JWT Header Viewer
Decode JWT header online in your browser. View alg, typ, and other header fields locally. Free, no upload, no account required.
Runs locally in your browser. Your input is not uploaded to DevToolMate servers.
Not signature verification
Decoding a JWT header does not prove the token is authentic. Treat alg and other fields as informational only.
Developer Quick Tips
- Paste a JWT and decode its header segment locally in your browser using Base64URL decoding. No upload, no external API calls.
- Inspect alg and typ fields in the JWT header to verify the algorithm and token type before debugging authentication issues.
- Compare header metadata across multiple tokens to spot algorithm mismatches or unexpected typ values in your auth flow.
- Remember: decoding the JWT header is informational only and does not verify the token signature or validate issuer trust.
- Your token input stays on your device for the current session; JWT Header Viewer runs entirely in your browser.
Frequently asked questions
How do I decode a JWT header online?
Paste your JWT into JWT Header Viewer. The tool decodes the first segment (header) from Base64URL to JSON in your browser. No account or upload required.
What does a JWT header contain?
A JWT header typically contains alg (the signing algorithm, e.g., HS256, RS256) and typ (usually 'JWT'). JWT Header Viewer extracts and displays these fields as JSON so you can inspect token metadata.
Why should I decode and inspect a JWT header?
Inspecting the header helps you verify the token uses the expected algorithm, spot unexpected typ values, and understand token structure before validating claims or debugging auth issues.
Does JWT Header Viewer verify JWT signatures?
No. This tool only decodes and displays the header JSON. It does not verify signatures, validate issuer trust, or confirm token authenticity. Decoding is informational only.
What is the difference between JWT header and payload?
The JWT header (first segment) contains metadata like alg and typ. The payload (second segment) contains claims and user data. JWT Header Viewer shows only the header; use a full JWT decoder to view payload.
Is my JWT token uploaded to DevToolMate servers?
No. JWT Header Viewer runs entirely in your browser. Your token input stays on your device for the current session and is never sent to any server.
Do I need an account to use JWT Header Viewer?
No. JWT Header Viewer is completely free and requires no login or account. Processing happens locally in your browser.
Related tools
- JWT Decoder: Decode JWT online and inspect bearer tokens locally in your browser. View header, payload, and OAuth 2.0 claims instantly. Free, no upload, no account.
- JWT Payload Viewer: Decode JWT payload online free. View claims like sub, exp, iat in your browser. No upload, no account, no signature verification.