fix: handle 404 and 406 gracefully in SSE stream initialization#1969
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nanookclaw wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
fix: handle 404 and 406 gracefully in SSE stream initialization#1969nanookclaw wants to merge 1 commit intomodelcontextprotocol:mainfrom
nanookclaw wants to merge 1 commit intomodelcontextprotocol:mainfrom
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Widen the response status check in StreamableHTTPClientTransport._startOrAuthSse() from 405-only to include 404 and 406. Servers that only support POST-based communication (no GET SSE stream) commonly return 404 (no handler) or 406 (Accept header rejected) instead of the spec-defined 405. All three statuses indicate the server does not offer an SSE stream at the GET endpoint. Fixes modelcontextprotocol#1635
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@modelcontextprotocol/client
@modelcontextprotocol/server
@modelcontextprotocol/express
@modelcontextprotocol/fastify
@modelcontextprotocol/hono
@modelcontextprotocol/node
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Problem
StreamableHTTPClientTransport._startOrAuthSse()only handles HTTP 405 as a graceful exit from the GET SSE stream attempt. Many MCP servers return 404 (no GET handler) or 406 (Accept header rejected) instead, causing the client to throw and preventing POST-based communication that would otherwise work.Fix
Widen the status check in
_startOrAuthSse()to treat 404, 405, and 406 identically — all three indicate the server does not offer an SSE stream at the GET endpoint.Change:
packages/client/src/client/streamableHttp.tsline 285:response.status === 405→response.status === 404 || response.status === 405 || response.status === 406Impact
Affects Gemini CLI (#5268, #2787), MCP Inspector (#1150), LibreChat, and any client consuming this SDK against servers that only support POST.
Closes #1635