How Do AI Agents Pay for Things?

Micropayments explained

A micropayment is a small payment for a specific digital resource or access window. In a machine-to-machine flow, an agent can read a payment challenge and respond programmatically instead of sending an operator through a browser checkout. The payer still needs funds and authority to sign the payment.

x402 uses an HTTP 402 response to describe the requested amount, asset, network, destination, and timeout. For Zambo's current Day Pass route, the challenge is for $1.49 USDC on Base and the pass lasts 24 hours after confirmed settlement. It is an access window, not a separate $1.49 charge for every MCP tool call.

The x402 flow step by step

1. POST to /api/x402/day-pass without an X-Payment header. The server responds with an HTTP 402 payment challenge.
2. Read the challenge and check its amount, USDC asset, Base network, destination, and timeout.
3. Sign the EIP-3009 TransferWithAuthorization typed data for the stated sender, recipient, value, validity window, and unique nonce.
4. Base64-encode the JSON payment envelope and retry the same request with it in the X-Payment header.
5. Wait for on-chain settlement confirmation. The successful response activates the Day Pass and returns an access_key for subsequent MCP calls.

After confirmed settlement, the success response also returns a spend_receipt bound to that payment. The x402 agent payments guide documents the live endpoint contracts and response fields.

Spending safely

• Before signing, compare the challenge's amount, asset, network, destination, and timeout with the payment you intend to authorize.
• After confirmation, compare the receipt's resource, chain, asset, payTo, and maxAmount with the original challenge.
• Inspect settlementId on Base, check the 24-hour expiry, and open auditUrl to review the public receipt.
• Keep the returned access_key with the agent that will use it. The public spend receipt does not expose this bearer capability.

An HTTP 402 refusal is not a successful payment or an active pass. If a client cannot use EIP-3009, the separate manual fallback pairs a payment intent with a verified transaction hash. For receipt and price context, see AI agent receipt pricing.

Start free, pay per use

The free MCP tier allows 20 calls per tool per day with no account. When an agent needs a paid access window, the current Day Pass is $1.49 USDC for 24 hours after settlement. Connect one Zambo MCP server through the install guide, and check pay-per-call pricing for current options.

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