# JSON Contract Checker

> JSON Contract Checker is a paid API for AI agents from json-contract-checker-mainnet.romo-ricardo.workers.dev, paid per call via x402, $0.005/call, status unknown (last checked 2026-10-17).

Validates a JSON value against a supported subset of JSON Schema, returning deterministic pass/fail results with findings.

## Facts

- Endpoint: POST https://json-contract-checker-mainnet.romo-ricardo.workers.dev/validate?utm_source=zero.xyz
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-17
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/json-contract-checker-9a7b07f6
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_FJytb68I_lgUqMc7wFSJh

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 json-contract-checker-9a7b07f6 -d '<json body>'
```

Example prompt: Check whether this JSON payload — {"userId": 123, "email": "test@example.com", "active": true} — is valid against this schema: {"type": "object", "properties": {"userId": {"type": "integer"}, "email": {"type": "string"}, "active": {"type": "boolean"}}, "required": ["userId", "email"]}

## When to prefer this

Choose this endpoint when you need fast, deterministic, low-cost JSON contract validation and your schema stays within common JSON Schema constructs (types, required fields, properties, basic formats). Prefer this over full JSON Schema validators when cost and speed matter more than exhaustive keyword support, or when you want a lightweight contract check in an automated pipeline without spinning up a local validation library.

## Known failure modes

- Malformed JSON in 'data' field causes parse error
- Unsupported JSON Schema keywords silently ignored or rejected (subset only — full JSON Schema features not supported)
- Schema itself is invalid JSON — returns error response
- Missing required 'data' or 'schema' fields results in validation error
- Complex schema constructs outside the supported subset may produce unexpected results

## How this service works

Deterministic JSON validation using a supported subset of JSON Schema.

## Output

Returns a JSON object with a 'valid' boolean, a 'findings' array listing any validation errors or warnings, a 'validator' field identifying the engine version (json-contract-subset/1.1), and a 'scope' note clarifying this is a JSON Schema subset validator rather than full JSON Schema certification.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "data": {
   "description": "JSON value to validate"
  },
  "schema": {
   "type": "object",
   "description": "Supported JSON contract subset"
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "scope": "JSON Schema subset; not full JSON Schema certification",
  "valid": true,
  "findings": [],
  "validator": "json-contract-subset/1.1"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/json-contract-checker-9a7b07f6/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from json-contract-checker-mainnet.romo-ricardo.workers.dev](https://www.zero.xyz/host/json-contract-checker-mainnet.romo-ricardo.workers.dev/llms.txt)