# X402 Payment Replay Key

> X402 Payment Replay Key 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).

Derives a stable, deterministic replay-detection key from x402 payment metadata (nonce, payer, resource) to prevent duplicate payment attacks.

## Facts

- Endpoint: POST https://api.delx.ai/api/v1/x402/x402-payment-replay-key?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-replay-key-66de6d92
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_22lLDiiql5w2carW1rcRy

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-replay-key-66de6d92 -d '<json body>'
```

Example prompt: Before I accept this x402 payment, derive a replay-detection key using nonce '0xabc123', payer '0xDeadBeef...', and resource '/api/v1/premium-data' so I can check for duplicate submissions.

## When to prefer this

Use this endpoint when implementing x402 payment acceptance logic and you need a fast, stateless, deterministic replay-detection key computed purely from payment metadata without relying on an external database or provider. Prefer this over building custom hashing logic when you need versioned, contract-aware replay detection that also returns normalized evidence and explicit accept/reject signals.

## Known failure modes

- Missing required fields (nonce, payer, or resource) may return a validation error or incomplete key
- Input values exceeding 8192 character limit per field will be rejected
- Malformed payer address format may produce a mismatch reason in the response
- Network timeout on the POST request returns no key and must be retried

## How this service works

X402 Payment Replay Key: X402 Payment Replay Key derives a stable non-secret replay-detection key from payment metadata from bounded caller-supplied values without an external provider. Call X402 Payment Replay Key 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 Replay Key as versioned dete…

## Output

Returns a stable, deterministic replay-detection key derived from the supplied nonce, payer, and resource fields, along with contract-specific validation checks, normalized evidence, any mismatch reasons, and an explicit acceptance or rejection status.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "nonce": {
   "type": "string",
   "maxLength": 8192,
   "description": "Nonce supplied to X402 Payment Replay Key; used only for this bounded calculation and processed in memory without retention."
  },
  "payer": {
   "type": "string",
   "maxLength": 8192,
   "description": "Payer supplied to X402 Payment Replay Key; used only for this bounded calculation and processed in memory without retention."
  },
  "resource": {
   "type": "string",
   "maxLength": 8192,
   "description": "Resource supplied to X402 Payment Replay Key; used only for this bounded calculation and processed in memory without retention."
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "result": {
   "key": "e3cc8d847b2f88337939a2fab5ac3409aebfe27f223855a64805eba4ea53a947",
   "fields": [
    "nonce",
    "payer",
    "resource"
   ],
   "secret": false
  },
  "schema": "delx/util-x402-payment-replay-key/v1",
  "status": "pass",
  "evidence": {
   "retained": false,
   "input_sha256": "07fbf5ea1e08edc254770787f1273ba864ab5603eb570e42969f774e980e8a0e",
   "external_calls": 0
  },
  "operation": "delivery_contract:x402_payment_replay_key"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/x402-payment-replay-key-66de6d92/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)