What Is a 505 Status Code?

The HTTP 505 Status Code stands for “HTTP Version Not Supported.” It means that the server refuses to handle the request because the HTTP version used in the request is not supported. This version mismatch creates a protocol-level incompatibility that stops the server from proceeding.

To understand this better, consider how browsers and APIs communicate with servers: every HTTP request includes a version declaration (e.g., HTTP/1.1, HTTP/2). If the server receives a version it does not recognize, or if it has been configured to reject specific versions, it returns a 505 error to signal this limitation.
This response is part of the 5xx class of HTTP status codes, which indicates server errors. However, unlike general 500-series errors that may stem from configuration issues or server crashes, a 505 error is specific and deliberate: the server knows the HTTP version but chooses not to or cannot support it.
According to the HTTP/1.1 standard defined in RFC 7231 Section 6.6.6, a 505 response should include a message body explaining why the version is unsupported, and possibly what alternative versions are available. This ensures the client receives meaningful feedback and guidance.

Key Characteristics:

Client-Side Triggered, Server-Side Response: The client sends a request using an unsupported HTTP version.
Intentional Refusal: The server consciously chooses not to process the request due to version mismatch.
Rare in Modern Web Use: Most modern clients and servers use HTTP/1.1 or HTTP/2, minimizing the frequency of 505 errors—yet it remains important for legacy system compatibility and debugging.

This status code is typically encountered in testing environments, when dealing with outdated infrastructure, or during API development where specific protocol versions are used deliberately.

505 CODE REFERENCES

The 505 HTTP Version Not Supported status code is recognized across most major web frameworks and programming languages. Each has its own constant or symbol to represent this response, making it easier for developers to identify and handle the error in server-side logic.

Below is a breakdown of how the 505 status code is represented in various platforms:

Common Language & Framework Constants:

Rails: :http_version_not_supported
Go: http.StatusHTTPVersionNotSupported
Symfony (PHP): Response::HTTP_VERSION_NOT_SUPPORTED
Python 2: httplib.VERSION_NOT_SUPPORTED
Python 3+: http.client.VERSION_NOT_SUPPORTED
Python 3.5+: http.HTTPStatus.VERSION_NOT_SUPPORTED
.NET: HttpStatusCode.HttpVersionNotSupported
Rust: http::StatusCode::HTTP_VERSION_NOT_SUPPORTED
Java: HttpURLConnection.HTTP_VERSION
Apache HttpComponents Core: org.apache.hc.core5.http.HttpStatus.SC_HTTP_VERSION_NOT_SUPPORTED
Angular: HttpStatusCode.HttpVersionNotSupported

These references allow developers to build proper logic for fallback or redirection when a 505 status code is encountered. It also aids in debugging when integrating third-party APIs or updating legacy systems.

If you’re handling HTTP responses programmatically, knowing the appropriate constant for your platform is crucial for writing robust, error-tolerant code.

505 Status Code Example

Understanding how a 505 HTTP Version Not Supported error unfolds in practice can help identify, replicate, and fix the issue faster. Below is a realistic scenario that demonstrates how this error appears in an HTTP request and response cycle.

Example Request

PUT /example HTTP/2.0
Host: www.example.com
Content-Type: text/plain
Content-Length: 42

Hello, this is an example request body.

Example Response

HTTP/1.1 505 HTTP Version Not Supported
Date: Wed, 24 Mar 2023 14:28:00 GMT
Server: Apache/2.4.48 (Unix)
Connection: close
Content-Type: text/html; charset=iso-8859-1

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html>
<head>
<title>505 HTTP Version Not Supported</title>
</head>
<body>
<h1>HTTP Version Not Supported</h1>
<p>The server does not support the HTTP protocol version used in the request.</p>
</body>
</html>

What’s Happening Here?

The client is making a PUT request using HTTP/2.0.
The server, however, only supports up to HTTP/1.1, and does not recognize or support HTTP/2.0.
As a result, the server responds with a 505 status code, indicating the protocol mismatch.
The response includes an HTML error message clearly stating the problem.

This example illustrates how a mismatch in protocol versions between client and server can cause the 505 error. It also highlights the importance of protocol negotiation and server compatibility when working with modern APIs or client libraries.

How to Fix a 505 Status Code

The 505 HTTP Version Not Supported status code indicates that the server refuses to fulfill the request using the HTTP protocol version provided by the client. To resolve this, you need to ensure compatibility between the client and server protocol versions. Here’s how:

1. Verify the HTTP Protocol Version Used

Check the request headers or your HTTP client configuration (like Postman, curl, or your app’s HTTP library) to ensure it’s using a supported version—typically HTTP/1.1. If it’s set to HTTP/2.0 or HTTP/3, and the server doesn’t support them, a 505 error will occur.

2. Modify the Client’s Request

Update the client’s request configuration to use a compatible protocol. For example, many HTTP libraries allow specifying the HTTP version—set it explicitly to HTTP/1.1. Most web servers today still rely on HTTP/1.1 for backward compatibility.

3. Update or Reconfigure the Server

If the server is outdated or not configured to support newer HTTP versions:

Upgrade to a newer server version (e.g., NGINX, Apache) that supports modern protocols.
Check if the HTTP version negotiation is enabled.
Review server settings that block or ignore unsupported HTTP versions.

4. Use a Proxy or Gateway

If you don’t control the server or cannot make changes, consider using a reverse proxy that can handle newer HTTP versions on the client side and downgrade to HTTP/1.1 for communication with the server.

5. Contact Hosting or Server Admin

If it’s a hosted environment or third-party API, reach out to the provider to ask about protocol compatibility or available workarounds.

What Is the Difference Between a 500 Status Code and a 505 Status Code?

While both 500 and 505 are part of the HTTP 5xx server error category, they represent fundamentally different issues in the request-handling process.

500 Internal Server Error

Cause: A generic error indicating the server encountered an unexpected condition that prevented it from fulfilling the request.
Common Scenarios: Misconfigured scripts, server overload, application bugs, insufficient memory, or broken permissions.
Meaning: The server attempted to process the request but failed due to an internal error.

505 HTTP Version Not Supported

Cause: The client’s HTTP request uses a protocol version that the server does not support.
Common Scenarios: The client sends a request using HTTP/2 or HTTP/3, but the server only supports HTTP/1.1.
Meaning: The server rejects the request outright because it doesn’t recognize or support the protocol version used.

Summary Table:

Feature	500 Internal Server Error	505 HTTP Version Not Supported
Error Nature	Internal server problem	Protocol version incompatibility
Request Processing Attempt	Yes	No
Server Capability Issue	Yes (execution failed)	Yes (protocol not recognized or rejected)
Client Fix Possible	Sometimes (depends on error cause)	Yes (change HTTP version used)
Server Fix Involved	Yes (debug and resolve internal issue)	Yes (add protocol support or redirect)

FAQs - 505 Status Code

What does a 505 HTTP Version Not Supported error mean?
It means the server does not support the HTTP protocol version used in the request. This often happens when the client uses HTTP/2 or HTTP/3, but the server only supports HTTP/1.1. The server refuses to process the request with that version.

What causes a 505 error?
A 505 error typically occurs when the client’s HTTP request uses a protocol version the server does not recognize or support. It can result from outdated server configurations, incompatible software, or misconfigured proxy settings rejecting newer protocol versions.

How do I fix a 505 HTTP Version Not Supported error?
You can fix it by switching to a supported HTTP version, updating your server to support newer protocols, or configuring a proxy that supports the desired HTTP version. If you’re unsure, consult the server admin or hosting provider.

Can a 505 error affect SEO?
Yes, a 505 error can impact SEO if search engines encounter it while crawling your website. Pages returning 505 errors may be deindexed or ranked lower due to poor accessibility and user experience.

Is the 505 status code common?
No, the 505 error is rare. Most modern servers and clients use compatible HTTP versions like 1.1 or 2.0. It typically appears on outdated or misconfigured servers that reject certain protocol versions.

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