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406 Not Acceptable

4xx client error

The server can't produce a response matching the request's Accept headers.

406 is failed content negotiation. The client said which formats, languages or encodings it accepts (Accept, Accept-Language, Accept-Encoding), and the server has nothing that fits. Most servers ignore an unsatisfiable Accept header and send their default instead, so a 406 usually comes from a strict API framework or a firewall.

Class4xx Client Error
Retry?No — change the request first
Cacheable by defaultNo — only with explicit Cache-Control
Response bodyAllowed
Defined inRFC 9110 §15.5.7 — 406 Not Acceptable
Example response
HTTP/1.1 406 Not Acceptable
Content-Type: application/json
Content-Length: 26

{"error":"Not Acceptable"}

Common causes

  • Requesting Accept: application/xml from a JSON-only API.
  • A client library sending an unusually narrow Accept header.
  • Some WAFs, notably mod_security, use 406 to block suspicious requests.

How to fix it

  1. Send Accept: application/json, or */*.
  2. On the server, consider falling back to a default representation instead of failing.
  3. If mod_security sent it, its audit log names the rule that fired.

Send a 406 from your server

Express
res.status(406).json({ error: "Not Acceptable" });
Next.js route handler
return Response.json({ error: "Not Acceptable" }, { status: 406 });
Flask
return {"error": "Not Acceptable"}, 406
Go net/http
http.Error(w, "Not Acceptable", http.StatusNotAcceptable)
nginx
return 406;

Check what a URL returns

Print just the status line from the command line, or check a whole list of URLs at once, with their response headers, in the HTTP Status Checker.

curl -sI https://example.com/your-path | head -n 1

Frequently asked questions

406 vs 415?

406 is about the response format the client will accept (Accept). 415 is about the request format the client sent (Content-Type).

Headers used with 406
Related status codes