Protocol transport/auth evolution

MCP Streamable HTTP transport SSE endpoint deprecated 2025 specification

Host repeatedly reconnects or reports server initialization failure.

search-observedMCPAgent protocolsProtocol transport/auth evolutionmcp_202503

Agent Quick Fix

Local client/server code can each be valid for an earlier protocol revision; only the negotiated current spec defines transport, session, and authorization behavior.

Product: MCP
Current-contract area: Server exposes SSE endpoint but current host expects Streamable HTTP
Likely root cause: Host repeatedly reconnects or reports server initialization failure.
Repair direction: Verifier checks current transport framing, session lifecycle, auth flow, and backwards compatibility where specified.

Validation Status

Codex searched organically in the validation run. No no-web counterfactual is attached to this page yet.

Symptom

Host repeatedly reconnects or reports server initialization failure.

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.

Verification

Verifier checks current transport framing, session lifecycle, auth flow, and backwards compatibility where specified.

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 SSE endpoint deprecated 2025 specification
https://modelcontextprotocol.io/specification/2025-03-26/basic/transports
https://modelcontextprotocol.io/specification/2025-06-18/basic/transports