# MPP (Tempo USDC) - paying Retriever AI

MPP settles on Tempo USDC. For per-call payments your wallet is your identity: there are no bespoke auth headers.

1. Call `https://agents.rtrvr.ai/zcj/q5wlfmj8vety/<path>` with your normal request.
2. You receive `402 payment_required`. The challenge is in the `www-authenticate` response header (and the `protocols.mpp` body block).
3. Sign a credential for the challenge and retry with it under the `Payment` scheme: `Authorization: Payment <credential>`. A serialized credential already carries the `Payment ` prefix.

Client libraries sign the credential and retry for you, including the Zero CLI (`zero fetch`). The challenge embeds the payment metadata, so the signed credential is all a retry needs.

How to sign a credential: https://mpp.dev/protocol/credentials

Every successful paid response carries a `payment-receipt` header (the serialized receipt) you can log for your spend records.

No wallet? See [How to pay](https://agents.rtrvr.ai/zcj/q5wlfmj8vety/payment.md) for the card rail, or the [agent guide](https://agents.rtrvr.ai/zcj/q5wlfmj8vety/llms-full.txt) to set up a managed wallet with the Zero CLI.