Summary

Fixes #924.

MCPServerStreamableHttp uses streamablehttp_client which returns a 3-tuple (read, write, get_session_id). However, the connect() method in the base class _MCPServerWithClientSession discarded the third element:

# Before (base class connect())
read, write, *_ = transport  # get_session_id silently dropped ❌

This made it impossible to retrieve the server-assigned MCP session ID, which is required to resume a stateful session in stateless worker setups (e.g. serverless functions, load-balanced workers).

Changes

src/agents/mcp/server.py

1. _MCPServerWithClientSession.__init__ — initializes _get_session_id: GetSessionIdCallback | None = None so the attribute is always present on the instance.

2. _MCPServerWithClientSession.connect() — captures the callback instead of discarding it:

   # After
   read, write, *rest = transport
   # Capture the session-id callback when present (streamablehttp_client only).
   self._get_session_id = rest[0] if rest and callable(rest[0]) else None

   This is backward-compatible: SSE transport returns a 2-tuple, so rest will be empty and _get_session_id stays None.

3. MCPServerStreamableHttp.session_id property — new public property that delegates to the callback:

   @property
   def session_id(self) -> str | None:
       if self._get_session_id is None:
           return None
       return self._get_session_id()

Usage example

# Worker 1: start a session and capture its ID
async with MCPServerStreamableHttp(params={"url": mcp_url}) as server:
    await runner.run(agent, "Do something")
    session_id = server.session_id  # e.g. "550e8400-e29b-41d4-a716-446655440000"
    # persist session_id to Redis / DB

# Worker 2 (new process / serverless invocation): resume the session
async with MCPServerStreamableHttp(
    params={"url": mcp_url, "headers": {"Mcp-Session-Id": session_id}}
) as server:
    await runner.run(agent, "Continue where we left off")

Tests

Added tests/mcp/test_streamable_http_session_id.py with 6 test cases:

All 120 existing MCP tests continue to pass.

Notes

• The _isolated_client_session helper (used for per-call isolated sessions in MCPServerStreamableHttp) intentionally keeps *_ — those ephemeral sessions are not meant to be resumed.
• No changes to __all__ or public API exports are needed; session_id is accessed directly on the instance.