# X402 Idempotency Check

> X402 Idempotency 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).

Compares payment and delivery idempotency keys for an x402 operation to detect duplicates and verify keys match before accepting or paying for agent-delivered results.

## Facts

- Endpoint: POST https://api.delx.ai/api/v1/x402/x402-idempotency-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-idempotency-check-7e06f314
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_dOl8IPmYCx006AFzVxAvF

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-idempotency-check-7e06f314 -d '<json body>'
```

Example prompt: Before accepting this x402 agent delivery, check whether the payment key 'pay_abc123xyz' and delivery key 'del_xyz789abc' are consistent and there are no mismatches or duplicate operation signals.

## When to prefer this

Use this endpoint when operating in the x402 payment protocol ecosystem and needing a deterministic, stateless check that payment and delivery idempotency keys correspond before committing to a payment or accepting an agent result. Prefer this over custom key comparison logic when you need normalized evidence and explicit accept/reject verdicts with mismatch explanations included.

## Known failure modes

- Missing or empty payment_key or delivery_key fields causes validation failure
- Keys exceeding 8192 character limit are rejected
- Mismatched keys return rejection status with detailed mismatch reasons
- Malformed or non-string key values may result in error response

## How this service works

X402 Idempotency Check: X402 Idempotency Check compares payment and delivery idempotency keys for one operation from bounded caller-supplied values without an external provider. Call X402 Idempotency 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 Idempotency Check as versioned deterministic…

## Output

Returns an explicit acceptance or rejection status for the operation, contract-specific validation checks, normalized evidence from the provided keys, and any mismatch reasons detected between the payment and delivery idempotency keys.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "payment_key": {
   "type": "string",
   "maxLength": 8192,
   "description": "Payment Key supplied to X402 Idempotency Check; used only for this bounded calculation and processed in memory without retention."
  },
  "delivery_key": {
   "type": "string",
   "maxLength": 8192,
   "description": "Delivery Key supplied to X402 Idempotency Check; used only for this bounded calculation and processed in memory without retention."
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "result": {
   "matched": true,
   "payment_key": "job-42",
   "delivery_key": "job-42"
  },
  "schema": "delx/util-x402-idempotency-check/v1",
  "status": "pass",
  "evidence": {
   "retained": false,
   "input_sha256": "a9aa562b36a16cb899fdccfd5bcae9beef661bc9955a23c2dab0076b08c36ff5",
   "external_calls": 0
  },
  "operation": "delivery_contract:x402_idempotency_check"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/x402-idempotency-check-7e06f314/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)