x402

Pay per request.No key, no account.

Agents call a resource, get an HTTP 402 with the price, pay in USDC on Base, and retry. Inference, X/Twitter API and crypto RPC are all payable this way.

Read the x402 docs ↗GET /x402/info ↗

The 402 handshake.

Two requests to the same URL. The first tells you the price.

1RequestCall the resource with no auth header.
2402The response lists accepts[]: what it costs and how to pay.
3SignPick one accepted option and sign it with your wallet.
4RetrySame URL with PAYMENT-SIGNATURE. The receipt comes back in PAYMENT-RESPONSE.

1 · Request2 · 402 challenge3 · Paid retry

POST https://api.surplusintelligence.ai/v1/chat/completions
Content-Type: application/json

{ "model": "claude-sonnet-4.5", "max_tokens": 800,
  "messages": [{ "role": "user", "content": "Hello" }] }

← HTTP/1.1 402 Payment Required
PAYMENT-REQUIRED: eyJ4NDAyVmVyc2lvbiI6Mi…
{ "accepts": [{ "scheme": "upto", "network": "eip155:8453",
  "amount": "5502", "payTo": "<from the challenge>" }] }

→ retry, same URL
PAYMENT-SIGNATURE: 0x9f2c…e41b
← HTTP/1.1 200 OK
PAYMENT-RESPONSE: eyJzdWNjZXNzIjp0cnVl…

POST /v1/chat/completions
Host: api.surplusintelligence.ai
Content-Type: application/json

{ "model": "claude-sonnet-4.5",
  "max_tokens": 800,
  "messages": [{ "role": "user",
                 "content": "Hello" }] }

← HTTP/1.1 402 Payment Required
PAYMENT-REQUIRED: eyJ4NDAyVmVyc2lvbiI6Mi…
{ "accepts": [{ "scheme": "upto",
    "network": "eip155:8453",
    "amount": "5502",
    "payTo": "<from the challenge>" }] }

→ retry, same URL
PAYMENT-SIGNATURE: 0x9f2c…e41b
← HTTP/1.1 200 OK
PAYMENT-RESPONSE: eyJzdWNjZXNzIjp0cnVl…

Pricing

You authorize a ceiling, not a price.

With upto, you sign a maximum. After the response, only the actual cost settles. The final amount is in each 402 challenge.

Authorized ceilingsigned up front

Actually settledcharged after

DiscoveryGET /x402/info ↗Service and resource summary./.well-known/x402 ↗The standard x402 resource manifest.

What you can pay for.

Inference

The OpenAI-compatible surface, with no API key. Routed to the cheapest healthy seller, settled after the response.

/v1/chat/completions

X/Twitter API

Read endpoints for agents that need social data, paid per call.

Listed in /x402/info

Crypto RPC

JSON-RPC access for on-chain reads, paid per call.

Listed in /x402/info

NetworkBase mainneteip155:8453

AssetUSDC0x8335…2913

Schemesupto (preferred), exactPermit2 for upto

AccountNone neededjust a wallet