# Agent Endpoint Preflight (x402 Interoperability Checker)

> Agent Endpoint Preflight (x402 Interoperability Checker) is a paid API for AI agents from api.x402preflight.com, paid per call via x402, $0.05/call, status unknown (last checked 2026-09-15).

Checks a public endpoint's x402 payment challenge, network/asset/amount terms, origin binding, canonical V2 structure, and common integration failures without actually making a payment.

## Facts

- Endpoint: POST https://api.x402preflight.com/v1/preflight
- Price: $0.05/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/agent-endpoint-preflight-x402-interoperability-checker-f6a7b6a8
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_HUFzzq80NUg_C-KausJVJ

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability agent-endpoint-preflight-x402-interoperability-checker-f6a7b6a8 -d '<json body>'
```

Example prompt: Can you preflight check https://api.myservice.com/premium-data to see if its x402 payment setup is valid — I want to know the verdict, any reason codes, and whether the payment terms look canonical before I try to integrate?

## When to prefer this

Use this endpoint when you need a read-only, non-paying diagnostic of any public x402 endpoint — especially before integrating an agent payment flow, debugging 402 negotiation failures, auditing canonical V2 structure compliance, or verifying payment terms (asset, network, amount) without committing real USDC. Prefer this over manually parsing 402 headers or attempting a live payment when the correctness of the remote endpoint is uncertain.

## Known failure modes

- TARGET_UNREACHABLE verdict when the target URL is offline or DNS fails
- TIMEOUT verdict when the target endpoint does not respond within the allowed window
- NOT_X402 verdict when the target does not return an HTTP 402 response at all
- REJECT verdict when the endpoint fails critical x402 structural requirements
- Payment of $0.05 USDC is consumed even if the result is WARN, REJECT, NOT_X402, TARGET_UNREACHABLE, or TIMEOUT — PASS is not guaranteed
- Malformed or unexpected paymentTerms fields if the target uses a non-standard x402 implementation

## How this service works

Read-only x402 endpoint interoperability preflight. Checks a public endpoint's payment challenge, network/asset/amount terms, origin binding, canonical V2 structure, and common integration failures without making a payment. Payment purchases one bounded assessment attempt. PASS DOES NOT MEAN SAFE. WARN, REJECT, NOT_X402, TARGET_UNREACHABLE, and TIMEOUT may be valid paid results.

## Output

A JSON object containing a verdict string (PASS, WARN, REJECT, NOT_X402, TARGET_UNREACHABLE, or TIMEOUT), a canonical boolean, a reachable boolean, the observed HTTP status, an observedAt timestamp, an array of reasonCodes, structured paymentTerms (asset contract address, payTo address, amount, scheme, network, maxTimeoutSeconds), a challengeHash, originBinding validity, a redirectChain array, schemaFindings array, protocolVersion integer, and a securityFindings array.

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "verdict": "WARN",
  "canonical": false,
  "reachable": true,
  "httpStatus": 402,
  "observedAt": "2026-01-01T00:00:00.000Z",
  "reasonCodes": [
   "NONCANONICAL_RESOURCE"
  ],
  "paymentTerms": {
   "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
   "payTo": "0x0000000000000000000000000000000000000000",
   "amount": "50000",
   "scheme": "exact",
   "network": "eip155:8453",
   "maxTimeoutSeconds": 300
  },
  "challengeHash": "sha256:example",
  "originBinding": {
   "valid": true
  },
  "redirectChain": [],
  "schemaFindings": [
   "NONCANONICAL_RESOURCE"
  ],
  "protocolVersion": 2,
  "securityFindings": []
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/agent-endpoint-preflight-x402-interoperability-checker-f6a7b6a8/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.x402preflight.com](https://www.zero.xyz/host/api.x402preflight.com/llms.txt)