Step-up authentication for AI agents

An MCP server can use long-lived OAuth authorization for routine actions while requiring a separate, fresh human approval for sensitive operations such as deleting environments, rotating keys, granting admin access, or sending broadcasts. Because RFC 9470 mid-session reauthentication is not yet broadly supported by MCP clients or fully available through the relevant AuthKit Connect flow, the described approach uses an out-of-band browser approval process: the server pauses a sensitive tool call, creates a short-lived approval record, and returns a link for the user to open in a web application. In the browser, the user reviews the exact action and arguments, completes step-up authentication if their identity was not recently verified, and explicitly approves or denies the request. Approval records are tied cryptographically to the authenticated user, tool name, and canonicalized arguments, expire after five minutes, and are consumed atomically to prevent reuse, cross-user redemption, or substitution of a different target. The MCP agent only relays the approval identifier and is instructed not to retry automatically, while the browser remains the authoritative place for identity verification and informed consent. The design also emphasizes validating redirect and freshness parameters, checking authentication freshness both when rendering and approving the page, maintaining audit information, and treating MCP elicitation as a useful intent-confirmation feature that does not replace browser-based reauthentication.

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