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A legacy server may reject server/discover with HTTP 200 and a JSON-RPC error while setting Mcp-Session-Id. The streamable client stored that ID and sent it on the fallback initialize, so a server that assigned a new session ID made Connect fail with "mismatching session IDs". server/discover is sessionless in 2026-07-28, and under 2025-11-25 the session ID belongs to the response carrying the InitializeResult, so the client no longer records a session ID from a server/discover response. Fixes modelcontextprotocol#1522 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| if hadSessionID == "" { | ||
| c.sessionID = sessionID | ||
| } |
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| if hadSessionID == "" && requestMethod == methodInitialize { | |
| c.sessionID = sessionID | |
| } |
According to spec sessionId should be set up only during initialize
A server MAY assign a session ID "at initialization time". It does this by putting it in an MCP-Session-Id header on the HTTP response that carries the InitializeResult
Adding this check here should be more clean
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A legacy server may reject
server/discoverwith HTTP 200 and a JSON-RPC error while settingMcp-Session-Id. The streamable client stored that ID and sent it on the fallbackinitialize, so a server that assigned a new session ID madeConnectfail withmismatching session IDs.server/discoveris sessionless in 2026-07-28, and under 2025-11-25 the session ID belongs to the response carrying theInitializeResult, so the client no longer records a session ID from aserver/discoverresponse.The new table test covers the HTTP 200 / -32601 case and four HTTP 400 rejection shapes seen from real legacy servers (plain text, -32600 with an unmatched id, -32600 with legacy-only
data.supported), each with a session ID on the discover response.Fixes #1522
AI disclosure: written with Claude Code; I reviewed the change and can explain it.