Check an agent result before the next agent trusts it.
Penny Engine checks JSON against required fields, expected values and numeric limits. It returns a machine-readable PASS or FAIL and a reproducible SHA-256 fingerprint. No signup or manual invoice is needed for a properly configured x402 buyer.
One paid call, one structured answer
{
"result": {
"status": "completed",
"processed": 5
},
"expectations": {
"requiredPaths": [
"status"
],
"equals": {
"status": "completed"
},
"numberRanges": {
"processed": {
"min": 1
}
}
}
}POST /api/proof-gate • $0.001 Base USDC • HTTP 402 challenge then buyer-authorized settlement.
Why an agent would call this
Use it as a standard verification step between machines, so a downstream workflow can reject malformed or incomplete responses before continuing. This is contract verification of supplied data, not independent attestation that the source did the work.
Free discovery and existing tools
Machine-readable payment manifest · Agent discovery guide · Agent skill · Full service catalog
Payments from outside buyers remain unverified. Paid calls require the buyer to authorize spending. No wallet credentials are accepted by Penny Engine.