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Add DecompressionMethods.Zstandard for HTTP automatic decompression #123531

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Add DecompressionMethods.Zstandard for HTTP automatic decompression #123531

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d681162
Initial plan
Copilot Jan 23, 2026
c4db1e4
Add DecompressionMethods.Zstandard enum and handler support
Copilot Jan 23, 2026
144e380
Add Zstandard decompression tests
Copilot Jan 23, 2026
e2b4db7
Add System.IO.Compression.Zstandard project reference
Copilot Jan 23, 2026
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31 changes: 28 additions & 3 deletions src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.Decompression.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,7 @@ public abstract class HttpClientHandler_Decompression_Test : HttpClientHandlerTe
public HttpClientHandler_Decompression_Test(ITestOutputHelper output) : base(output) { }

public static IEnumerable<object[]> DecompressedResponse_MethodSpecified_DecompressedContentReturned_MemberData() =>
from compressionName in new[] { "gzip", "GZIP", "zlib", "ZLIB", "deflate", "DEFLATE", "br", "BR" }
from compressionName in new[] { "gzip", "GZIP", "zlib", "ZLIB", "deflate", "DEFLATE", "br", "BR", "zstd", "ZSTD" }
from all in new[] { false, true }
from copyTo in new[] { false, true }
from contentLength in new[] { 0, 1, 12345 }
Expand All @@ -40,9 +40,9 @@ public static IEnumerable<object[]> DecompressedResponse_MethodSpecified_Decompr
public async Task DecompressedResponse_MethodSpecified_DecompressedContentReturned(string compressionName, bool all, bool useCopyTo, int contentLength)
{
if (IsWinHttpHandler &&
(compressionName is "br" or "BR" or "zlib" or "ZLIB"))
(compressionName is "br" or "BR" or "zlib" or "ZLIB" or "zstd" or "ZSTD"))
{
// brotli and zlib not supported on WinHttpHandler
// brotli, zlib, and zstd not supported on WinHttpHandler
return;
}

Expand All @@ -64,6 +64,12 @@ public async Task DecompressedResponse_MethodSpecified_DecompressedContentReturn
methods = all ? DecompressionMethods.Brotli : _all;
break;

case "zstd":
case "ZSTD":
compress = s => new ZstandardStream(s, CompressionLevel.Optimal, leaveOpen: true);
methods = all ? DecompressionMethods.Zstandard : _all;
break;

case "zlib":
case "ZLIB":
compress = s => new ZLibStream(s, CompressionLevel.Optimal, leaveOpen: true);
Expand Down Expand Up @@ -133,6 +139,13 @@ public static IEnumerable<object[]> DecompressedResponse_MethodNotSpecified_Orig
DecompressionMethods.Deflate | DecompressionMethods.GZip,
useCopyTo
};
yield return new object[]
{
"zstd",
new Func<Stream, Stream>(s => new ZstandardStream(s, CompressionLevel.Optimal, leaveOpen: true)),
DecompressionMethods.Deflate | DecompressionMethods.GZip | DecompressionMethods.Brotli,
useCopyTo
};
#endif
}
}
Expand Down Expand Up @@ -177,6 +190,7 @@ await server.AcceptConnectionAsync(async connection =>
#if !NETFRAMEWORK
[InlineData("deflate", DecompressionMethods.Deflate)]
[InlineData("br", DecompressionMethods.Brotli)]
[InlineData("zstd", DecompressionMethods.Zstandard)]
#endif
[SkipOnPlatform(TestPlatforms.Browser, "AutomaticDecompression not supported on Browser")]
public async Task DecompressedResponse_EmptyBody_Success(string encodingName, DecompressionMethods methods)
Expand Down Expand Up @@ -205,6 +219,10 @@ await server.AcceptConnectionAsync(async connection =>
[InlineData(DecompressionMethods.Brotli, "br", "br")]
[InlineData(DecompressionMethods.Brotli, "br", "gzip")]
[InlineData(DecompressionMethods.Brotli, "br", "gzip, deflate")]
[InlineData(DecompressionMethods.Zstandard, "zstd", "")]
[InlineData(DecompressionMethods.Zstandard, "zstd", "zstd")]
[InlineData(DecompressionMethods.Zstandard, "zstd", "gzip")]
[InlineData(DecompressionMethods.Zstandard, "zstd", "gzip, deflate, br")]
#endif
[InlineData(DecompressionMethods.GZip, "gzip", "")]
[InlineData(DecompressionMethods.Deflate, "deflate", "")]
Expand All @@ -228,6 +246,12 @@ public async Task GetAsync_SetAutomaticDecompression_AcceptEncodingHeaderSentWit
return;
}

// Zstandard only supported on SocketsHttpHandler.
if (IsWinHttpHandler && (encodings.Contains("zstd") || manualAcceptEncodingHeaderValues.Contains("zstd")))
{
return;
}

await LoopbackServer.CreateServerAsync(async (server, url) =>
{
HttpClientHandler handler = CreateHttpClientHandler();
Expand Down Expand Up @@ -266,6 +290,7 @@ await LoopbackServer.CreateServerAsync(async (server, url) =>
[Theory]
#if NET
[InlineData(DecompressionMethods.GZip | DecompressionMethods.Deflate | DecompressionMethods.Brotli, "gzip; q=1.0, deflate; q=1.0, br; q=1.0", "")]
[InlineData(DecompressionMethods.GZip | DecompressionMethods.Deflate | DecompressionMethods.Brotli | DecompressionMethods.Zstandard, "gzip; q=1.0, deflate; q=1.0, br; q=1.0, zstd; q=1.0", "")]
#endif
[InlineData(DecompressionMethods.GZip | DecompressionMethods.Deflate, "gzip; q=1.0, deflate; q=1.0", "")]
[InlineData(DecompressionMethods.GZip | DecompressionMethods.Deflate, "gzip; q=1.0", "deflate")]
Expand Down
1 change: 1 addition & 0 deletions src/libraries/System.Net.Http/src/System.Net.Http.csproj
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Expand Up @@ -486,6 +486,7 @@
<ProjectReference Include="$(LibrariesProjectRoot)System.Threading.Channels\src\System.Threading.Channels.csproj" />
<ProjectReference Include="$(LibrariesProjectRoot)System.Threading.ThreadPool\src\System.Threading.ThreadPool.csproj" />
<ProjectReference Include="$(LibrariesProjectRoot)System.IO.Compression.Brotli\src\System.IO.Compression.Brotli.csproj" />
<ProjectReference Include="$(LibrariesProjectRoot)System.IO.Compression.Zstandard\src\System.IO.Compression.Zstandard.csproj" />
</ItemGroup>

<ItemGroup Condition="'$(TargetPlatformIdentifier)' != 'browser' and '$(TargetPlatformIdentifier)' != 'wasi' ">
Expand Down
20 changes: 19 additions & 1 deletion src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/DecompressionHandler.cs
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Expand Up @@ -19,9 +19,11 @@ internal sealed class DecompressionHandler : HttpMessageHandlerStage
private const string Gzip = "gzip";
private const string Deflate = "deflate";
private const string Brotli = "br";
private const string Zstd = "zstd";
private static readonly StringWithQualityHeaderValue s_gzipHeaderValue = new(Gzip);
private static readonly StringWithQualityHeaderValue s_deflateHeaderValue = new(Deflate);
private static readonly StringWithQualityHeaderValue s_brotliHeaderValue = new(Brotli);
private static readonly StringWithQualityHeaderValue s_zstdHeaderValue = new(Zstd);

/// <summary>Header value for all enabled decompression methods, e.g. "gzip, deflate".</summary>
private readonly string _acceptEncodingHeaderValue;
Expand All @@ -34,14 +36,15 @@ public DecompressionHandler(DecompressionMethods decompressionMethods, HttpMessa
_decompressionMethods = decompressionMethods;
_innerHandler = innerHandler;

List<string?> methods = [GZipEnabled ? Gzip : null, DeflateEnabled ? Deflate : null, BrotliEnabled ? Brotli : null];
List<string?> methods = [GZipEnabled ? Gzip : null, DeflateEnabled ? Deflate : null, BrotliEnabled ? Brotli : null, ZstandardEnabled ? Zstd : null];
methods.RemoveAll(item => item is null);
_acceptEncodingHeaderValue = string.Join(", ", methods);
Comment on lines +39 to 41
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@stephentoub stephentoub Jan 23, 2026

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Span<string?> methods = ["", "", "", ""];
int count = 0;
if (GZipEnabled) methods[count++] = Gzip;
if (DeflateEnabled) methods[count++] = Deflate;
if (BrotliEnabled) methods[count++] = Brotli;
if (ZstandardEnabled) methods[count++] = Zstandard;
_acceptEncodingHeaderValue = string.Join(", ", methods.Slice(0, count);

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@stephentoub stephentoub Jan 23, 2026

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@copilot

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Copilot AI Jan 23, 2026

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The comment appears incomplete - could you clarify what change you'd like to see for lines 39-41? Are you suggesting to use Span<string> with stackalloc instead of the List pattern?

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@stephentoub stephentoub Jan 23, 2026

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@copilot, incomplete how? I'm suggesting the code should look like this:

Span<string?> methods = ["", "", "", ""];
int count = 0;
if (GZipEnabled) methods[count++] = Gzip;
if (DeflateEnabled) methods[count++] = Deflate;
if (BrotliEnabled) methods[count++] = Brotli;
if (ZstandardEnabled) methods[count++] = Zstandard;
_acceptEncodingHeaderValue = string.Join(", ", methods.Slice(0, count);

It initializes a span with a collection expression, ensuring enough space for the maximum number of encodings. Then it fills in just the encodings that are enabled. And then it concatenates those strings into a single string.

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The code suggestion appears to be cut off after "Span" - could you provide the complete code you'd like to see for lines 39-41?

}

internal bool GZipEnabled => (_decompressionMethods & DecompressionMethods.GZip) != 0;
internal bool DeflateEnabled => (_decompressionMethods & DecompressionMethods.Deflate) != 0;
internal bool BrotliEnabled => (_decompressionMethods & DecompressionMethods.Brotli) != 0;
internal bool ZstandardEnabled => (_decompressionMethods & DecompressionMethods.Zstandard) != 0;

private static bool EncodingExists(HttpHeaderValueCollection<StringWithQualityHeaderValue> acceptEncodingHeader, string encoding)
{
Expand Down Expand Up @@ -81,6 +84,11 @@ internal override async ValueTask<HttpResponseMessage> SendAsync(HttpRequestMess
{
acceptEncoding.Add(s_brotliHeaderValue);
}

if (ZstandardEnabled && !EncodingExists(acceptEncoding, Zstd))
{
acceptEncoding.Add(s_zstdHeaderValue);
}
}

HttpResponseMessage response = await _innerHandler.SendAsync(request, async, cancellationToken).ConfigureAwait(false);
Expand All @@ -105,6 +113,10 @@ internal override async ValueTask<HttpResponseMessage> SendAsync(HttpRequestMess
{
response.Content = new BrotliDecompressedContent(response.Content, encodings);
}
else if (ZstandardEnabled && string.Equals(last, Zstd, StringComparison.OrdinalIgnoreCase))
{
response.Content = new ZstandardDecompressedContent(response.Content, encodings);
}
}

return response;
Expand Down Expand Up @@ -430,5 +442,11 @@ private sealed class BrotliDecompressedContent(HttpContent originalContent, stri
protected override Stream GetDecompressedStream(Stream originalStream) =>
new BrotliStream(originalStream, CompressionMode.Decompress);
}

private sealed class ZstandardDecompressedContent(HttpContent originalContent, string[] contentEncodings) : DecompressedContent(originalContent, contentEncodings)
{
protected override Stream GetDecompressedStream(Stream originalStream) =>
new ZstandardStream(originalStream, CompressionMode.Decompress);
}
}
}
2 changes: 1 addition & 1 deletion src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/SocketsHttpHandler.cs
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Expand Up @@ -569,7 +569,7 @@ private void EnsureDecompressionHandlerFactory()
}

// Not stored as a constant on the DecompressionHandler to allow it to get trimmed.
private const DecompressionMethods SupportedDecompressionMethods = DecompressionMethods.GZip | DecompressionMethods.Deflate | DecompressionMethods.Brotli;
private const DecompressionMethods SupportedDecompressionMethods = DecompressionMethods.GZip | DecompressionMethods.Deflate | DecompressionMethods.Brotli | DecompressionMethods.Zstandard;

protected internal override HttpResponseMessage Send(HttpRequestMessage request,
CancellationToken cancellationToken)
Expand Down
1 change: 1 addition & 0 deletions src/libraries/System.Net.Primitives/ref/System.Net.Primitives.cs
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Expand Up @@ -120,6 +120,7 @@ public enum DecompressionMethods
GZip = 1,
Deflate = 2,
Brotli = 4,
Zstandard = 8,
}
public partial class DnsEndPoint : System.Net.EndPoint
{
Expand Down
1 change: 1 addition & 0 deletions src/libraries/System.Net.Primitives/src/System/Net/DecompressionMethods.cs
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Expand Up @@ -10,6 +10,7 @@ public enum DecompressionMethods
GZip = 0x1,
Deflate = 0x2,
Brotli = 0x4,
Zstandard = 0x8,
All = ~None
}
}
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