HTTP Content Negotiation

Updated 18 Sep 2026

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What it is
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  selectinng a resource representation sing preferences or capabilities expressed by the client
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Request fields
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  Accept—media types
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  Accept-Language—languages
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  Accept-Encoding—content codings
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Media-type selection
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  omitted q defaults to $1$ ; q=0 rejects a matching representation
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  determine a representation’s effective preference using the most specific matching media range
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  then compare available representations according to the selection policy
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Example
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  image/avif;q=0,image/*;q=0.8 rejects AVIF despite accepting other images through the wildcard
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  in the assignment’s highest-q policy, image/jpeg;q=1,image/webp;q=0.1 selects JPEG when available
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Response
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  send a Content-Type matching the actual encoded bytes
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  use Vary: Accept when representation selection depends on Accept
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Caution
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  AVIF -> WebP -> JPEG is the sample implementation’s tie-break policy, not an HTTP rule
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  that implementation chooses 406 when nothing available is acceptable
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  its restricted parser and particular error policies are not a complete HTTP implementation
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References
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  https://www.rfc-editor.org/rfc/rfc9110.html#section-12.5.1

Linked references 2