SSE Mode /messages/?session_id=... Returns "Invalid Content-Type header" with Correct Headers Description

[abokyoy]

mcp_20250924-103932.txt

When I am running Serena MCP server in SSE mode (--transport sse --port 9121), the /sse endpoint correctly returns a dynamic session_id (e.g., /messages/?session_id=2091628429e544b283d6fe06575bb05b). However, accessing /messages/?session_id=... consistently returns Invalid Content-Type header, even with correct headers (Accept: text/event-stream and Content-Type: text/event-stream).
Steps to Reproduce

Run Serena:uvx --from git+https://github.com/oraios/serena serena-mcp-server --transport sse --port 9121 --context ide-assistant --project E:\LLM\LectureLearnLoop --log-level DEBUG

Test /sse:curl -H "Accept: text/event-stream" http://localhost:9121/sse

Output: event: endpoint\ndata: /messages/?session_id=2091628429e544b283d6fe06575bb05b
Test /messages/ with session_id:curl -v -H "Accept: text/event-stream" -H "Content-Type: text/event-stream" http://localhost:9121/messages/?session_id=2091628429e544b283d6fe06575bb05b

Output: HTTP/1.1 400 Bad Request\nInvalid Content-Type header

Expected Behavior
The /messages/?session_id=... endpoint should return SSE stream data (e.g., tool list or MCP messages) when provided with correct headers.
Actual Behavior
Returns Invalid Content-Type header regardless of headers used.
Environment

OS: Windows
Serena Version: 0.1.4-ec050699-dirty (via uvx)

Related Issues

#196: SSE Transport Mode Fails with Dynamic Client Registration Error
#487: How to set messagePath in sse mode

Streamable HTTP mode (--transport streamable-http) also fails with 404 on /mcp/tools.
Windsurf client shows "0 tools" and "Loading MCP servers..." due to endpoint issues.

[MischaPanch]

Serena uses the official python mcp sdk, we are not responsible for what it does or doesn't do. I suppose the same problems apply to all python MCP servers, right?

[abokyoy]

Thank you so much for your response and insights! I’ve learned a lot from your project. For now, I’ve switched to --transport stdio as a workaround, and it’s working perfectly with Windsurf, resolving my issue.

Thanks again for your amazing work and support!