# JWT Header Shape Validator

> JWT Header Shape Validator is a paid API for AI agents from agentshelf.syntexa.ch, paid per call via x402, $0.005/call, status unknown (last checked 2026-10-02).

Checks whether a given string has a valid JWT header shape (three-part base64url structure) without performing any signature verification.

## Facts

- Endpoint: POST https://agentshelf.syntexa.ch/v1/check-jwt-shape?utm_source=zero.xyz
- Price: $0.005/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-02
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/jwt-header-shape-validator-36ceeac6
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_ccqLRj5srkLgxB8YBI0rc

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 jwt-header-shape-validator-36ceeac6 -d '<json body>'
```

Example prompt: Can you check if this string looks like a valid JWT token based on its header shape — just the structure, no signature verification needed: 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c'?

## When to prefer this

Use this endpoint when you need a fast, lightweight structural check to determine if a string is shaped like a JWT — without the overhead or complexity of full signature verification. Ideal for pre-filtering, input validation pipelines, or debugging token format issues. Prefer the free sandbox endpoint POST /v1/sandbox/check-jwt-shape if available to avoid the $0.005 USDC cost.

## Known failure modes

- Empty or missing value field returns validation error
- Value exceeding 20,000 character limit is rejected
- Malformed request body returns 400-level error
- Payment of $0.005 USDC not provided results in 402 Payment Required

## How this service works

Call when an agent needs whether a token has a JWT header shape. No signature check. Exact $0.005 USDC. Prefer unpaid POST /v1/sandbox/check-jwt-shape first.

## Output

Returns a boolean or structured result indicating whether the provided string conforms to a valid JWT header shape — specifically whether it follows the three-part dot-delimited base64url encoding pattern expected of a JWT. No cryptographic signature validation is performed.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "data": {
   "description": "JSON value for schema-check SKUs. Omit it on string checks such as email or IBAN."
  },
  "value": {
   "type": "string",
   "examples": [
    "not-a-jwt"
   ],
   "maxLength": 20000,
   "minLength": 1,
   "description": "Value to check. The route description names the format: email, phone, IBAN, postal code, VAT, national id, or similar."
  },
  "schema": {
   "description": "JSON Schema document for schema-check SKUs. Omit it on string checks."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/jwt-header-shape-validator-36ceeac6/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from agentshelf.syntexa.ch](https://www.zero.xyz/host/agentshelf.syntexa.ch/llms.txt)