Definition
Pay-Per-Call
A pricing model where each individual API call is billed as a separate micro-transaction, rather than through subscriptions or rate-limited tiers.
Pay-per-call is a pricing model where each API request is an independent financial transaction. Instead of buying a monthly subscription with a fixed number of calls, or managing rate-limited API keys, agents pay the exact cost of each call — often fractions of a cent. This model is ideal for AI agents because their usage is unpredictable: a research agent might make 10 calls one day and 10,000 the next. x402 makes pay-per-call practical by automating the payment negotiation inline with each HTTP request.
Related terms
x402 Protocol
An open payment protocol that uses HTTP 402 Payment Required to enable pay-per-call API transactions for AI agents.
HTTP 402 Payment Required
An HTTP status code reserved for future use since 1997, now used by x402 to signal that a request requires payment before processing.
API Marketplace
A curated directory of APIs that AI agents can discover, evaluate, and pay for on a per-call basis.
Micro-Transaction
A very small financial transaction, typically under $1, enabled by low-fee blockchain networks for pay-per-call API pricing.