[Client][Auth] SEP-2352: Key DCR registrations and tokens by AS issuer; reject cross-AS credential reuse #361

Implements SEP-2352 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #338.

Spec summary

Clarifies that clients must key DCR/pre-registered credentials by AS issuer, maintain separate registration state per AS, and error on AS mismatch. CIMD client_ids remain portable. Clients MUST NOT assume cross-AS credential validity.

PHP SDK changes

• TokenStorage ([Client] Add OAuth 2.0 credential storage interface (TokenStorageInterface) #315) and DCR ([Client] Implement Dynamic Client Registration (RFC 7591) + pre-registered client support #320) must be indexed by AS issuer URL, not just by MCP server URL.
• Token cache key should be (resource_server, as_issuer) tuples.
• PRM/AS metadata discovery ([Client] Implement Protected Resource Metadata discovery (RFC 9728) #317, [Client] Implement Authorization Server Metadata discovery (RFC 8414) #318) must drive the binding.
• Client must error if a stored credential's bound issuer != the issuer currently advertised by the resource.
• Server-side: minimal — server PRM metadata document in src/Server/Transport/Http/OAuth/ must continue to advertise its AS issuer unambiguously.

Related

• Umbrella [2026-07-28] Authorization hardening (OAuth/OIDC) #338
• Extends [Client] Add OAuth 2.0 credential storage interface (TokenStorageInterface) #315 (TokenStorage) and [Client] Implement Dynamic Client Registration (RFC 7591) + pre-registered client support #320 (DCR) with per-issuer keying