JWT Decoder

JWT Decoder splits a JSON Web Token into its three parts — header, payload, and signature — and decodes the header and payload back into readable JSON, entirely on your device.

What is JWT Decoder and Why is it Useful?

A JSON Web Token (JWT) is a compact, base64url-encoded string made of three dot-separated parts: a header (algorithm and token type), a payload (the claims/data), and a signature. Developers frequently need to inspect what's actually inside a token — for example to debug an authentication flow or check claim values — without having access to (or needing) the signing key. This tool decodes the header and payload for you instantly.

Key Features

• Header & Payload Decoding: Paste a token and get both the header and payload decoded into readable, formatted JSON.
• Raw Signature Display: The signature segment is shown as-is for reference, clearly labeled as not verified.
• No Signature Verification: This tool does not and cannot validate the signature — that requires the secret or public key used to sign the token, which should never be entered into a browser tool. If you need to verify a signature, do it server-side with the actual signing key.
• Fully Local, Nothing Transmitted: The token you paste is decoded using only atob/JSON.parse in your browser. It is never sent to any server or third party — important since JWTs often contain user IDs, emails, or session data.
• Format Validation: If the input isn't a valid 3-part JWT or the parts aren't valid base64url JSON, the tool shows a clear error instead of partial/garbled output.

How to Use JWT Decoder

1. Paste your JWT (the full header.payload.signature string) into the input field.
2. Click Decode Token.
3. Read the decoded header and payload as formatted JSON.
4. The raw signature is shown below for reference — remember it is not verified by this tool.

Why Choose Our Client-Side Tool?

Many online JWT tools silently transmit the token you paste to their backend, which is risky since JWT payloads often contain user identifiers, roles, or session details. Because this decoder runs entirely in your browser, your token never leaves your device.

Frequently Asked Questions (FAQ)

Does this tool verify the JWT signature?

No. This tool only decodes the header and payload so you can read their contents. It does not check whether the signature is valid, since that requires the secret or public key, which should not be used in a browser-based tool. To verify a signature, use a server-side library with the actual signing key.

Is my token sent to a server?

No. Decoding happens entirely in your browser using JavaScript's built-in base64 and JSON utilities. The token never leaves your device.

Why did I get a decode error?

This usually means the input isn't a valid JWT — it must have exactly three dot-separated parts, and the header and payload parts must be valid base64url-encoded JSON.

Can I trust the claims in a decoded token without verifying it?

No — anyone can craft a token with arbitrary header/payload content. Decoding only tells you what a token claims; only signature verification (done server-side, with the correct key) tells you whether it can be trusted.