How AI agents pay for APIs with x402
x402 is an open payment protocol built on the long-dormant HTTP 402 status code. A server answers an unpaid request with a 402 challenge, the client settles a stablecoin payment, and retries. It lets an autonomous age…
What x402 is, The request → 402 → pay → 200 flow, Why it fits autonomous agents
What x402 is
x402 is an open payment protocol built on the long-dormant HTTP 402 status code. A server answers an unpaid request with a 402 challenge, the client settles a stablecoin payment, and retries. It lets an autonomous agent pay for a resource per request — no API key, no account, no human-managed billing in the loop.
The request → 402 → pay → 200 flow
An agent POSTs its inputs and receives HTTP 402 with a payment challenge: amount, asset, network, and recipient. Its x402 client signs a USDC-on-Base authorization and resends the request with the payment header. The server verifies the payment, runs the work, and returns 200 with the result. The whole exchange is one extra round trip.
Why it fits autonomous agents
Agents can't fill out signup forms, accept terms, or rotate API keys mid-task. x402 turns 'pay for this call' into a primitive the agent performs itself. A finance or research agent can discover a tool and use it the moment it needs it, paying only for the calls it actually makes.
Where to start
Hermes Plant's finance and quant endpoints are all x402-metered and cost a few cents per call. Browse the suite, point your agent's x402 client at an endpoint, and it will negotiate payment on the first 402 automatically.