How to charge per API call in USDC on Algorand with x402

To charge per API call in USDC on Algorand with x402, an endpoint returns HTTP 402 with an Algorand accept quote for USDC ASA 31566704 on MainNet, the client pays that exact amount, the server verifies the payment, then returns the data.

1. The 402 response. On an unpaid request the endpoint answers with HTTP 402 and an accepts[] list. Each accept names scheme exact, network algorand, asset 31566704 (Circle USDC on Algorand MainNet), payTo, maxAmountRequired in micro-USDC (6 decimals), and often a feePayer. A live DeskCrew probe of POST https://deskcrew.io/api/x402/tools/deskcrew/draft_reply quoted amount 60000 ($0.06), payTo PDGWRU3R7VY4QNPNLATXY25NSIT3OF7RXQMVTEDJSXLSLQ6XHIJDKI5AB4, and feePayer ZMFK2OI7ZBD2U27ISERZC4S6LKM6WMFJPZQ4MYNJDZ2VNBNMBA67RA22AA. Catalog terms for Algorand also live at https://deskcrew.io/.well-known/x402.

2. The client paying. The paying wallet builds an Algorand group: an optional fee PaymentTxn (note x402-fee-payer) plus an AssetTransferTxn of exactly maxAmountRequired micro-USDC to payTo (note x402-payment-v2), using algosdk.transaction.AssetTransferTxn, PaymentTxn, and calculate_group_id. The client retries the same HTTP request with an X-PAYMENT header carrying the signed payload.

3. Verifying. The server (or facilitator) confirms the group settled on MainNet: correct ASA 31566704 amount, receiver, and notes. Only then is the call treated as paid.

4. Returning the data. After verification the endpoint returns the tool JSON (for example a draftId). Settlement of the USDC charge is separate from any later prize payout on another network.

What does an x402 402 response contain?

Scheme, network, asset id, payTo, maxAmountRequired, timeout, and optional feePayer extras. Those fields are the price tag for that single call.

Does the client need ALGO to pay?

The paying wallet must hold USDC ASA 31566704 (opted in) and keep Algorand’s small ALGO minimum balance. Someone else can cover the fee via feePayer, but that does not remove the payer’s own ALGO minimum or the need to send the USDC axfer.

How do I verify an x402 payment on Algorand?

Look up the group on a MainNet indexer: confirm the axfer of ASA 31566704 for the quoted micro-amount to payTo, matching x402 notes, and that the fee leg (if any) was covered. Reject mismatches; only then return the paid response.

Plain path: unpaid call → 402 quote on ASA 31566704 → exact signed pay → verify on MainNet → return data. No invented links; probe /.well-known/x402 and one unpaid 402 before hard-coding amounts.