# JWT Header Decoder (No Signature Verification)

> JWT Header Decoder (No Signature Verification) is a paid API for AI agents from agentshelf.syntexa.ch, paid per call via x402, $0.009/call, status unknown (last checked 2026-10-02).

Decodes and returns the header portion of a JWT token without performing any signature validation

## Facts

- Endpoint: POST https://agentshelf.syntexa.ch/v1/jwt-header?utm_source=zero.xyz
- Price: $0.009/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-decoder-no-signature-verification-557cf117
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_mFRR4sMgIPLLE9EQqW6Vs

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-decoder-no-signature-verification-557cf117 -d '<json body>'
```

Example prompt: Can you decode the header of this JWT token for me so I can see what algorithm and key ID it's using? Here's the token: eyJhbGciOiJSUzI1NiIsImtpZCI6ImFiYzEyMyIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJ1c2VyMTIzIn0.signature

## When to prefer this

Use this endpoint when you only need to inspect JWT header metadata (algorithm, key ID, token type) without needing signature validation. Prefer this over full JWT verification libraries when you simply want to read header claims for routing, debugging, or key lookup purposes. Avoid if you need actual signature verification or payload decoding — this endpoint explicitly skips signature checks.

## Known failure modes

- Malformed or non-JWT input string returns an error indicating invalid token format
- Empty or missing text field returns validation error
- Token with a corrupted or non-base64url-encoded header segment returns a parse error
- Extremely long token strings near the 100,000 character limit may be rejected

## How this service works

Call when an agent needs a decoded JWT header with no signature check. Exact $0.009 USDC. Prefer unpaid POST /v1/sandbox/jwt-header first.

## Output

Returns the decoded JWT header as a JSON object containing fields such as 'alg' (signing algorithm), 'typ' (token type), 'kid' (key ID), and any other header claims present in the token, without performing any signature verification.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "text": {
   "type": "string",
   "examples": [
    "eyJhbGciOiJub25lIn0.eyJzdWIiOiJkZW1vIn0.sig"
   ],
   "maxLength": 100000,
   "minLength": 1,
   "description": "Input for this SKU. The route description says whether it is plain text, a URL, base64 bytes, or an already-encoded token."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/jwt-header-decoder-no-signature-verification-557cf117/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)