What Is a 400 Bad Request?

HTTP status codes are signals a website sends to your browser to explain what’s happening. They range from success (like 200 for “OK”) to errors (like 404 for “Not Found”). One common error is the 400 Bad Request, which means the server couldn’t understand the request your browser sent. Knowing about this error is important for users trying to browse, developers building websites, and website owners to ensure a smooth experience.

What Is a 400 Bad Request?

A 400 Bad Request happens when a server (the computer where a website lives) can’t understand the request sent by your device, like your phone, tablet, or computer. This usually means something is wrong on your side — not the website’s.

The 400 error is part of a group of 4xx status codes, all about problems due to the user’s request. Other common 4xx errors include 404 (Page Not Found) when a webpage doesn’t exist and 403 (Forbidden) when you’re not allowed to access something.

A 400 Bad Request can happen for many reasons. For example:

• You might have typed the website address (URL) wrong.
• You might have sent damaged or incorrect information without realizing it.
• Your internet browser might have old data that’s confusing the server.

Common Causes of a 400 Bad Request

You might see a 400 Bad Request error when visiting a website for many reasons. Most of the time, the problem happens because something went wrong with the request your device sent to the server. Let’s go through the common causes one by one:

Malformed Request Syntax

When you type a website address into your browser or click a link, your browser sends a “request” to the server. The server cannot understand if this request has mistakes — like missing parts, wrong formatting, or strange characters. This often happens if:

• The URL (website link) has typos.
• There are missing characters like a slash (/) or colon (:).
• The structure of the request is broken.

Example: Typing www.example..com or wwwexample.com can confuse the server.

Invalid or Corrupted Cookies

Cookies are small data saved by websites to help remember you (like keeping you logged in). But sometimes, these cookies get old, broken, or don’t match the server’s current information. When the server reads these bad cookies, it can’t figure out what you want, sending back a 400 error.

This can happen if:

• You recently updated your account on a website.
• The website changed its cookie rules.
• The cookie files got damaged by
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  a browser crash.

Large File Size

If you are trying to upload a big file (like a huge video, photo, or document), the server may block it if it’s too large. Every server has a size limit for uploads. If your file is bigger than allowed, the server cannot process it properly and shows a 400 error instead.

Tip: Before uploading, check the website’s upload size limits (like 5 MB, 50 MB, etc.).

URL String Errors

Sometimes, the link you click on or type includes special symbols like %, #, &, or spaces. The server cannot understand the request if these symbols are not correctly used or encoded. Broken or poorly written URLs are a very common reason for 400 errors.

Examples of URL problems:

• Using spaces instead of %20.
• Extra punctuation like double question marks ??.
• Missing important parts like query parameters.

DNS Lookup Errors

DNS (Domain Name System) helps turn easy website names like google.com into a computer-friendly IP address. If your computer or internet service provider (ISP) can’t find the correct IP address, it may send a wrong request to the server. The server, unable to figure it out, may return a 400 error.

Common causes of DNS errors:

• Outdated DNS records.
• Problems with your ISP’s DNS servers.
• Switching between networks (like from Wi-Fi to mobile data).

Browser Cache Issues

Your browser saves bits of websites you visit (called cache) to load them faster next time. However, this saved data can become old, incomplete, or damaged. If your browser tries to use insufficient cached data to visit a website, the server might not recognize the request correctly, leading to a 400 Bad Request.

Signs that cache is the problem:

• Other people can access the site, but you can’t.
• The error disappears after clearing your browser cache.

Bad or Expired Request Headers

Request headers carry extra information when your browser asks for a webpage. If these headers are incorrect, too long, or expired, the server might not understand what your browser asks and show a 400 error.

This can happen if:

• You’re using an old browser version.
• Browser extensions are messing with headers.

Client-Side Network Issues

Sometimes, the problem isn’t your website or browser— your internet connection.

Bad Wi-Fi signals, unstable mobile networks, or router problems can cause incomplete requests, which confuse the server and trigger a 400 error.

Third-Party Plugins or Extensions

Some browser extensions, like ad blockers, security tools, or VPNs, can change how your browser talks to websites. If they modify your request too much, the server may not understand it and return a 400 error.

How to Fix a 400 Bad Request

The good news is that most 400 Bad Request errors are easy to fix! You can usually solve the problem yourself by following a few simple steps. Let’s go through them one by one:

Double-check the URL

One of the most common causes of a 400 error is typing the wrong website address (URL).

Make sure:

• There are no typos.
• There are no extra spaces or wrong symbols.
• If needed, you didn’t leave out important parts like “https://.”

Example: Instead of typing www.example..com, it should be www.example.com.

Clear Your Browser’s Cookies and Cache

Old or broken cookies and cached data can cause 400 errors. Clearing them gives your browser a fresh start and often fixes the problem.

Here’s how to clear cookies and cache on different browsers:

Google Chrome:

• Click the three dots (top right) > Settings > Privacy and Security > Clear Browsing Data.
• Select Cookies, other site data, Cached images, and files, then click Clear Data.

Mozilla Firefox:

• Click the three lines (top right) > Settings > Privacy & Security.
• Under Cookies and Site Data, click Clear Data.

Microsoft Edge:

• Click the three dots (top right) > Settings > Privacy, Search, and Services.
• Under Clear Browsing Data, click Choose what to clear, select Cookies and Cached files, then click Clear.

After clearing, restart your browser and try opening the website again.

Reload the Webpage

Sometimes, the error happens because of a temporary glitch.

Simply press F5 on your keyboard or click the refresh button in your browser.

This sends a fresh request to the server and might fix the problem immediately.

Check File Upload Size

If you try to upload a file (like a picture, document, or video) and see a 400 error, the file might be too big.

• Look for any upload size rules on the website.
• Try compressing the file or making it smaller.
• If the site allows only up to 5MB and your file is 10MB, it will cause an error.

Keeping your file within the limit can help the upload go smoothly.

Remove the DNS Cache

Sometimes, your computer’s memory of website addresses (the DNS cache) gets outdated or wrong, which can cause 400 errors.

You can clear (remove) the DNS cache to fix it:

On Windows:

• Press the Windows key + R, type cmd, and press Enter to open Command Prompt.
• Type this command: ipconfig /flushdns
• Press Enter.
• You will see a message saying the DNS cache was successfully flushed.

On Mac:

• Open the Terminal app.
• Type this command: sudo dscacheutil -flush cache
• Press Enter.
• You might need to enter your computer password.

After removing, try revisiting the website.

Try a Different Browser

Sometimes, the problem is with the browser you are using.

You can:

• Open the same website in a different browser (for example, if you use Chrome, try Firefox or Edge).
• See if the website works there.
• If it works, the issue might be related to browser extensions, settings, or saved data in your original browser.

Contact the Website

If none of the above solutions work, the problem might not be on your side. There could be something wrong with the website itself.

What you can do:

• Look for a Contact Us page on the website.
• Send them a message or email explaining that you are getting a 400 Bad Request error.
• Include details like the page you tried to visit and what steps you already tried.

They may be able to fix the problem or guide you further.

Quick Tip:

Most 400 Bad Request errors are easy to fix on your own. Start by checking the URL and clearing your cookies. If that doesn’t work, just follow the other steps individually — you’ll likely fix it without much help!

How Developers Can Prevent 400 Errors

While many 400 Bad Request errors occur due to user mistakes, developers can also take steps to prevent these errors and make websites easier to use. Here are some simple ways developers can help:

Add Server-Side Validation

Before the server tries to use the information users send (like a form submission, a file upload, or a login request), it should check if the information is correct. This is called server-side validation.

Good server-side validation can:

• Catch problems like missing information.
• Find errors like wrongly typed website addresses (URLs).
• Stop very large files from being uploaded if they’re too big for the server to handle.

Example:

If someone uploads a 200MB video to a site that only allows 10MB files, the server should immediately say, “Sorry, this file is too big,” instead of returning a confusing 400 error.

Why it matters: It protects the server and improves the user’s experience.

Provide Clear and Helpful Error Messages

If something goes wrong, it’s important not to just show a “400 Bad Request” without any explanation.

Instead, the website should tell users clearly what happened and how they can fix it.

Good error messages should:

• Explain the problem in simple words.
• Give hints or steps to fix it.
• Avoid confusing technical jargon.

Example:

Instead of just showing “400 Bad Request,” the website could show:

“Oops! It looks like the file you uploaded is too large. Please upload a file smaller than 10MB.”

This way, users feel guided and know exactly what to do next.

Regularly Test Forms, Inputs, and Uploads

Developers should test the website’s forms, search bars, upload tools, and any place where users can send data to the server.

Testing helps make sure that:

• The website handles both correct and incorrect data correctly.
• Users can’t accidentally crash the server by sending bad information.
• Error handling works smoothly for common mistakes.

Testing tips for developers:

• Try submitting empty forms.
• Upload files that are too large.
• Enter strange characters into input boxes.
• Use very long or very short text.

By testing often, developers can find problems before real users do, keeping the website safer and more user-friendly.

Conclusion

A 400 Bad Request error might look scary at first, but it’s usually caused by small and fixable problems, like a typing mistake in the URL, old cookies, or a file that’s too large.

Most of the time, you don’t need any technical skills to fix it — simple actions like checking the website address, clearing your browser’s cookies and cache, or refreshing the page can solve the issue quickly.

For users, learning how to troubleshoot a 400 error saves time and avoids frustration. For developers, adding strong checks, giving clear instructions, and testing websites carefully can prevent many of these errors from happening in the first place.

In short, understanding what causes a 400 Bad Request and how to fix it helps everyone — whether you’re just browsing the internet, running a website, or building one.