---
title: Accept
summary: null
url: https://www.fastly.com/documentation/reference/http/http-headers/Accept
---

The `Accept` request HTTP header indicates which content formats, expressed as MIME types, the client is able to understand. A server can use this information to select a suitable format for the response and informs the client of the choice with the `Content-Type` response header. Browsers set required values for this header based on the context of the request. For example, a browser uses different values in a request when fetching a CSS stylesheet, compared with an image, video, or an HTML page.

> **HINT:** `Accept`, `Accept-Encoding` and `Accept-Language` are designed to allow for [content negotiation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation).  For content negotiation to work properly on content served through Fastly, the `Vary` header must be correctly set.  See our blog post [Getting the most out of Vary with Fastly](https://www.fastly.com/blog/getting-most-out-vary-fastly) for a discussion of the best practices and patterns that you may find useful.

Fastly ignores this header but preserves it on requests.  Origin servers that take notice of the value of `Accept` on any request to a given URL and respond with content in a format matching that requested must include a `Vary: Accept` on all responses for requests to that same URL.  This enables Fastly to store each differently-formatted version of the content separately and ensure each user receives the right one.

A common use case for content-negotiation based on `Accept` is to return an HTML "single page app" in response to a browser page navigation request, and subsequently to return JSON data in response to a JavaScript client within the single-page-app fetching the same URL with an `Accept: application/json` header.

## Normalization

The number of possible permutations of `Accept` values is very large, and if responses include `Vary: Accept`, cache performance may noticeably decline.  Normalizing the value of `Accept` on inbound requests can dramatically improve cache hit ratios in this situation.

Fastly does not normalize `Accept` by default, but we provide utilities to make it easy to do so in VCL services.  See `accept.charset_lookup` and `accept.media_lookup` for more details.