JWT Header Viewer
The JWT Header Viewer isolates the first segment of a JSON Web Token and pretty-prints it as formatted JSON, so you can immediately see the signing algorithm, token type and any key identifiers the token declares.
What This Tool Does
The header is a small JSON object that describes how the token was signed. Typical fields are alg (the signing algorithm, such as HS256 or RS256) and typ (usually "JWT"), plus optional fields like kid (key ID) used by issuers that rotate keys. This tool decodes just that segment and formats it readably.
How to Use
- Paste your JWT into the input field.
- The decoded header appears automatically as formatted JSON.
- Check alg to confirm the expected signing algorithm.
- Copy the header JSON if you need it for debugging or documentation.
Key Features
- Extracts only the header segment of the token
- Pretty-printed JSON output
- Surfaces alg, typ, kid and any custom header fields
- Fast malformed-token error reporting
- Copy, download, sample and reset actions
- Runs entirely client-side
Common Use Cases
- Confirming which algorithm a token was signed with
- Finding the kid to select the right verification key from a JWKS
- Checking for suspicious headers such as alg "none"
- Debugging token generation code
- Teaching the structure of a JWT segment by segment
Why the Algorithm Header Matters
The alg field tells verifiers how the signature was produced. Verifiers should never blindly trust it: pin the expected algorithm server-side, otherwise an attacker may downgrade to alg "none" or exploit confusion between HMAC and RSA keys.
The kid field is a hint that identifies which key signed the token when an issuer publishes several — common with JWKS endpoints and key rotation.
Frequently Asked Questions
What is inside a JWT header?
Typically alg (signing algorithm) and typ (token type). Issuers may add kid (key ID) or other fields such as cty.
What does alg "none" mean?
It marks an unsecured JWT with no signature. Most libraries reject it by default, and you should not accept such tokens in production.
What is the kid field used for?
kid identifies which key signed the token, letting verifiers pick the right public key when an issuer rotates keys or publishes a JWKS.
Can the header be trusted on its own?
No. Like the payload, the header is only Base64URL-encoded. It becomes trustworthy only after the signature is verified.
Does this tool read the payload too?
No — it decodes only the first segment. Use the JWT Decoder or Claims Viewer for the payload.
Is the token sent to a server?
No. Decoding happens locally in your browser.
Related JWT Tools
- JWT Decoder — Decode JWT tokens to view header and payload
- JWT Signature Verifier — Verify JWT signature with secret or public key
- JWT Inspector — Comprehensive analysis of JWT token structure and claims
- JWT Payload Formatter — Pretty print JWT payload with proper indentation