406 Not Acceptable

Meaning & Description

Common Causes

• Mismatch between client `Accept` headers and server supported Content-Types
• Misconfigured HTTP clients sending overly restrictive `Accept` headers
• Strict content negotiation rules enabled on the web framework

How to fix a 406 error

1. Check the `Accept` header sent by the client.
2. Compare it with the content types the server is capable of returning (e.g., application/json).
3. Update the client request to accept the server's supported formats, or update the server to support the requested format.
4. Relax server strictness to return a default format if the `Accept` header is unsupported.

Browser & SEO Behaviour

Code Examples

app.get('/api/data', (req, res) => {
  if (!req.accepts('json')) {
    return res.status(406).send('Not Acceptable: This API only returns JSON.');
  }
  res.json({ message: 'Success' });
});

from fastapi import FastAPI, Request, HTTPException

app = FastAPI()

@app.get("/data")
def get_data(request: Request):
    accept = request.headers.get("accept", "")
    if "application/json" not in accept and "*/*" not in accept:
        raise HTTPException(status_code=406, detail="Only application/json is supported")
    return {"data": "Success"}

func handler(w http.ResponseWriter, r *http.Request) {
    accept := r.Header.Get("Accept")
    if !strings.Contains(accept, "application/json") && accept != "*/*" {
        http.Error(w, "Not Acceptable", http.StatusNotAcceptable)
        return
    }
    w.Header().Set("Content-Type", "application/json")
    w.Write([]byte(`{"message":"ok"}`))
}

// In ASP.NET Core, if you configure MvcOptions.ReturnHttpNotAcceptable = true,
// the framework automatically returns 406 when the client requests an unsupported format.
builder.Services.AddControllers(options =>
{
    options.ReturnHttpNotAcceptable = true;
});

curl -i -X GET https://api.example.com/data \
  -H "Accept: application/xml"

Raw HTTP Response Example

HTTP/1.1 406 Not Acceptable
Content-Type: application/json
Content-Length: 51

{
  "error": "This endpoint only supports JSON"
}

Real-world Examples

Frequently Asked Questions

Developer Tips

• Unless you have a specific reason to strictly enforce content negotiation, it is often better to fall back to a default format (like JSON) instead of returning a 406.
• If you do return a 406, include a payload that lists the available representations (e.g., `["application/json"]`) to help the developer fix their request.
• Check if your web framework has a setting like `ReturnHttpNotAcceptable`. Many default to false and just send JSON regardless of the Accept header.

Interview Questions

These are questions you might face in a backend or API design interview that touch on HTTP 406.