Abstract API: add server transport capability request option

Sources/AHCHTTPClient/AHC+HTTPClient.swift

@@ -18,15 +18,16 @@ import Foundation
 import HTTPTypes
 import NIOCore
 import NIOHTTP1
+public import NetworkTypes
 import Synchronization
 
 @available(macOS 26.2, iOS 26.2, watchOS 26.2, tvOS 26.2, *)
 extension AsyncHTTPClient.HTTPClient: HTTPAPIs.HTTPClient {
     public typealias RequestWriter = RequestBodyWriter
     public typealias ResponseConcludingReader = ResponseReader
 
-    public struct RequestOptions: HTTPClientCapability.RequestOptions {
-
+    public struct RequestOptions: HTTPClientCapability.ServerTransportCapabilityHint {
+        public var serverSupportedTransports: Set<NetworkTypes.TransportVersion> = []
     }
 
     public struct RequestBodyWriter: AsyncWriter, ~Copyable {

Sources/HTTPAPIs/Client/HTTPClientCapability+ServerTransportCapabilityHint.swift

@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift HTTP API Proposal open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift HTTP API Proposal project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+public import NetworkTypes
+
+@available(macOS 26.2, iOS 26.2, watchOS 26.2, tvOS 26.2, visionOS 26.2, *)
+extension HTTPClientCapability {
+    /// A protocol for HTTP request options that hint at the transports a
+    /// server is known to support.
+    ///
+    /// Providing server transport information allows the client to optimize
+    /// connection establishment. For example, if a server is known to support
+    /// QUIC, the client can attempt an HTTP/3 connection directly instead of
+    /// falling back to TCP-based negotiation.
+    public protocol ServerTransportCapabilityHint: RequestOptions {
+        /// The transports that the target server is known to support.
+        ///
+        /// An empty set indicates no prior knowledge of server capabilities,
+        /// and the client uses its default protocol negotiation behavior.
+        var serverSupportedTransports: Set<TransportVersion> { get set }
+    }
+}

Sources/HTTPClient/DefaultHTTPClient.swift

@@ -137,12 +137,12 @@ public final class DefaultHTTPClient: HTTPAPIs.HTTPClient {
         options: HTTPRequestOptions,
         responseHandler: (HTTPResponse, consuming ResponseConcludingReader) async throws -> Return
     ) async throws -> Return {
-        // TODO: translate request options
-        let options = self.client.defaultRequestOptions
+        var translatedOptions = self.client.defaultRequestOptions
+        translatedOptions.serverSupportedTransports = options.serverSupportedTransports
         let body = body.map {
             HTTPClientRequestBody<ActualHTTPClient.RequestWriter>(other: $0) { RequestWriter(actual: $0) }
         }
-        return try await self.client.perform(request: request, body: body, options: options) { response, body in
+        return try await self.client.perform(request: request, body: body, options: translatedOptions) { response, body in
             try await responseHandler(response, ResponseConcludingReader(actual: body))
         }
     }

Sources/HTTPClient/HTTPRequestOptions.swift

@@ -10,9 +10,12 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+public import NetworkTypes
 
 /// The options for the default HTTP client implementation.
 @available(macOS 26.2, iOS 26.2, watchOS 26.2, tvOS 26.2, visionOS 26.2, *)
-public struct HTTPRequestOptions: HTTPClientCapability.RequestOptions {
+public struct HTTPRequestOptions: HTTPClientCapability.ServerTransportCapabilityHint {
+    public var serverSupportedTransports: Set<TransportVersion> = []
+
     public init() {}
 }

Sources/NetworkTypes/TransportVersion.swift

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift HTTP API Proposal open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift HTTP API Proposal project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// An enumeration that represents a transport protocol used to carry HTTP
+/// traffic.
+///
+/// ``TransportVersion`` provides type-safe access to supported transport
+/// protocols, allowing clients and servers to communicate transport
+/// capabilities. New transports may be added in future releases, so client
+/// code must handle unknown cases.
+@nonexhaustive
+public enum TransportVersion: Sendable, Hashable {
+    /// Plaintext TCP transport.
+    case tcp
+
+    /// TCP with TLS transport.
+    case tcpWithTLS
+
+    /// QUIC transport.
+    ///
+    /// QUIC is defined in RFC 9000 and is the transport used by HTTP/3
+    /// (RFC 9114).
+    case quic
+}

Sources/URLSessionHTTPClient/URLSessionHTTPClient.swift

@@ -279,7 +279,7 @@ public final class URLSessionHTTPClient: HTTPClient, IdleTimerEntryProvider {
         }
         request.allowsExpensiveNetworkAccess = options.allowsExpensiveNetworkAccess
         request.allowsConstrainedNetworkAccess = options.allowsConstrainedNetworkAccess
-        request.assumesHTTP3Capable = options.assumesHTTP3Capable
+        request.assumesHTTP3Capable = options.serverSupportedTransports.contains(.quic)
         if let stallTimeout = options.stallTimeout {
             request.timeoutInterval = stallTimeout / .seconds(1)
         } else {

Sources/URLSessionHTTPClient/URLSessionRequestOptions.swift

@@ -19,7 +19,8 @@ public import NetworkTypes
 public struct URLSessionRequestOptions:
     HTTPClientCapability.RedirectionHandler,
     HTTPClientCapability.TLSSecurityHandler,
-    HTTPClientCapability.TLSVersionSelection
+    HTTPClientCapability.TLSVersionSelection,
+    HTTPClientCapability.ServerTransportCapabilityHint
 {
     public var redirectionHandler: (any HTTPClientRedirectionHandler)? = nil
 
@@ -30,7 +31,7 @@ public struct URLSessionRequestOptions:
     public var maximumTLSVersion: TLSVersion = .v1_3
     public var allowsExpensiveNetworkAccess: Bool = true
     public var allowsConstrainedNetworkAccess: Bool = true
-    public var assumesHTTP3Capable: Bool = false
+    public var serverSupportedTransports: Set<TransportVersion> = []
     public var stallTimeout: Duration? = nil
 
     public init() {}