modelcontextprotocol / modelcontextprotocol/python-sdk
MCP client doesn't not initialize new session when getting 404 session not found
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Description
Initial Checks
- I confirm that I'm using the latest version of MCP Python SDK
- I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue
Description
Per https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#session-management
When a client receives HTTP 404 in response to a request containing an MCP-Session-Id, it MUST start a new session by sending a new InitializeRequest without a session ID attached.
This is not the case with the python client. It just sends a session terminated error in streamable_http.py
Example Code
Python & MCP Python SDK
mcp: 1.22.0
python: 3.14.0
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with src/mcp/client/streamable_http.py around line 277, where a 404 response containing an MCP-Session-Id is handled, and compare that behavior with the session-management requirement linked in the issue. Trace the client’s initialization entry point and verify the completed behavior by exercising a 404 response: the client should send a new InitializeRequest without a session ID instead of returning a session-terminated error.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- api
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 70/100