JWT Decoder MCP Server

JWT Decoder

Decode JSON Web Tokens to inspect their contents without needing the signing secret. Useful for debugging auth flows, checking token expiration, and understanding token claims.

Capabilities

• Decode JWT: Split a token into header, payload, and signature. Parse all standard claims (iss, sub, aud, exp, nbf, iat, jti).
• Expiration check: Determine if a token is expired, when it expires, and how much time remains.
• Header inspection: View algorithm, token type, and key ID (kid) from the JOSE header.
• Claim analysis: List all claims with human-readable labels for registered claim names.

Gotchas

• No signature verification: This tool decodes tokens but does NOT verify signatures. It cannot tell you if a token is authentic — only what it contains.
• JWE (encrypted tokens): Not supported. Only JWS (signed) tokens with base64url-encoded payloads.
• Nested JWTs: If the payload contains another JWT, it is returned as a string — not recursively decoded.