The 400 error code, called Bad Request, means the server received your request but could not make sense of it, so it refused to go further. Unlike most 5xx errors, this one often starts on your side: a damaged link, a stale cookie, a very long address or a file that is too awkward for the form.
You may see "400 Bad Request", "HTTP Error 400", "Bad Request. Your browser sent a request that this server could not understand" or "Request header or cookie too large". The fixes below go from the quickest checks to the deeper ones, and there is a short part for developers who see 400 in an API.
Common causes
1Old or corrupted cookies
How to tellOne site only, and an incognito tab opens it fine
What fixes itWipe that site's stored data, then sign in again
2Mistyped or broken link
How to tellAddress has odd characters, spaces or a cut off ending
What fixes itRetype the address or open the site from its home page
3Cookie or header too large
How to tellMessage says request header or cookie too large
What fixes itClear that site's cookies, since too many have piled up
4File upload rejected
How to tellError shows right after choosing a photo or document
What fixes itUse a smaller file or a common format and try again
5Outdated app version
How to tellApp shows 400 while the website works in a browser
What fixes itInstall the newest app version and sign in again
6Bad data sent by a script or API call
How to tellDeveloper sees 400 with a message about a missing or wrong field
What fixes itCheck the request body, headers and field names against the docs
How to fix it, step by step
Check the address you typed
Look for extra characters, spaces, percent signs or a missing part at the end. A link copied from a chat app often carries a stray bracket or full stop. Remove it and press Enter again.
Test in a fresh incognito tab
Paste the address into a tab that does not use your saved data. When the page appears there, old cookies are to blame, and the next step cures it.
Clear cookies for the one site
In Chrome, open the site settings from the lock or tune icon in the address bar and remove stored data for that site. On Android, Settings, then Apps, then your browser, then Storage also has a clear cache option. You will be signed out of that site only.
Try a smaller file or fewer details
If the error came while uploading or filling a long form, retry with a smaller photo, a shorter text or fewer attachments. Servers set limits on size and some reject unusual file names with symbols.
Update the browser or the app
Old versions can send requests in a format the server no longer accepts. Open your phone's app store, update the browser or the app, and try again.
Pause extensions and any VPN for a test
Ad blockers, privacy add ons and VPNs can change what is sent. Pause them and reload. If the error goes, enable them again one at a time to catch the culprit.
Still failing? Write to the site
Send the page address and a screenshot. If the same 400 appears on other devices, the website's form or setup is at fault, and only its owner can correct it.
What a 400 error code means in plain words
Codes beginning with 4 mean the request had a problem. A 400 is the broadest of them. The server is not saying you lack permission or that the page is gone. It is saying the message itself was malformed, incomplete or confusing. Think of posting a letter with a smudged address: the post office cannot even decide where to send it.
You paste a long link from a message and the browser shows 400 Bad Request. Request Header Or Cookie Too Large. The site stored too many cookies for you. Clearing that site's cookies makes the page load again.
400 compared with other 4xx errors
| Code | What it says | Your next move |
|---|---|---|
| 400 | The request was malformed | Fix the link, cookies or data |
| 401 | You are not signed in | Log in |
| 403 | You are not allowed | Ask for access |
| 404 | Page not found | Check the address |
| 422 | Data was readable but invalid | Correct the form values |
Details are in the guides for 401 Unauthorized, 403 Forbidden, 404 Not Found and 422 Unprocessable Content.
HTTP error code 400 in a REST API
Developers meet this code when a client sends a body the API cannot parse. Typical reasons are invalid JSON (a trailing comma or unescaped quote), a missing required field, a wrong data type, a wrong Content-Type header or an unreadable query string. A well built API returns a short message naming the field. Read that message first, compare your payload with the documentation, and validate the JSON before sending. Use 422 only if the format is fine but the values break a rule.
Bad request errors inside apps
Video, social and shopping apps sometimes show a 400 on a screen when their saved login data breaks. Try the app's own sign out and sign in. If that is not possible, clear the app's cache from Android app storage settings, or on an iPhone remove and reinstall the app. More ideas are collected in apps and websites not working.
Stop 400 errors from coming back
- Clear cookies for sites you use rarely, so old data does not pile up.
- Keep your browser and apps updated.
- Share short, clean links rather than very long tracked ones.
- Site owners should set sensible header and upload size limits, and show a helpful message instead of a bare code.
Frequently asked questions
What does error code 400 mean?
It means the server could not understand the request it received. The cause is usually in the link, the cookies or the data sent, not in the server hardware.
How to fix 400 bad request error on a website?
Check the address, open it in a private window, clear cookies for that site, update the browser and try a smaller upload. These steps solve most cases.
Who causes a 400, my phone or the website?
Either can be the cause. Bad cookies and broken links are on your side, while a faulty form or wrong server setting is the website's problem. An incognito test helps you tell which.
What is the difference between 400 and 404?
A 400 says the request itself was malformed. A 404 says the request was fine but the page does not exist at that address.
Why do I get error 400 in an app but not on the website?
The app may be old or may hold damaged saved data. Install its latest version, sign out and in, and clear its cache. A working website proves your account and internet are fine.