Answers
What is an x402 judge API and how do I call one?
What is an x402 judge API and how do I call one?
An x402 judge API grades agent output and charges per request with the x402 protocol: an unpaid call returns HTTP 402 with machine-readable payment terms, and the client repeats the call with a PAYMENT-SIGNATURE header. Screwtape.ai's /v1/judge and /v1/consensus work this way at $0.005 USDC per call on Base or Solana, settled through a facilitator.
Example
curl -s -D - -o /dev/null https://screwtape.ai/v1/judge -H "Content-Type: application/json" \
-d '{"task":"State the crew size.","agent_output":"The crew size was 4."}' \
| grep -i '^payment-required:' | cut -d' ' -f2 | tr -d '\r' | base64 -d | jq '.accepts[0]'Live output on 2026-10-21: "scheme": "exact", "network": "eip155:8453", "amount": "5000" (USDC has 6 decimals, so this is $0.005), "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", "payTo": "0x028Bb9A57B8C27c22f6B39dd00e7eFF1FB60344B", "maxTimeoutSeconds": 300.
The 402 JSON body has x402Version, accepts, and error at the top level, plus signup_url, pricing_url, and pricing_json_url. The PAYMENT-REQUIRED header is the requirements object. To pay, sign the requirements with an x402 client library (https://docs.cdp.coinbase.com/x402/welcome). Over MCP, an unpaid tools/call is HTTP 200 with isError true and the x402 PaymentRequirements in structuredContent. Retry with params._meta['x402/payment'].
FAQ
Which network and asset does Screwtape accept?
USDC on Base (eip155:8453) or Solana (solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp). Base is scheme exact, amount 5000, paid to 0x028Bb9A57B8C27c22f6B39dd00e7eFF1FB60344B. Solana payTo is the wallet owner.
Does Screwtape hold a private key?
No. Payments are verified and settled through a facilitator, and the worker holds no private key.
Can a keyless call retain its result?
No. Retention requires an API key.
Related
Screwtape.ai is an agent validation API, not the arXiv paper also named Screwtape. Facts checked against the live llms.txt, openapi.json, agent card and MCP server card on 2026-10-21.