debug("[PATCH] Closed write stream via close()")
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else:
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logger.debug("[PATCH] Session has no _write_stream")
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logger.info(f"[PATCH] Successfully invalidated session {session_key}")
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except Exception as close_err:
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logger.warning(f"[PATCH] Could not close streams for {session_key}: {close_err}")
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except Exception as invalidate_err:
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logger.error(f"[PATCH] Error invalidating sessions: {invalidate_err}", exc_info=True)
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else:
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logger.debug("[PATCH] Session manager has no _sessions attribute")
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else:
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logger.debug(f"[PATCH] Object {type(self).__name__} has no _mcp_session_manager attribute")
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# Re-raise the exception so the original decorator can handle retry
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raise
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return wrapper
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def apply_mcp_session_patch() -> None:
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"""Apply the monkey-patch to the retry_on_errors decorator.
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This should be called once during application initialization before
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any MCP tools are created.
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"""
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global _original_retry_on_errors
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if _original_retry_on_errors is None:
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logger.info("Applying MCP session manager patch for server restart handling")
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# Store the original decorator
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from google.adk.tools.mcp_tool import mcp_session_manager
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from google.adk.tools.mcp_tool.mcp_toolset import McpToolset
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_original_retry_on_errors = mcp_session_manager.retry_on_errors
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# Replace the decorator in the module
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mcp_session_manager.retry_on_errors = _patched_retry_on_errors
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# Re-decorate the methods in McpToolset that use @retry_on_errors
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# Find all methods that were decorated and re-decorate them
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for attr_name in dir(McpToolset):
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if not attr_name.startswith("_"):
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attr = getattr(McpToolset, attr_name)
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if callable(attr) and hasattr(attr, "__wrapped__"):
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# This is likely a decorated method
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# Re-decorate it with our patched decorator
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original_func = attr.__wrapped__
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setattr(McpToolset, attr_name, _patched_retry_on_errors(original_func))
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logger.debug(f"Re-decorated McpToolset.{attr_name}")
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logger.info("MCP session manager patch applied successfully")
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else:
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logger.warning("MCP session manager patch already applied")
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def remove_mcp_session_patch() -> None:
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"""Remove the monkey-patch and restore original behavior.
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This is primarily for testing purposes.
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"""
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global _original_retry_on_errors
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if _original_retry_on_errors is not None:
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logger.info("Removing MCP session manager patch")
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from google.adk.tools.mcp_tool import mcp_session_manager
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mcp_session_manager.retry_on_errors = _original_retry_on_errors
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if hasattr(mcp_toolset, "retry_on_errors"):
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mcp_toolset.retry_on_errors = _original_retry_on_errors
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_original_retry_on_errors = None
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logger.info("MCP session manager patch removed")

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