What Is a 501 Status Code?
The 501 Not Implemented status code is an HTTP server response defined in RFC7231 Section 6.6.2. It indicates that the server does not recognize the request method or lacks the capability to support the functionality needed to fulfill the request. In short, the server understands the request was made, but it doesn’t know how—or isn’t configured—to execute it.
This response is distinct from a 405 Method Not Allowed, which means the method is understood but not permitted for the specific resource. In contrast, a 501 status reflects a fundamental lack of implementation—the server literally does not support the feature or method at all, for any resource.
When Is 501 Typically Used?
This status code is rare in modern production environments but may occur:
When using nonstandard HTTP methods (e.g.,
PATCHon an old server)When a REST API is deployed without full method support
On outdated or minimal server setups lacking required modules or handlers
During reverse proxy requests where the proxy doesn’t understand the forwarded method
501 Is Cacheable
According to the specification, 501 responses are cacheable by default, unless overridden by cache-control headers. This means intermediate systems (like CDNs or browsers) can store and serve this error unless explicitly told not to.
The 501 Not Implemented status code indicates:
- The server does not support the HTTP method or feature.
- It’s a server-side limitation, not a client-side error.
- Fixes typically require server-side upgrades, configuration, or capability extensions.
- What is a 101 Status Code?
- What is a 102 Status Code?
- What is a 200 Status Code?
- What is a 201 Status Code?
- What is a 202 Status Code?
- What is a 208 Status Code?
- See All Status Code
501 Code References Across Frameworks
To help developers understand and implement error handling for the 501 Not Implemented status code, most modern programming languages and web frameworks define standard constants or symbols for it. Below is a breakdown of how the 501 status is referenced across popular platforms:
| Platform / Framework | 501 Reference |
|---|---|
| Ruby on Rails | :not_implemented |
| Go (Golang) | http.StatusNotImplemented |
| Symfony (PHP) | Response::HTTP_NOT_IMPLEMENTED |
| Python 2 (httplib) | httplib.NOT_IMPLEMENTED |
| Python 3+ (http.client) | http.client.NOT_IMPLEMENTED |
| Python 3.5+ (http.HTTPStatus) | http.HTTPStatus.NOT_IMPLEMENTED |
| .NET / C# | HttpStatusCode.NotImplemented |
| Rust | http::StatusCode::NOT_IMPLEMENTED |
| Java (HttpURLConnection) | HttpURLConnection.HTTP_NOT_IMPLEMENTED |
| Apache HttpComponents Core | HttpStatus.SC_NOT_IMPLEMENTED |
| Angular (HttpStatusCode Enum) | HttpStatusCode.NotImplemented |
These predefined constants make it easier to respond with or check against a 501 error without having to hard-code numeric values like 501, ensuring clarity and code maintainability.
501 Status Code Example
To better understand the 501 Not Implemented status, let’s break down a practical HTTP request and response scenario that triggers this status code.
Example Scenario
Client Request:
GET https://example.com/unsupported-feature HTTP/1.1
Host: example.com
In this example, the client is attempting to retrieve a resource that depends on a feature the server doesn’t recognize or support.
Server Response:
HTTP/1.1 501 Not Implemented
Date: Wed, 16 Mar 2023 12:00:00 GMT
Server: Apache
Content-Type: text/plain;charset=UTF-8
Content-Length: 28
This feature is not supported.
Explanation:
The client made a valid request, but the server returned a
501 Not Implementedstatus code because the requested functionality (such as a specific method or protocol) is not supported.The response includes:
Date header: When the response was generated.
Server header: Server details (optional but common).
Content-Type and Content-Length: Define the type and size of the message body.
Plain-text Message: Provides context to the user or developer.
This is the correct server behavior when a client attempts to use a feature or method that is unknown or not available on the server.
How to Fix a 501 Status Code
The HTTP 501 Not Implemented error indicates that the server doesn’t support the functionality required to fulfill the request. It’s typically a server-side issue, but resolving it can involve both server and client configurations.
Steps to Fix a 501 Error:
Verify Server Capability
Ensure the server supports the HTTP method or feature being requested. For example, some legacy servers might not support modern HTTP verbs like PATCH or OPTIONS.Upgrade or Configure the Server
If the server is outdated or misconfigured, upgrade it or adjust settings. This may include updating server software (e.g., Apache, Nginx) or enabling specific modules or handlers that support the missing feature.Review Client Requests
Confirm that the client (browser, API call, bot, etc.) is sending a valid request using supported methods. Switching from an unsupported method (like CONNECT) to a standard method (like GET or POST) may solve the issue.Check Application Code
If the request is made by an application, debug the backend to see whether unsupported methods are being issued due to a logic error or outdated API implementation.Use Fallbacks or Workarounds
If the server truly cannot support a specific method, consider re-routing the request to an alternative path that delivers equivalent functionality using supported methods.Provide a User-Friendly Error Message
If the 501 response is unavoidable, return a helpful message in the response body to guide users or developers on what to do next.
Do 501 Status Codes Affect Search Engine Optimization (SEO)?
Yes, 501 errors can negatively impact SEO because they signal to search engines that your site has server-level problems, which can reduce trust, crawlability, and ranking potential.
Why 501 Errors Are Problematic for SEO:
Crawler Experience:
When search engines like Googlebot encounter a 501 status code, they treat it as a server failure. This may cause the crawler to stop indexing that page — or deprioritize crawling your site altogether if errors are frequent.Loss of Indexing:
If 501 responses persist, important URLs could be deindexed over time. This directly affects your organic visibility.Reduced Crawl Budget Efficiency:
Search engines assign a limited “crawl budget” to each site. 501 responses waste that budget on failed attempts, leaving valuable content potentially undiscovered.User Trust and Engagement:
If users land on pages that trigger a 501 error, their experience is disrupted. High bounce rates and low engagement can further impact rankings indirectly.
SEO-Friendly Practices to Prevent 501-Related Damage:
Set up error monitoring tools (like Google Search Console or server logs).
Fix 501s quickly to avoid repeated crawler failures.
Serve a 503 (Service Unavailable) instead, if the unavailability is temporary — it tells crawlers to come back later.
Implement custom error pages to maintain user experience even when errors occur.
FAQs
What is a 501 status code used for?
A 501 status code indicates that the server does not recognize or support the HTTP method used in the request. It’s a signal that the requested functionality is not implemented on the server.
Is a 501 error a server or client issue?
A 501 error is a server-side issue. It means the server lacks the ability to fulfill the request, typically due to unsupported features or misconfigured server settings.
How is a 501 different from a 500 error?
While both are server errors, 501 means the functionality is not implemented, whereas 500 indicates a generic internal server error due to an unexpected condition or bug.
Can a 501 error be fixed by users?
No, users typically can’t fix a 501 error. The resolution must come from the server side, such as implementing support for the requested method or updating server configurations.
Does a 501 error impact SEO?
Yes, frequent 501 errors can negatively affect SEO by preventing crawlers from indexing your pages. Fixing these errors promptly helps maintain search engine visibility and crawl budget efficiency.
Don’t Let Your Server Stay Silent
The 102 Status Code is your server’s way of saying “Working on it!”—but like any good communication, it needs follow-through.
If you’re unsure whether your server responses are SEO-friendly or crawlable, our SEO specialists in Dubai are here to help.
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