Protocol transport/auth evolution
Model Context Protocol Streamable HTTP POST response JSON SSE 202 Accepted specification
Tool invocation hangs when the new server legitimately returns a non-streaming response.
Agent Quick Fix
Repair against the current MCP contract, then keep the change narrow and source-backed.
Product: MCP
Current-contract area: Client assumes every POST yields an SSE stream
Likely root cause: Tool invocation hangs when the new server legitimately returns a non-streaming response.
Repair direction: Verifier checks current transport framing, session lifecycle, auth flow, and backwards compatibility where specified.Symptom
Tool invocation hangs when the new server legitimately returns a non-streaming response.
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.
Model Context Protocol Streamable HTTP POST response JSON SSE 202 Accepted specification
site:modelcontextprotocol.io spec streamable http POST response SSE JSON MCP
https://modelcontextprotocol.io/specification/2025-06-18/basic/transports