Accept-Encoding header
Baseline
Widely available
*
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
* Some parts of this feature may have varying levels of support.
The HTTP Accept-Encoding request and response header indicates the content encoding (usually a compression algorithm) that the sender can understand.
In requests, the server uses content negotiation to select one of the encoding proposals from the client and informs the client of that choice with the Content-Encoding response header.
In responses, it provides information about which content encodings the server can understand in messages to the requested resource, so that the encoding can be used in subsequent requests to the resource.
For example, Accept-Encoding is included in a 415 Unsupported Media Type response if a request to a resource (e.g., PUT) used an unsupported encoding.
Even if both the client and the server support the same compression algorithms, the server may choose not to compress the body of a response if the identity value is also acceptable.
This happens in two common cases:
- The data is already compressed, meaning a second round of compression will not reduce the transmitted data size, and may actually increase the size of the content in some cases. This is true for pre-compressed image formats (JPEG, for instance).
- The server is overloaded and cannot allocate computing resources to perform the compression. For example, Microsoft recommends not to compress if a server uses more than 80% of its computational power.
As long as the identity;q=0 or *;q=0 directives do not explicitly forbid the identity value that means no encoding, the server must never return a 406 Not Acceptable error.
Note:
IANA maintains a list of official content encodings.
The bzip and bzip2 encodings are non-standard, but may be used in some cases, particularly for legacy support.
| Header type | Request header, Response header |
|---|---|
| Forbidden request header | Yes |
Syntax
Accept-Encoding: gzip
Accept-Encoding: compress
Accept-Encoding: deflate
Accept-Encoding: br
Accept-Encoding: zstd
Accept-Encoding: dcb
Accept-Encoding: dcz
Accept-Encoding: identity
Accept-Encoding: *
// Multiple algorithms, weighted with the quality value syntax:
Accept-Encoding: deflate, gzip;q=1.0, *;q=0.5
Directives
gzip-
A compression format that uses the Lempel-Ziv coding (LZ77) with a 32-bit CRC.
compress-
A compression format that uses the Lempel-Ziv-Welch (LZW) algorithm.
deflate-
A compression format that uses the zlib structure with the deflate compression algorithm.
br-
A compression format that uses the Brotli algorithm.
zstd-
A compression format that uses the Zstandard algorithm.
dcb-
A format that uses the Dictionary-Compressed Brotli algorithm. See Compression Dictionary Transport.
dcz-
A format that uses the Dictionary-Compressed Zstandard algorithm. See Compression Dictionary Transport.
identity-
Indicates the identity function (that is, without modification or compression). This value is always considered as acceptable, even if omitted.
*(wildcard)-
Matches any content encoding not already listed in the header. This is the default value if the header is not present. This directive does not suggest that any algorithm is supported but indicates that no preference is expressed.
;q=(qvalues weighting)-
Any value is placed in an order of preference expressed using a relative quality value called weight.
Examples
Default Accept-Encoding values
Browser navigation typically has the following Accept-Encoding request header value:
GET /en-US/ HTTP/2
Host: developer.mozilla.org
Accept-Encoding: gzip, deflate, br, zstd
Weighted Accept-Encoding values
The following header shows Accept-Encoding preferences using a quality value between 0 (lowest priority) and 1 (highest-priority).
Brotli compression is weighted at 1.0, making br the client's first choice, followed by gzip at 0.8 priority, and then any other content encoding at 0.1:
Accept-Encoding: br;q=1.0, gzip;q=0.8, *;q=0.1
Specifications
| Specification |
|---|
| HTTP Semantics # field.accept-encoding |
Browser compatibility
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Accept-Encoding
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br
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dcb
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dcz
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zstd
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Legend
Tip: you can click/tap on a cell for more information.
- Full support
- Full support
- No support
- No support
- Experimental. Expect behavior to change in the future.
- See implementation notes.
See also
415 Unsupported Media Type- HTTP content negotiation
- A header with the result of the content negotiation:
Content-Encoding - Other similar headers:
TE,Accept,Accept-Language - Brotli compression
- GZip compression
- Zstandard compression
- Compression Dictionary Transport guide