Pay-per-call API

A pay-per-call API prices every request on its own and takes payment with the request, rather than through a subscription, a quota or an API key. The price is quoted in an HTTP 402 before anything is paid, and a failed call is not charged.

One request, one price

A pay-per-call API charges for each request individually. There is no plan to pick and no monthly minimum: the caller pays the stated price for the call it makes, and a caller who makes one request pays for one request. Prices on this server start at $0.001 per call.

That shape suits callers whose usage is irregular or unpredictable, which describes most agents. It also suits sellers, because a stranger can buy a single call with no onboarding.

The price comes first

Every priced route answers an unpaid request with a 402 carrying its price, so a caller always knows the cost before paying. Catalogs list the same prices ahead of time: /api/pricing and /openapi.json on this server, and /api/find returns the price with each match.

$ curl -s https://agent402.tools/api/find?q=hash+some+text
# each match carries route, price, input schema and a ready example

$ curl -s -o /dev/null -w '%{http_code}\n' -X POST https://agent402.tools/api/hash \
    -H 'Content-Type: application/json' -d '{"text":"hello world"}'
402   # the quote: amount "1000" = $0.001 in USDC (6 decimals), nothing charged yet

Some routes price a request by its size rather than a flat fee. The metered model routes quote each request from its body, and a buyer paying through a prepaid key or a Permit2 allowance settles at actual usage under that quote.

What is charged, and what is not

  • A successful answer is charged once, and carries a receipt.
  • A failed call (an error, a timeout, an upstream outage) is not charged, because settlement runs after the tool and only on success.
  • A retry with the same Idempotency-Key and the same credential replays the first answer instead of paying again.
  • Reading the 402 quote is free.

Paying at volume

Signing and settling every request on chain is the right shape for occasional calls and the wrong one for thousands. Two options avoid it: a prepaid credits key (one card purchase, then a Bearer header on each call, debited per success) or, with a wallet on Base, a one-time Permit2 approval so metered calls settle at actual usage. Both keep the per-request price; only the settlement changes.

Related

More explainers: x402 · HTTP 402 Payment Required · MPP (Machine Payments Protocol) · Agent payments · MCP payments