HTTP 406 – Not Acceptable

The HTTP 406 Not Acceptable status code indicates that the server cannot produce a response matching the list of acceptable values defined by the client in the request headers. This typically occurs when the client specifies certain requirements for the response (e.g., content type, language, or encoding) that the server cannot fulfill.

Key Points

• The client sends an Accept, Accept-Language, Accept-Encoding, or similar header to specify what it can handle.
• The server cannot generate a response that meets these requirements.
• This status code is often used in content negotiation, where the client and server attempt to agree on the format or characteristics of the response.

Common Causes

1. Unsupported Content Type: The client requests a specific content type (e.g., application/xml) that the server cannot provide.
2. Unsupported Language: The client requests content in a specific language (e.g., Accept-Language: fr) that the server does not support.
3. Unsupported Encoding: The client requests a specific encoding (e.g., Accept-Encoding: gzip) that the server cannot use.
4. Misconfigured Server: The server is not configured to handle the requested content type, language, or encoding.

How to Fix

1. Check Client Headers: Ensure the Accept, Accept-Language, or Accept-Encoding headers in the request are reasonable and align with what the server supports.
2. Server-Side Configuration: If you control the server, ensure it can handle the requested content types, languages, or encodings.
3. Fallback Mechanism: Implement a fallback mechanism on the server to return a default response (e.g., application/json) if the requested format is not available.
4. Content Negotiation: Use proper content negotiation to dynamically determine the best response format based on client preferences and server capabilities.

Example

If a client sends a request with Accept: application/xml, but the server only supports application/json, the server might respond:

HTTP/1.1 406 Not Acceptable
Content-Type: application/json

{
  "error": "Not Acceptable",
  "message": "The server cannot produce a response in the requested format (application/xml). Supported formats: application/json."
}

Client-Side Handling

• Review the Accept, Accept-Language, and Accept-Encoding headers in your request.
• Ensure the headers align with what the server supports (check API documentation).
• Handle the 406 response gracefully by either retrying with different headers or informing the user.

Server-Side Handling

• Return a 406 status code with a clear error message indicating why the request was not acceptable.
• Include a list of supported formats, languages, or encodings in the response body or headers.
• Use content negotiation to dynamically select the best response format.

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