Is 406 a client error or server error?

It is a 4xx client error — the client requested a format the server cannot provide. Fix it by adjusting your Accept header.

How do I know what formats an API supports?

Check the API documentation. You can also send an OPTIONS request or try common formats like application/json and text/html.

HTTP Content Negotiation — 406 Not Acceptable and Accept Header Errors

Warning4xx client error

Overview

Fix HTTP 406 Not Acceptable errors caused by content negotiation failures when the server cannot produce a response matching the client's Accept headers.

Key Details

Content negotiation allows clients to request specific formats via Accept, Accept-Language, Accept-Encoding headers
HTTP 406 Not Acceptable means the server cannot produce a response matching any of the client's acceptable formats
Server-driven negotiation uses client headers; agent-driven negotiation uses 300 Multiple Choices
The Accept header specifies media types: application/json, text/html, image/webp, etc.
Quality values (q=0.9) let clients express preference ordering

Common Causes

Client requesting a format the server does not support (e.g., Accept: application/xml on a JSON-only API)
Overly restrictive Accept header excluding the server's available formats
API versioning via Accept header with an unsupported version string
Misconfigured reverse proxy stripping or modifying Accept headers

Steps

1Check which formats the API supports in its documentation
2Set Accept: */* to accept any format, or Accept: application/json, text/html for common web formats
3If using API versioning via Accept header, verify the version string matches supported versions
4Test the request with curl adding -H 'Accept: application/json' to isolate the issue
5Check proxy and CDN configuration to ensure Accept headers are forwarded correctly