JWT Decoder API

Decode and inspect JWT tokens without verification. Extracts header, payload, claims, expiry, and signature algorithm. Pay-per-call via x402 (USDC on Base L2) -- no API key, no signup, no rate-limit wall.

Part of the klymax402 marketplace -- 100 x402 micropayment APIs for AI agents, one wallet, USDC on Base.

Quickstart -- MCP

Add to your MCP client config (Claude Desktop, Cursor, ElizaOS, etc.):

{
  "mcpServers": {
    "jwt-decoder": {
      "url": "https://jwt-decoder.api.klymax402.com/mcp"
    }
  }
}

Quickstart -- HTTP (x402)

curl -X POST "https://jwt-decoder.api.klymax402.com/api/decode" \
  -H "Content-Type: application/json" \
  -d '{"token":"..."}'
# -> 402 Payment Required, with an x402 payment challenge in the response body

Any x402-aware client (@x402/fetch, x402-agent-tools, ATXP) handles the 402 -> sign -> retry cycle automatically.

Tools

security_decode_jwt

Use this when you need to decode and inspect a JWT token without verifying its signature. Returns the full header, payload, and expiration status.

Parameters

Example response:

{"header":{"alg":"RS256","typ":"JWT"},"payload":{"sub":"user123","exp":1720000000},"issuedAt":"2025-01-01T00:00:00Z","expiresAt":"2025-07-03T00:00:00Z","isExpired":false}

When to use: debugging authentication issues, inspecting token claims before API calls, or verifying token expiry. Use this BEFORE making authenticated requests to check if a token needs refreshing.

Not for: hashing data (use crypto_generate_hash), base64 encoding/decoding (use utility_encode_base64), password analysis (use security_check_password).

Example agent prompts

• "Decode and inspect a JWT token without verifying its signature"

Payment

• Protocol: x402 -- HTTP-native pay-per-call, no signup, no API key
• Network: Base L2 (eip155:8453)
• Asset: USDC
• Facilitator: Coinbase CDP (primary), PayAI (fallback)
• Also reachable via ATXP (OAuth-wrapped x402, RFC 9728 protected-resource metadata)

Part of klymax402

100 x402 micropayment APIs for AI agents -- one wallet, USDC on Base, zero signup.

• Catalog: https://klymax402.com/llms.txt
• Full API reference: https://klymax402.com/llms-full.txt
• Live stats: https://klymax402.com/stats

License

MIT