Check PKCE compatibility before blaming the client.

Metadata signals

{
  "response_types_supported": ["code"],
  "grant_types_supported": ["authorization_code", "refresh_token"],
  "code_challenge_methods_supported": ["S256"]
}

Verifier and challenge

The client creates a high-entropy verifier, derives a SHA-256 challenge, sends the challenge during authorization, and presents the verifier at the token endpoint. The authorization server must bind the two parts of the same transaction.

Frequent integration errors

• S256 is enforced but absent from discovery metadata.
• The server advertises plain only.
• The token endpoint receives a verifier associated with a different authorization request.
• A proxy or session layer loses the stored challenge.
• The redirect URI differs between the authorization and token requests.