fix(client): handle 404 and 406 gracefully in GET SSE stream#1799
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4444J99 wants to merge 1 commit intomodelcontextprotocol:mainfrom
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fix(client): handle 404 and 406 gracefully in GET SSE stream#17994444J99 wants to merge 1 commit intomodelcontextprotocol:mainfrom
4444J99 wants to merge 1 commit intomodelcontextprotocol:mainfrom
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Widen the early-return guard in _startOrAuthSse() to include 404 and 406 alongside the existing 405 check. All three status codes indicate the server does not offer an SSE stream at the GET endpoint — the client falls back to POST-only communication. Fixes modelcontextprotocol#1635
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Motivation and Context
_startOrAuthSse()sends a GET request to open an SSE stream duringstart(). Per the MCP spec, the server MAY respond with405 Method Not Allowedif it doesn't support SSE streams, and the client correctly handles this by returning early and falling back to POST-only communication.However, many MCP servers return 404 Not Found or 406 Not Acceptable instead of 405:
Accept: text/event-stream(e.g., requires bothapplication/jsonandtext/event-stream)Both carry the same semantic meaning as 405: no SSE stream is available at the GET endpoint. The client currently treats them as fatal errors, preventing the connection even though POST-based communication would work fine.
This affects downstream consumers including Gemini CLI (google-gemini/gemini-cli#5268, google-gemini/gemini-cli#2787), MCP Inspector, and LibreChat.
Fixes #1635
How Has This Been Tested?
should handle 404 gracefully when server has no GET handlerandshould handle 406 gracefully when server rejects Accept header for GET) following the same pattern as the existing 405 testBreaking Changes
None.
Types of changes
Checklist