Protocol transport/auth evolution

MCP authorization PKCE state remote authorization flow session remains unauthorized

Authentication loops back successfully, yet the MCP session remains unauthorized.

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: PKCE/state handling mismatch in remote authorization flow
Likely root cause: Authentication loops back successfully, yet the MCP session remains unauthorized.
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

Authentication loops back successfully, yet the MCP session remains unauthorized.

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 authorization PKCE state remote authorization flow session remains unauthorized
site:modelcontextprotocol.io/specification authorization PKCE state MCP
Model Context Protocol specification authorization OAuth 2.1 PKCE state
https://modelcontextprotocol.io/specification/2025-03-26/basic/authorization
https://modelcontextprotocol.io/specification/2025-06-18/basic/authorization