# X402 Payment Nonce Check

> X402 Payment Nonce Check is a paid API for AI agents from api.delx.ai, paid per call via x402, $0.001/call, status unknown (last checked 2026-10-20).

Validates a payment nonce for presence, minimum length, and caller-known reuse against a bounded list of previously seen nonces, returning acceptance or rejection status with mismatch reasons.

## Facts

- Endpoint: POST https://api.delx.ai/api/v1/x402/x402-payment-nonce-check?utm_source=zero.xyz
- Price: $0.001/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-20
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/x402-payment-nonce-check-5ea00c01
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_a7WFZRMib0dGH67Avo0Lj

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 x402-payment-nonce-check-5ea00c01 -d '<json body>'
```

Example prompt: Before accepting this x402 payment, check if the nonce 'abc123xyz' is valid — it must be at least 16 characters long and confirm it hasn't appeared in this list of previously seen nonces: ['def456', 'ghi789'].

## When to prefer this

Use this endpoint when you need a lightweight, stateless, in-memory nonce validation step before accepting or paying for an x402 payment or agent delivery — especially when you maintain your own seen-nonces list on the caller side and want deterministic, contract-specific rejection reasons without an external database or provider dependency. Prefer it over custom nonce logic when you need normalized evidence and versioned results for audit trails.

## Known failure modes

- Nonce absent or empty string — rejected with presence failure reason
- Nonce shorter than minimum_length — rejected with length mismatch reason
- Nonce found in seen_nonces list — rejected with reuse/replay detection reason
- seen_nonces array exceeds 256 items — request may be rejected or truncated
- nonce or seen_nonces values exceed 8192 character limit — validation error

## How this service works

X402 Payment Nonce Check: X402 Payment Nonce Check checks a payment nonce for presence, length, and caller-known reuse from bounded caller-supplied values without an external provider. Call X402 Payment Nonce Check before paying for or accepting an x402 or agent delivery assembled from caller-supplied challenge and result metadata. Returns contract-specific checks, normalized evidence, mismatch reasons, and an explicit acceptance or rejection status for X402 Payment Nonce Check as versioned det…

## Output

Returns a structured verdict including an explicit acceptance or rejection status, contract-specific check results (presence, length conformance, reuse detection), normalized evidence of the checks performed, and mismatch reasons explaining any failures — all deterministically computed in memory without external calls or data retention.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "nonce": {
   "type": "string",
   "maxLength": 8192,
   "description": "Nonce supplied to X402 Payment Nonce Check; used only for this bounded calculation and processed in memory without retention."
  },
  "seen_nonces": {
   "type": "array",
   "items": {
    "type": "string",
    "maxLength": 8192
   },
   "maxItems": 256,
   "description": "Seen Nonces supplied to X402 Payment Nonce Check; used only for this bounded calculation and processed in memory without retention."
  },
  "minimum_length": {
   "type": "integer",
   "description": "Minimum Length supplied to X402 Payment Nonce Check; used only for this bounded calculation and processed in memory without retention."
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "result": {
   "reused": false,
   "accepted": true,
   "nonce_length": 18
  },
  "schema": "delx/util-x402-payment-nonce-check/v1",
  "status": "pass",
  "evidence": {
   "retained": false,
   "input_sha256": "5b10e7096699f6b6a1dcbc3414bdd28a9a8deeb87ccf9e505917b2a855abe5e2",
   "external_calls": 0
  },
  "operation": "delivery_contract:x402_payment_nonce_check"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/x402-payment-nonce-check-5ea00c01/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.delx.ai](https://www.zero.xyz/host/api.delx.ai/llms.txt)