fix(client): reinitialize expired streamable HTTP sessions

Author: nankingjing

src/mcp/client/_transport.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from contextlib import AbstractAsyncContextManager
-from typing import Protocol
+from typing import Final, Protocol
 
 from mcp.shared._stream_protocols import ReadStream, WriteStream
 from mcp.shared.message import SessionMessage
@@ -12,6 +12,11 @@
 
 TransportStreams = tuple[ReadStream[SessionMessage | Exception], WriteStream[SessionMessage]]
 
+# SDK-private signal emitted by StreamableHTTPTransport after a request with an
+# established MCP session receives HTTP 404. It never crosses the wire.
+SESSION_EXPIRED: Final = -32003
+SESSION_EXPIRED_MARKER: Final = "mcp.client.session_expired"
+
 
 class Transport(AbstractAsyncContextManager[TransportStreams], Protocol):
     """Protocol for MCP transports.