# JWT Decoder & Validator MCP for AI Agents MCP

> JWT Decoder & Validator MCP. It lets you inspect, decode, and verify JSON Web Tokens instantly. It handles signature checks for HS256, RS256, and ES256, and checks if tokens are expired or active. Perfect for debugging auth flows without manual decoding.

## Overview
- **Category:** security
- **Price:** Free
- **Endpoint:** https://edge.vinkius.com/vk_preview_E47Wl1pMoDCfXmcNGBlWo7ceQ1TGjYjWMSJWkgQy/mcp
- **Tags:** jwt, security, cryptography, auth, token-validation

## Description

When you're debugging authentication, the last thing you want to do is manually paste a long string into a web tool and hope it doesn't leak data to a third party. This MCP lets you stay inside your workspace while your agent handles the heavy lifting of inspecting tokens. You can ask your agent to see what's actually inside a JWT header or payload, check if a signature is legit, or see if a token is still valid based on its timestamp. It takes the guesswork out of security testing. Because it's hosted on Vinkius, you can plug it into your existing setup and let your agent act as a security analyst. It’s about getting immediate answers on whether a token is structurally sound or cryptographically valid without leaving your terminal or IDE.

## Tools

### verify_jwt_signature
Validates that the signature matches the header and payload using your provided key. It supports HS256, RS256, and ES256 algorithms.

### decode_jwt_components
Extracts the readable JSON content from the JWT header and payload. Use this to see what's inside a token without manual decoding.

### parse_jwt_string
Checks if a string actually follows the three-part JWT format. It's the first step to see if a token is even valid.

### validate_jwt_temporal_claims
Checks standard time-based claims like 'exp' and 'nbf' against your system time. It tells you if a token is currently active.

## Prompt Examples

**Prompt:** 
```
Is this token valid? eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
```

**Response:** 
```
I've analyzed the token for you:

*   **Structure**: Valid (3 segments)
*   **Payload**: Contains user info for "John Doe"
*   **Status**: The token is structurally sound, but I'll need your public key to verify the signature.
```

**Prompt:** 
```
Decode the payload of this JWT: eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ
```

**Response:** 
```
Here is the decoded payload from that token:

```json
{
  "sub": "1234567890",
  "name": "John Doe",
  "iat": 1516239022
}
```
```

**Prompt:** 
```
Check if this payload is still valid: {"exp": 1600000000}
```

**Response:** 
```
I checked that timestamp against the current time.

**Status**: ❌ **Expired**

The 'exp' claim has already passed, so this token is no longer valid for authentication.
```

## Capabilities

### Decode JWT headers and payloads
Extract readable JSON data from tokens.

### Verify cryptographic signatures
Check if a token was signed correctly using specific keys.

### Validate expiration dates
Ensure a token isn't expired or active too early.

### Check structural integrity
Confirm a string follows the three-part JWT format.

## Use Cases

### Debugging Login Failures
A backend dev gets a 401 Unauthorized and asks the agent to check if the token is expired or if the signature is actually valid.

### Security Auditing
A security engineer needs to see what claims are inside a production token to ensure no sensitive data is being leaked in the payload.

### Key Rotation Testing
A dev is switching from HS256 to RS256 and needs to verify the new signature is valid using the `verify_jwt_signature` tool.

### QA Edge Cases
A tester wants to confirm that a token with a future 'nbf' claim is correctly rejected by the system.

## Benefits

- Stay in your IDE: You don't have to switch tabs to a web-based decoder. Use `decode_jwt_components` to see payloads instantly.
- Secure debugging: Keep your tokens out of third-party websites. This MCP processes everything locally through your agent.
- Faster auth troubleshooting: Quickly identify if a failure is due to an expired token using `validate_jwt_temporal_claims`.
- Signature verification: Confirm your keys are working correctly with `verify_jwt_signature` for HS256, RS256, and ES256.
- Structural checks: Use `parse_jwt_string` to immediately catch malformed tokens before wasting time on deeper analysis.

## How It Works

The bottom line is you get instant, secure validation of authentication tokens without leaving your dev environment.

1. Provide a JWT string to your AI client.
2. The agent uses the MCP to inspect the specific parts of the token you need.
3. You get a clear breakdown of the JSON content or a simple pass/fail on the signature.

## Frequently Asked Questions

**How can I use the JWT Decoder & Validator MCP to debug authentication?**
You can ask your agent to decode a token's payload or check its signature. It provides a clear breakdown of the JSON content so you can see exactly what's inside without manual effort.

**Can the JWT Decoder & Validator MCP check if a token is expired?**
Yes, it checks standard time-based claims like 'exp' and 'nbf' against your current system time to tell you if a token is still active.

**Is it safe to use the JWT Decoder & Validator MCP with production tokens?**
Yes, it's safer than using web-based decoders because the processing happens through your agent in your own environment rather than on a third-party website.

**What signature algorithms does the JWT Decoder & Validator MCP support?**
It supports the most common cryptographic algorithms, including HS256, RS256, and ES256, making it versatile for different security setups.

**Can the JWT Decoder & Validator MCP decode headers and payloads?**
It can extract and display the readable JSON content from both the header and the payload of any standard JWT.

**How does the JWT Decoder & Validator MCP help with security audits?**
It allows you to quickly inspect claims and verify signatures, helping you ensure that tokens aren't leaking sensitive data and that your keys are correctly configured.

**What algorithms are supported for signature verification?**
The `verify_jwt_signature` tool supports HS256, RS256, and ES256 algorithms.

**Can I check if a token has expired?**
Yes, by using the `validate_jwt_temporal_claims` tool with the decoded payload and the current timestamp.

**Does this server verify the authenticity of the token?**
The `verify_jwt_signature` tool can validate the signature if you provide the appropriate secret or public key.