mcp: limit SSE discover supportedVersions to SSE-defined revisions - #1121
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mcp: limit SSE discover supportedVersions to SSE-defined revisions#1121arimu1 wants to merge 1 commit into
arimu1 wants to merge 1 commit into
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HTTP+SSE is only defined by older MCP revisions. SSEServerTransport now implements ProtocolVersionSupporter so server/discover does not advertise 2026-07-28 (stdio + Streamable HTTP only). Also avoid marking the session initialized from discover when the requested version is not transport-supported, so Modern-first clients can fall back to the legacy initialize handshake over SSE. Fixes modelcontextprotocol#1112
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Summary
NewSSEHandlersessions were advertising2026-07-28inserver/discoversupportedVersions. That revision only defines stdio and Streamable HTTP, not the deprecated HTTP+SSE binding, so Modern-first clients could negotiate a revision the transport cannot honour.Changes
ProtocolVersionSupporteronSSEServerTransportsofilterSupportedVersionsexcludes>= 2026-07-28(matching the streamable transport's version gate pattern).Server.discover, only set sessionInitializeParamswhen the requested version is actually supported by the transport. Without this, a discover probe that returns only legacy versions still marked the session initialized, and the client's fallbackinitializefailed withduplicate "initialize" received.Fixes #1112
Test plan
go test ./...go test ./mcp/ -run 'TestSSE|TestInMemory_E2E_Discover|TestStreamableStateful_AcceptsDiscover|TestStreamableHTTP_E2E_Discover|TestClientConnectDiscover'Assistance: human-reviewed AI assistance used for implementation.