400 Bad request error: 6 Quick fixes you should try now

A 400 bad request error can appear without warning and instantly block access to a website, page, or online service. In many cases, the problem is caused by something simple such as corrupted browser cache, invalid cookies, or a broken request sent to the server. 

However, server configuration issues and WordPress plugin conflicts can also trigger the error. The good news is that most 400 errors are easier to fix than they seem. Below are 6 practical solutions that can help you resolve the issue and prevent it from happening again.

What is a 400 bad request error?Link to heading

A 400 Bad Request error is an HTTP status code that appears when the request sent from a user’s browser to the website server is considered invalid. Because the server cannot understand or process the request correctly, the user is unable to access the webpage. Depending on the website system, browser, or server configuration, the error may appear in several different forms, such as:

• Bad Request: Error 400
• HTTP Error 400 – Bad Request
• 400 – Bad Request. The server could not understand the request because of invalid syntax. The request should not be repeated without modification
• HTTP Error 400. The request hostname is invalid
• Bad Request – Invalid URL
• Bad Request. Your browser sent a request that this server could not understand
• 400 Bad Request

>>> Learn more: What is a 429 Too Many Requests error? How to fix error 429

What causes a 400 bad request error?Link to heading

In most situations, there are several common reasons why a 400 Bad Request error may occur when accessing a website:

• Invalid URL syntax: A simple typo or an unsupported character anywhere in the URL is enough to trigger a 400 error. Double-check the address bar for misplaced slashes, spaces, or special characters that do not belong in a valid URL.
• Corrupted browser cache or cookies: Your browser stores temporary files and cookies to speed up page loads, but when those files become outdated or corrupted, they can send garbled data to the server. An expired cookie or a damaged cache entry is one of the most common culprits behind a 400 error.
• Conflicting browser extensions: Some extensions modify outgoing requests in the background without you realizing it. An ad blocker, a security plugin, or even a shopping assistant can unintentionally strip or alter request headers, causing the server to reject the request entirely.
• Stale DNS cache: Your device keeps a local record of DNS lookups to avoid re-querying the same domains repeatedly. If that record is out of date and no longer matches the domain's current DNS registration, the request can break before it even reaches the server.
• Oversized file upload: Every web server enforces an upload size limit. If the file you are trying to send exceeds that threshold, even by a small margin, the server will reject the request and return a 400 error rather than attempting a partial upload.
• Server-side misconfiguration: Although a 400 error typically points to a client-side problem, a misconfigured server or a brief technical glitch on the hosting end can produce the same result. It is less common, but worth keeping in mind if every other fix comes up empty.

>>> Learn more: What is a 403 Forbidden error? Causes and how to fix it

How to fix a 400 bad request errorLink to heading

Check the URL carefullyLink to heading

One of the most common causes of a 400 Bad Request error is an incorrectly entered URL. Even a small typo or an unsupported special character can prevent the server from processing the request properly.

How to check the URL correctly

• Review the domain name, path, and query string in the URL.
• Look for spelling mistakes, extra slashes, or unusual characters.
• Carefully check for special characters such as “%”, “-”, “#”, “&”, or unnecessary spaces.

Example: If the correct URL is: www.yourbusiness.com/products but you enter: www.yourbusiness.com/prod%ucts the “%” character may cause the browser to display a 400 Bad Request error.

Use a URL checking tool

Some URLs contain encoded special characters, so you can use a URL Decoder/Encoder tool to check whether the address includes invalid characters.

Find the website another way

If you are unsure whether the URL is correct, try finding the page through a search engine instead of typing the address manually.

Example:
Type: site:yourwebsite.com keyword

This method helps you access the correct page without entering the full URL yourself.

Clear your browser cache and cookiesLink to heading

Corrupted or outdated cache and cookies can cause the browser to send invalid data to the server, which may trigger a 400 Bad Request error.

On Google Chrome

1. Click the three-dot icon in the upper-right corner.
2. Select More Tools > Clear Browsing Data.
3. Check:
   • Cookies and other site data
   • Cached images and files
4. Under the time range section, select All time to remove all stored data.
5. Click Clear data and wait for the process to finish.

On Mozilla Firefox

1. Click the three-line menu icon.
2. Go to Settings > Privacy & Security.
3. Find the Cookies and Site Data section.
4. Click Clear Data.
5. Check:
   • Cookies and Site Data
   • Cached Web Content
6. Click Clear to complete the process.

After clearing the cache and cookies, reload the website to see whether the error still appears.

Disable browser extensionsLink to heading

Some browser extensions or add-ons may interfere with requests sent to websites and cause a 400 Bad Request error.

How to disable extensions on Google Chrome

1. Click the three-dot icon.
2. Select Extensions > Manage Extensions.
3. Turn off all active extensions.

How to disable extensions on Firefox

1. Click the three-line menu icon.
2. Select Add-ons and Themes.
3. Disable all enabled extensions.

Test the website again

After disabling the extensions:

• Reload the webpage showing the error.
• If the website works normally, one of the extensions is likely causing the issue.

Identify the problematic extension

Enable the extensions one by one and test the website after each activation. This will help you identify exactly which extension is causing the conflict.

Flush the DNS cacheLink to heading

Sometimes, a 400 Bad Request error happens because your device is using outdated DNS information. Clearing the DNS cache can help your browser reconnect to the correct server and fix the issue.

On Windows

• Press Win + R on your keyboard.
• Type cmd and press Enter.
• In Command Prompt, type the following command:

ipconfig /flushdns

• Press Enter to clear the DNS cache.

On Mac

• Open Terminal.
• Enter the following command:

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

• Press Enter.
• Enter your Mac password if requested.

On Linux

• Open Terminal.
• Type the following command:

sudo systemd-resolve --flush-caches

• Press Enter to continue.

Clear DNS cache in Chrome

If you use Google Chrome, you can also clear the browser DNS cache manually:

• Open Chrome.
• Type the following into the address bar:

chrome://net-internals/#dns

• Press Enter.
• Click Clear host cache.

After clearing the DNS cache, reload the website to check if the 400 Bad Request error has disappeared. 

Check the uploaded file sizeLink to heading

A website may display a 400 Bad Request error if the file you are trying to upload exceeds the server’s allowed size limit.

Check the maximum upload limit

Some websites limit how large uploaded files can be. If you are using WordPress, you can usually see the upload limit by going to:

Media → Add New

Reduce file size before uploading

If your file is too large, try reducing its size before uploading it again.

• For images: Use compression tools like TinyPNG to reduce image size while keeping good quality.
• For documents: Compress files into a ZIP folder.

Test with a smaller file

Try uploading a smaller file to see whether the error still appears. This helps confirm if the issue is related to file size limits.

Increase the upload limit

If you manage the website yourself, you can increase the maximum upload size through server settings, hosting control panels, or by contacting your hosting provider for support.

Troubleshoot your device and internet connectionLink to heading

Sometimes, the problem is not related to the website itself. Your device, browser, or internet connection may also trigger a 400 Bad Request error.

Restart your device

Restarting your computer or phone can clear temporary memory, close background processes, and refresh system connections.

Restart your wi-fi router

Turn your router off for a few seconds and restart it. This simple step can fix temporary network problems.

Check your internet connection

Try switching between:

• Wi-Fi and wired connection
• Mobile network and home internet
• Another internet connection if available

This helps determine whether the issue is caused by your current network.

Disable VPN or proxy services

VPN services and proxy servers may occasionally disrupt communication between your browser and the website server. Disable them temporarily, then reload the website to check whether the issue has been resolved.

Run a network troubleshooter

On MacOS: Open the Wireless Diagnostics tool.

On Windows 11:

1. Go to Settings
2. Select System
3. Click Troubleshoot
4. Open Other troubleshooters
5. Run Network and Internet

If none of these methods solve the issue and the 400 Bad Request error continues appearing on multiple websites, consider contacting your internet service provider for additional support.

Monitoring and preventing 400 bad request errorsLink to heading

Taking preventive steps can reduce the risk of dealing with recurring website issues in the future. Regular monitoring helps you identify problems early before they affect visitors or website performance.

Use website auditing toolsLink to heading

Website auditing tools such as Google Search Console can help detect technical issues on your website, including pages returning a 400 Bad Request error. These tools regularly scan your website and provide detailed reports about broken pages, invalid URLs, and crawling problems.

Here are a few basic steps you can follow:

• Configure the auditing tool and connect it to your website properly
• Start a website scan or crawl to identify technical errors
• Review the results carefully to find pages showing a 400 status code
• Fix the detected issues by correcting URLs, repairing broken links, or adjusting server settings

Regularly update your websiteLink to heading

Keeping your website updated is one of the simplest ways to reduce technical errors and security risks. Outdated software, plugins, or themes can create compatibility problems that may trigger 400 errors and other website issues. Make sure your CMS, plugins, themes, and other website components are updated regularly to maintain stability, performance, and security.

Educate your teamLink to heading

Anyone responsible for managing website content should understand the best practices needed to avoid technical mistakes. Training your team to use proper URLs, upload correct file formats, and follow website management guidelines can help prevent unnecessary errors from appearing.

When to seek professional assistanceLink to heading

In some situations, the 400 Bad Request error may continue even after trying multiple troubleshooting methods. If the issue becomes difficult to identify or resolve, contacting a professional may be the best option.

Here are several signs that professional assistance may be necessary:

• Server-side problems: Errors caused by server configuration issues, database failures, or hosting-related problems may require technical expertise
• Large-scale impact: If many users experience the same error across different browsers, devices, or networks, the problem may need urgent professional attention
• Limited time or technical knowledge: Website owners often manage multiple responsibilities. If troubleshooting becomes too time-consuming or complex, working with an experienced technician can save time and prevent larger issues later

ConclusionLink to heading

A 400 Bad Request error looks intimidating at first, but in most cases, it comes down to something fixable. Working through the six fixes in this guide will resolve the issue in the vast majority of situations.

If the error keeps coming back, the problem likely runs deeper than a browser setting. At that point, checking your server configuration or contacting your hosting provider is the right move. The sooner you identify the root cause, the less disruption it will cause to your site and your visitors.

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