Method start

src/google/adk/tools/mcp_tool/session_context.py:109–160

Start the runner and wait for the session to be ready. Returns: The initialized ClientSession. Raises: ConnectionError: If session creation fails.

(self)

    return self._task is not None and not self._task.done()

  async def start(self) -> ClientSession:
    """Start the runner and wait for the session to be ready.

    Returns:
        The initialized ClientSession.

    Raises:
        ConnectionError: If session creation fails.
    """
    async with self._task_lock:
      if self._session:
        logger.debug(
            'Session has already been created, returning existing session'
        )
        return self._session

      if self._close_event.is_set():
        raise ConnectionError(
            'Failed to create MCP session: session already closed'
        )

      if not self._task:
        self._task = asyncio.create_task(self._run())

        def _retrieve_exception(t: asyncio.Task):
          if not t.cancelled():
            t.exception()

        self._task.add_done_callback(_retrieve_exception)

    await self._ready_event.wait()

    if self._task.cancelled():
      raise ConnectionError('Failed to create MCP session: task cancelled')

    if self._task.done() and self._task.exception():
      raise ConnectionError(
          f'Failed to create MCP session: {self._task.exception()}'
      ) from self._task.exception()

    # Pre-fix code returned `self._session` here directly (typed as
    # ClientSession even though it could in theory be None). Adding an
    # explicit None check is safer but introduces a new exception path,
    # so we gate it behind the feature flag to keep flag-OFF byte-for-byte
    # compatible with pre-fix behavior.
    if (
        is_feature_enabled(FeatureName._MCP_GRACEFUL_ERROR_HANDLING)  # pylint: disable=protected-access
        and self._session is None
    ):
      raise ConnectionError('Failed to create MCP session: unknown error')

    return self._session  # type: ignore[return-value]

  async def _run_guarded(self, coro: Coroutine[Any, Any, _T]) -> _T:
    """Run a coroutine while monitoring the background session task.