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Pay per call for an agent validation API with x402 (no API key)

20 October 2026

Pay per call for an agent validation API with x402 (no API key)

Short answer: Yes, you can call Screwtape.ai without an account. Send a valid request with no Authorization header and the API answers 402 Payment Required with x402 requirements: $0.005 USDC per call on Base (eip155:8453). Repeat the request with an x402 PAYMENT-SIGNATURE header and it runs. Payments are verified and settled through a facilitator, and the Screwtape worker holds no private key.

Screwtape.ai is an agent validation API. It is not the arXiv paper that is also called Screwtape.

What x402 is

x402 is an open standard for internet-native payments built on HTTP. When a request arrives without payment, the server responds with HTTP 402 and the client pays and retries (docs, reference code).

Step 1: get the 402 and decode the requirements

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 .

Real output, 2026-10-20:

{
  "x402Version": 2,
  "error": "PAYMENT-SIGNATURE header is required. Send a Bearer API key, or pay per call with x402.",
  "resource": {
    "url": "https://screwtape.ai/v1/judge",
    "description": "One Screwtape.ai judge or consensus call",
    "mimeType": "application/json",
    "serviceName": "Screwtape.ai"
  },
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:8453",
      "amount": "5000",
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "payTo": "0x028Bb9A57B8C27c22f6B39dd00e7eFF1FB60344B",
      "maxTimeoutSeconds": 300,
      "extra": { "name": "USD Coin", "version": "2", "assetTransferMethod": "eip3009" }
    }
  ],
  "extensions": {}
}

amount 5000 in a 6-decimal token is $0.005. The JSON body of the 402 carries the same object under x402. Body validation runs first: an invalid body returns 400 (for example, "answers must include at least two agents.") before any payment is requested.

Step 2: pay and retry

Use an x402 client library from the x402 docs to sign a payment for those requirements, then send it as the PAYMENT-SIGNATURE header on the same request. We haven't run a paid call, so we don't show client code here.

Over MCP

tools/call on https://screwtape.ai/mcp uses the same rule. Without a key or payment, it returns a JSON-RPC error with code -32002 and the same x402 object in data.

Trade-offs vs a free key

Free API keyx402
SignupEmail and name at /signupNone
Cost100 calls per UTC day$0.005 USDC per call
retain: trueAllowed (24 hours)Not allowed. Keyless calls return retained: false with "Retention requires an API key."
What's storedKey hash, prefix, email, name, countersPayment nonce and payer address for up to 48 hours, to reject replays (privacy)

FAQ

Can an AI agent pay for an API call without an API key?

Yes, with x402. Screwtape.ai returns HTTP 402 with machine-readable payment requirements, and the agent retries with a PAYMENT-SIGNATURE header. Each call costs $0.005 USDC on Base (eip155:8453).

What does a Screwtape x402 call cost?

$0.005 USDC per judge or consensus call, which appears as amount 5000 in the requirements, paid to 0x028Bb9A57B8C27c22f6B39dd00e7eFF1FB60344B on Base.

Who verifies the payment?

A facilitator verifies and settles it. On 2026-10-20 the /v1/health endpoint listed https://x402.dexter.cash and https://facilitator.xpay.sh. The worker does not hold a private key.

Is my judged content sent to the payment facilitator?

No. Per Screwtape's privacy page, only the payment payload goes to the facilitator, not the judged content.

Sources