# X402 Scheme Select

> X402 Scheme Select 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).

Selects the first buyer-supported payment scheme from a list of x402 challenge scheme options, returning deterministic acceptance or rejection status.

## Facts

- Endpoint: POST https://api.delx.ai/api/v1/x402/x402-scheme-select?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-scheme-select-f84680d6
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_xtZy3ipUMHKqqFjqiMvSG

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-scheme-select-f84680d6 -d '<json body>'
```

Example prompt: From this list of x402 challenge schemes — ['exact/v1','evm/v1','base/v1'] — pick the first one that matches my supported schemes ['evm/v1','base/v1'] and tell me whether I should accept or reject the payment.

## When to prefer this

Use this endpoint when implementing an x402 payment client or agent that needs to deterministically pick the correct payment scheme before authorizing or accepting a payment. It is the right choice when you have a list of schemes from a challenge response and a list of what your buyer supports, and you need a normalized, auditable selection decision without building the logic yourself. Prefer it over custom client-side logic when you need explicit mismatch reasons and a versioned JSON decision record for audit or debugging.

## Known failure modes

- No matching scheme found between offered and supported lists — returns rejection status with mismatch reasons
- Empty schemes or supported_schemes arrays — may return rejection or empty selection
- Malformed scheme strings or oversized arrays exceeding 256-item limit — may return validation error
- Payment failure (insufficient USDC balance for $0.001 fee) — request not processed

## How this service works

X402 Scheme Select: X402 Scheme Select selects the first buyer-supported payment scheme from challenge options from bounded caller-supplied values without an external provider. Call X402 Scheme Select 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 Scheme Select as versioned deterministic JSON. Pri…

## Output

Returns a deterministic JSON object indicating which scheme was selected (the first buyer-supported scheme found in the challenge list), an explicit acceptance or rejection status, normalized evidence, mismatch reasons if no compatible scheme was found, and contract-specific check results — all versioned and computed in-memory without data retention.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "schemes": {
   "type": "array",
   "items": {
    "type": "string",
    "maxLength": 8192
   },
   "maxItems": 256,
   "description": "Schemes supplied to X402 Scheme Select; used only for this bounded calculation and processed in memory without retention."
  },
  "supported_schemes": {
   "type": "array",
   "items": {
    "type": "string",
    "maxLength": 8192
   },
   "maxItems": 256,
   "description": "Supported Schemes supplied to X402 Scheme Select; used only for this bounded calculation and processed in memory without retention."
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "result": {
   "selected": "exact",
   "supported": [
    "exact"
   ]
  },
  "schema": "delx/util-x402-scheme-select/v1",
  "status": "pass",
  "evidence": {
   "retained": false,
   "input_sha256": "a045739927a7259d237f0c405b73130cd2fb17caabdca6ce41cffff6ac4ff2ed",
   "external_calls": 0
  },
  "operation": "delivery_contract:x402_scheme_select"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/x402-scheme-select-f84680d6/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)