A **pay-per-call API** charges for each request on its own instead of through a monthly plan or a prepaid balance. Each call has its own price, and you pay only for the calls you make. The model fits workloads that are irregular, experimental, or run by autonomous agents that should spend only what a task needs. ## How do the three billing models compare? Model You commit to Unused value Monthly plan a fixed fee per month lost at month end Prepaid credits a pack bought upfront kept until expiry Pay-per-call nothing none FastAF offers optional prepaid packs for API-key and MCP use, and true pay-per-call over x402. ## How does FastAF do pay-per-call? On `GET https://api.fastaf.co/x402/v1/` each request is quoted and settled on its own in USDC on Base. Prices are fixed per route, for example `serp` ($0.003), `walmart` ($0.005), `shopping` ($0.01) and `linkedin` ($0.06): see the [x402 price list](/pricing/x402-price-list/). Only successful calls are paid. ## Worked example A research agent runs 40 Google searches, reads 10 YouTube transcripts and fetches 5 Walmart search pages: * 40 × $0.003 = $0.12 * 10 × $0.005 = $0.05 * 5 × $0.005 = $0.025 Total: $0.195. If two of the searches fail upstream they are not settled, and the total drops by $0.006. No account, no leftover balance. ## What does a fixed price buy you? Because each route has one price at its default configuration, an agent can compute the maximum cost of a job before starting: calls × price. Options that would cost more are not sold on the pay-per-call path, so the quote never changes in the middle of a job. ## When is pay-per-call the wrong choice? * High steady volume, where a prepaid pack is simpler to manage. * MCP clients, which use an API key, not x402. * Options above the fixed route price, which require API-key billing. ## Checklist for a pay-per-call integration 1. Read the quote before paying. 2. Set a per-call and a per-job cap in the agent. 3. Log each settlement receipt. 4. Retry failures freely: they are not billed. More: [budget a scraping job before you run it](/how-to/budget-scraping-job-fixed-prices/). Related: [x402](/glossary/x402/) · [HTTP 402](/glossary/http-402/) ## Related terms * [x402](/glossary/x402/): x402 is a protocol for paying for HTTP resources using a payment challenge and a signed retry. * [MCP](/glossary/mcp/): Model Context Protocol (MCP) standardizes how AI applications discover and invoke tools and access context. * [HTTP 402](/glossary/http-402/): HTTP 402 Payment Required is an HTTP status code indicating that payment is required for a resource. What it costs: [FastAF pricing and credit packs](/pricing/) · [x402 price list per call](/pricing/x402-price-list/) ## Frequently asked questions ### What is a pay-per-call API? paying for each request individually, without an account or subscription. ## Try it on your own request 500 free credits every month after email verification, no card. [Start free](/signup/) ## Related * [subscription vs pay-per-call cost](/how-to/cost-per-agent-saas-vs-x402/) * [x402](/glossary/x402/) * [MCP](/glossary/mcp/)