HTTP 505 Response Status Code Guide: What is it, Usage, Methods

Jun 17 2024 by Oskay Günaçar

What Is HTTP 505 Status Code?

The HTTP 505 status code, also known as "HTTP Version Not Supported," indicates that the server refuses to support the major version of HTTP that was used in the request message. This error occurs when the server does not support or is not capable of handling the particular version of the HTTP protocol used by the client in the request header.

For instance, if a client sends a request using HTTP/2.0, but the server only supports HTTP/1.1 and not the newer versions, it will return an HTTP 505 error. This error response is the server’s way of informing the client that it needs to downgrade the HTTP version in its requests.

Common Causes of HTTP 505 Status Code

The HTTP 505 status code, indicating "HTTP Version Not Supported," is typically caused by mismatches between the HTTP versions supported by a server and those used by a client. Here are some common causes of this error:

  1. Client Using Unsupported HTTP Version: The most straightforward cause is that the client request includes an HTTP version newer or older than those the server supports. For instance, if a server is configured to handle HTTP/1.1 requests but receives an HTTP/2.0 request, this can lead to a 505 error.

  2. Misconfigured Server: In some cases, the server might be improperly configured, leading it to reject supported HTTP versions. This can happen during server upgrades or when settings are changed without proper testing.

  3. Proxy or Load Balancer Issues: If there is a proxy, load balancer, or other network device between the client and the server, these devices might not support the same HTTP versions as the server or client. This mismatch can cause HTTP requests to fail with a 505 error if the intermediate device cannot handle the HTTP version used.

  4. Software Bugs: Occasionally, bugs in the server software or in web frameworks/application code might incorrectly handle HTTP version negotiation, leading to errors.

  5. Strict Transport Security Settings: While not a direct cause, servers enforcing strict policies for secure connections might also scrutinize HTTP versions more closely, contributing to a higher occurrence of these errors if clients use deprecated or less secure versions.

Importance of HTTP 505 Status Code in SEO

Search Engine Crawling

The HTTP 505 status code can significantly impact how search engines interact with a website. If a crawler encounters a 505 error, it means that the server is rejecting requests from the crawler due to the HTTP version used. This can prevent search engines from accessing content on the website, leading to unindexed pages or outdated search results.

User Experience and Site Accessibility

From an SEO perspective, user experience is crucial. A site that frequently returns HTTP 505 errors can frustrate users and increase bounce rates. High bounce rates and poor user interaction signals can negatively affect the site's SEO performance because search engines interpret these as indicators of low-quality content or a poor user experience.

Website Reputation and Reliability

Frequent HTTP 505 errors can harm a website's reputation with search engines. Consistent accessibility issues may lead search engines to deem the site unreliable or of low quality, which can negatively impact its rankings. Ensuring stable and compatible server configurations is essential for maintaining good standing with search engines.

Mitigating HTTP 505 Errors for SEO

To minimize the impact of HTTP 505 errors on SEO, it's important for webmasters to ensure their server configurations support the HTTP versions commonly used by browsers and crawlers. Regularly testing website accessibility from different client environments can help identify and resolve potential HTTP version compatibility issues. Additionally, monitoring server logs for frequent HTTP errors and addressing them promptly can improve the overall health and SEO performance of a site.

How to Fix the HTTP 505 Error From the Website?

Confirm the HTTP Version Support

The first step in fixing an HTTP 505 error is to determine the HTTP versions supported by both the server and the client. Ensure that the server is configured to support the versions that are commonly used by modern web browsers and crawlers, such as HTTP/1.1 and HTTP/2.

Server Configuration Review

  • Server Software Update: Ensure that the server software (like Apache, Nginx, IIS) is up-to-date, as newer versions tend to support more recent HTTP protocols.
  • Configuration Settings: Review the server’s configuration files for any explicit directives that restrict HTTP protocol versions. Adjust these settings to include broader support where necessary.

Check Intermediary Network Devices

If there are proxies, load balancers, or other network devices between the client and the server, check their configurations as well. These devices need to support the HTTP versions used by the server and client to avoid introducing issues that could lead to a 505 error.

Testing and Validation

  • Testing with Different Clients: Test the server's response using different clients (web browsers, command-line tools like curl) to see if the error persists across various HTTP versions.
  • Use Developer Tools: Utilize developer tools in browsers to inspect the HTTP version being used and the response from the server. This can help in identifying if the error is consistent or a result of specific conditions.

Contacting Hosting Provider or IT Support

If you are not managing the server yourself or if the above steps do not resolve the issue, contact your hosting provider or IT support team. They can provide insights into server capabilities and may need to perform upgrades or patches to support the required HTTP versions.

Monitoring and Logs

Regularly monitor server logs for errors and anomalies related to HTTP version requests. Logs can provide clues on whether the 505 errors are widespread or isolated, helping in pinpointing the source of the problem.

How to Check HTTP 505 Status Code?

To identify and diagnose an HTTP 505 status code on your website, you can use various methods ranging from browser tools to command-line utilities. Here's how you can perform these checks:

Using Web Browsers

  1. Developer Tools:
    • Open your web browser and navigate to the website where you suspect the HTTP 505 error might occur.
    • Press F12 or right-click on the page and select "Inspect" to open the developer tools.
    • Go to the "Network" tab and reload the page.
    • Look for any network requests that display a 505 status code in the status column.

Using Command-Line Tools

  1. Curl:

    • Open a command-line interface (CLI) on your computer.
    • Type the following command to send a request to the server:
      curl -I
    • Replace with the URL of your website.
    • Review the output headers for the HTTP status code. If it returns HTTP/1.1 505 HTTP Version Not Supported, then the server is indeed sending a 505 response.
  2. HTTPie:

    • HTTPie is another handy tool for simpler HTTP interactions via command line. To check the status, you can use:
    • Look for a HTTP/1.1 505 HTTP Version Not Supported response in the output.

Automated Monitoring Tools

  • Uptime and Performance Monitors: Tools like Uptime Robot, Pingdom, or similar services can be configured to alert you when your website returns a 505 status code. These tools continually check your site and provide real-time alerts and analytics.

Logs Analysis

  • Server Logs: Check the server logs, which can provide detailed information about the requests processed by the server and their response status codes. Look for any entries with a 505 status to diagnose issues related to HTTP version support.

Checking Through API Testing Tools

  • Postman: If you are dealing with APIs or want a more interactive approach to testing HTTP requests, you can use tools like Postman. Configure the request with different HTTP versions to see how your server responds.

By employing these methods, you can effectively identify HTTP 505 errors and start working on the necessary configurations or updates needed to resolve them.

What is the Browser Compatibility of HTTP 505 Status Code?

The HTTP 505 status code, which indicates "HTTP Version Not Supported," is universally recognized and supported by all major web browsers. This compatibility does not typically concern the browser's ability to understand or display the error, but rather how the browser handles HTTP protocol versions during its communication with servers.

Key Points on Browser Compatibility:

HTTP Protocol Handling

  • All Modern Browsers: Modern web browsers like Google Chrome, Mozilla Firefox, Apple Safari, and Microsoft Edge support various versions of the HTTP protocol, including HTTP/1.1 and HTTP/2. When these browsers encounter a 505 error, it generally means that the server is unable to process the HTTP version used in the request sent by the browser.

Browser Behavior

  • Error Display: Browsers display an error page when they receive a 505 response from a server, informing the user that the HTTP version used in the request is not supported by the server. The specific design and information on this error page can vary between browsers.

Protocol Negotiation

  • Fallback Mechanisms: While HTTP/2 and newer versions offer improved performance, browsers are typically designed to fall back to older HTTP versions (like HTTP/1.1) if they encounter issues like the 505 status code. This ensures better compatibility and website accessibility even if newer protocol versions fail.

Developer Tools

  • Network Inspection: Developers can use built-in tools in browsers to inspect network traffic and responses, including any HTTP 505 errors. This feature helps in debugging issues related to HTTP protocol versions used by websites.

In summary, the HTTP 505 error is well-handled across all browsers, and compatibility issues more commonly arise from server-side configuration rather than the browser itself. The key for web developers and administrators is to ensure that servers are properly configured to handle the HTTP versions that their audience's browsers are likely to use.


Understanding and resolving the HTTP 505 status code is crucial for maintaining a reliable and efficient online presence. This error, indicating "HTTP Version Not Supported," highlights a fundamental mismatch between the HTTP protocol versions used by the client and those supported by the server. By addressing this issue, web administrators ensure that their sites remain accessible and functional across all user platforms and devices.

For SEO purposes, mitigating HTTP 505 errors is particularly important as it affects how search engines crawl and index web pages, impacting overall site visibility and user engagement. Regularly updating server configurations, ensuring compatibility with current HTTP standards, and utilizing comprehensive testing and monitoring tools are essential steps in preventing these errors and ensuring optimal website performance.

Ultimately, the compatibility of HTTP 505 with all modern browsers means that the resolution typically lies in server-side adjustments rather than client-side fixes. By understanding the root causes and employing the appropriate diagnostic tools, web developers and administrators can enhance site reliability, improve user experience, and maintain strong search engine rankings. Thus, proactive management of HTTP versions supports a robust digital strategy, ensuring that your website remains competitive and compliant with current web standards.

Oskay Günaçar
Oskay Günaçar is a Technical SEO expert and backend developer. His main areas of interest are back-end programming, Python, Django, Go, semantic SEO, technical SEO. With more than 5 years of SEO experience, he has held SEO specialist positions at İkiler, Webtures, Boosmart, and companies, and has been involved in numerous projects as an outsourced expert. Through the TechSEOHub website, he aims to share the expertise and experience he has gained from managing and developing (SEO) processes for many successful and large websites in the industry, and to produce content for easy access to accurate and high-quality information.