Accept
Indicates which content types, expressed as MIME types, the client is able to understand.
This header is defined by an external standard.
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. 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 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.
