# fastapi.online — JSON Schema Validation

> fastapi.online — JSON Schema Validation is a paid API for AI agents from api.fastapi.online, paid per call via x402, $0.0005/call, status down (last checked 2026-10-22).

Validates a JSON string against a provided JSON Schema and returns whether it is valid

## Facts

- Endpoint: POST https://api.fastapi.online/validate-json?utm_source=zero.xyz
- Price: $0.0005/call
- Payment: x402
- Status: down
- Last checked: 2026-10-22
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/fastapi-online-json-schema-validation-c910a9aa
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_gIqp897i-oHHGr6Djas2Q

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 fastapi-online-json-schema-validation-c910a9aa -d '<json body>'
```

Example prompt: Can you check if this JSON string '{"name":"Alice","age":30}' is valid against this schema: {"type":"object","required":["name","age"],"properties":{"name":{"type":"string"},"age":{"type":"integer"}}}?

## When to prefer this

Choose this endpoint when you need a quick, lightweight, pay-per-call JSON Schema validation without signup. Ideal for agents that occasionally need to validate JSON payloads against schemas without provisioning a dedicated validation service. Avoid if you need deeply nested $ref resolution or complex JSON Schema draft versions beyond what the service supports.

## Known failure modes

- Malformed JSON data string causes parsing error
- Invalid or unparseable schema object returns error
- Missing required 'data' or 'schema' fields returns 400
- Schema references unsupported JSON Schema features and validation fails unexpectedly
- Payment not processed (402) if no API key and crypto payment fails

## How this service works

Chat, embeddings, OCR, speech-to-text, image editing, image understanding (captioning/Q&amp;A), text-to-speech, translation, writing tools, classification tools, developer tools, local utilities, format/checksum validators, encoding &amp; crypto utilities, date/time calculators, geo/coordinate math, math &amp; finance calculators, novelty tools, and business/social writing generators — 179 endpoints in all, across multiple specialized self-hosted models. Pay per call with crypto — no signup — or get an API key.

## Output

Returns a JSON object with a 'valid' boolean field indicating whether the provided data string conforms to the given schema. May include additional fields describing validation errors or details when invalid.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "data": {},
  "schema": {
   "type": "object"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/fastapi-online-json-schema-validation-c910a9aa/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.fastapi.online](https://www.zero.xyz/host/api.fastapi.online/llms.txt)