What is the difference between 401 and 403?
401 means the server does not know who you are or your credentials failed, so authenticating may help. 403 means it knows and still refuses, so new credentials for the same user will not help.
HTTP 401 UNAUTHORIZED
Authentication is missing or failed. Must send WWW-Authenticate. Really means "unauthenticated". 401 belongs to the 4xx client error class and is defined in RFC 9110.
Loading the workbench.
About HTTP 401
What Does HTTP 401 Unauthorized Mean?
Authentication is missing or failed. Must send WWW-Authenticate. Really means "unauthenticated". The code is registered with IANA and defined in RFC 9110.
Client error: the request has a problem, so it cannot be fulfilled as sent. Every code from 400 to 499 shares that general meaning, so a client that does not recognise 401 must treat it like 400.
When Do You See a 401 Response?
401 Unauthorized really means unauthenticated: the request lacked valid credentials. Causes include a missing or expired token, a wrong API key, a malformed Authorization header, or a session cookie that was not sent. In browsers, a 401 with Basic authentication triggers the built-in login dialog. It differs from 403, where the server knows who you are but still refuses, so logging in again will not help there.
How Do You Fix a 401?
Servers must include a WWW-Authenticate header naming the scheme, such as Bearer realm="api", and may add an error description for expired tokens. Clients should refresh the token or log in again, then retry once. Check that the header is spelled correctly and that a proxy or CORS preflight is not stripping it. The 2http Header decoder shows the scheme and a JWT's expiry with the token masked.
What Does a 401 Response Look Like?
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="api", error="invalid_token"
Content-Type: application/json401 is not cacheable by default: a browser or CDN stores it only when Cache-Control or Expires explicitly allows it.
To inspect a real one without sending it anywhere:
- Copy the response headers from your browser Network tab, or from
curl -i. - Paste them into the Header decoder to see every header explained.
- Check caching in the Cache lane.
Which Status Codes Are Related to 401?
- 403 Forbidden: The server knows who you are and refuses anyway. Logging in again will not help.
- 407 Proxy Authentication Required: Like 401, but the proxy needs credentials.
- 511 Network Authentication Required: A captive portal, such as hotel Wi-Fi, needs you to log in first.
Browse every code on the HTTP status codes page.
What Other Lanes Are in the Reference Section?
- HTTP status codes: Every IANA-registered status code, with a page per code.
- MIME types: File extensions to Content-Type, searchable both ways, plus a Content-Type parser.
- HTTP methods: GET, POST, PUT, PATCH, DELETE and the rest: safe, idempotent, cacheable.
Elsewhere in 2http: Header decoder, Cookie decoder, Cache-Control decoder, CORS checker, User-agent parser, and all eleven lanes on the 2http home page. Part of the 2suite.io privacy-first ecosystem.
Frequently Asked Questions
Is WWW-Authenticate required on a 401?
Yes. RFC 9110 requires at least one challenge in WWW-Authenticate on every 401, so the client knows which authentication scheme to use.
Is HTTP 401 an error?
Yes. 401 is a client error response: the problem is in the request, so repeating it unchanged will usually fail again.
Is a 401 response cacheable by default?
No. Caches store a 401 only when Cache-Control or Expires explicitly allows it.
How Can You Verify That Nothing Leaves This Browser?
You do not have to take our word that your headers stay here. Developer tools show every request a page makes, so you can watch this page work and confirm that nothing is sent. The check takes under a minute.
- Open your browser developer tools with
F12, orCmd+Option+Ion a Mac, and select the Network tab. - Reload the page and let it finish. You will see the page's own files, its web fonts, and one anonymous page-view request. Nothing has been typed yet, so none of them can contain your headers.
- Click the clear icon in the Network tab so that the log is empty.
- Paste real headers into any lane and read the result.
- Look at the Network tab again. It is still empty. For extra proof, turn on offline mode after the page loads and every lane keeps working.
Only two things can add a line to that log, and neither carries your headers. Submitting the feedback form sends the vote and comment you typed. Typing characters in a script the page has not displayed yet, such as Cyrillic or Greek, can make the browser fetch the matching font file, and that request names only the font.
2http never sends a request on your behalf: the request builder writes code for you to run, and the meta inspector reads source you paste instead of fetching a URL. The production site also sends a Content-Security-Policy header that tells your browser to refuse connections to anywhere except this site and the anonymous analytics endpoint.