MCP: Scope step-up is not authentication step-up

MCP’s existing Step-Up Authorization Flow handles insufficient OAuth scopes, allowing servers to return a 403 insufficient_scope challenge and clients to request broader permissions, but it does not address whether a human user has recently re-authenticated or remains present for a sensitive action. That distinct authentication step-up problem is covered by RFC 9470 through concepts such as insufficient_user_authentication, acr_values, max_age, and auth_time, none of which MCP currently adopts. This gap is especially significant for long-running, unattended, and resumable agent sessions, where a valid token with durable permissions may be tied to an authentication event from hours or days earlier. MCP’s URL mode elicitation can support an out-of-band browser-based approval process and requires servers to verify the identity of the user completing it, but it does not provide a standard way to require authentication freshness. Until MCP and authorization servers support RFC 9470-style challenges, servers handling destructive operations should implement their own short-lived, single-use approvals bound to the verified user identity, the exact operation and its arguments, and a fresh authentication event.

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