Protocol transport/auth evolution
MCP Streamable HTTP transport replaces HTTP+SSE 2025-03-26 old SSE endpoint /sse /mcp
Client reports a generic connection or 404 failure even though both endpoints are individually healthy.
Agent Quick Fix
Repair against the current MCP contract, then keep the change narrow and source-backed.
Product: MCP
Current-contract area: Old HTTP+SSE client cannot connect to Streamable HTTP server
Likely root cause: Client reports a generic connection or 404 failure even though both endpoints are individually healthy.
Repair direction: Verifier checks current transport framing, session lifecycle, auth flow, and backwards compatibility where specified.Symptom
Client reports a generic connection or 404 failure even though both endpoints are individually healthy.
Why This Happens
Local client/server code can each be valid for an earlier protocol revision; only the negotiated current spec defines transport, session, and authorization behavior.
Common Wrong Fixes
- Changing local code without checking the current external contract.
- Retrying the same install, build, or API call with no version/source change.
- Applying a broad unsafe bypass when a narrow compatibility fix is available.
Codex Search Keywords
These are the search terms observed in a neutral Codex validation run for this failure shape.
MCP Streamable HTTP transport replaces HTTP+SSE 2025-03-26 old SSE endpoint /sse /mcp
site:modelcontextprotocol.io specification streamable HTTP transport SSE deprecated
https://modelcontextprotocol.io/specification/2025-03-26/basic/transports