How to tell an AI agent that its user needs to authorize

A transparent AI-agent token proxy must distinguish its own request errors, a user’s missing or invalid provider connection, and third-party API responses while preserving upstream statuses, headers, and bodies unchanged. Because 401, 403, and 404 can originate either from the proxy’s authentication and routing logic or from providers, the design uses 402 Payment Required for the recoverable condition in which a user must authorize a provider, returning a machine-readable error code and an authorization URL so an agent can prompt the user and retry. An X-Token-Proxy-Upstream-Status header marks responses that reached the provider, resolving otherwise ambiguous statuses such as 404. The approach has limitations: an incorrect organization-scope header can look identical to a missing user connection, and 402 is increasingly associated with the x402 payment protocol, making response-body parsing essential and suggesting that a different status code may be preferable in the future.

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