HTTP 406 Not Acceptable — Meaning, Causes & How to Fix

Code

406

Name

Not Acceptable

Class

4xx Client Error

Specification

RFC 9110 §15.5.7

Cacheable by default

Only with explicit Cache-Control / Expires

What HTTP 406 means

406 Not Acceptable is a content-negotiation failure: the client asked (via Accept, Accept-Language or Accept-Encoding) for a format the server cannot produce. Most servers prefer to send a default format instead of a 406.

Common causes

• An API client sending Accept: application/xml to a JSON-only API.
• Some security modules (such as mod_security) returning 406 for blocked requests.

How to fix it

For site owners

• Consider falling back to a default representation.
• If a WAF is producing it, review the rule that fired.

For visitors & developers

• Send an Accept header the API supports, e.g. application/json.

SEO impact

Rare for pages; crawlers send broad Accept headers. A WAF returning 406 to crawlers keeps those pages out of the index.

Example 406 response

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

{"error":"supported types: application/json"}

Related status codes

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