My AI agent says payment is required or I'm out of units. What do I do?

On the self-serve platform, a payment error carries a payment link: a person opens it and pays, or waits for a usage window to refill, then the agent retries.

1. Units ran out: REST answers 402 payment_required with an upgrade_url and a Link header with rel="payment"; a platform MCP server answers a payment error with the same link (error page).
2. An Agent usage window is used up and no credits remain: REST answers 429 usage_limit_reached with the refill time, a Retry-After header and an upgrade_url to buy credits; MCP answers a payment error with the same members.
3. The spending cap is reached: 402 payment_required with the reason spending_cap_reached, until the period ends or the cap is raised.

Once a person has paid, the agent polls GET /v1/account/payment-links/{id} about every 5 seconds until the status is completed, then retries.

The agent cannot pay by itself (can an agent pay?); credits top up automatically only if a person turned on auto top-up. Whether an assistant must reconnect after paying is not published. See handling a 402 and credits.

On the Casafari MCP server, Request limit reached for this product. is different: see quota versus units.