Bootstrap a testing harness target.

Package.swift

@@ -68,29 +68,33 @@ let package = Package(
     var result = [Product]()
 
 #if os(Windows)
-    result.append(
+    result += [
       .library(
         name: "Testing",
         type: .dynamic, // needed so Windows exports ABI entry point symbols
         targets: ["Testing"]
       )
-    )
+    ]
 #else
-    result.append(
+    result += [
       .library(
         name: "Testing",
         targets: ["Testing"]
       )
-    )
+    ]
 #endif
 
-    result.append(
+    result += [
       .library(
         name: "_TestDiscovery",
         type: .static,
         targets: ["_TestDiscovery"]
-      )
-    )
+      ),
+      .executable(
+        name: "swift-testing-harness",
+        targets: ["swift-testing-harness"]
+      ),
+    ]
 
 #if DEBUG
     // Build _TestingInterop for debugging/testing purposes only. It is
@@ -116,6 +120,12 @@ let package = Package(
     // specified semantic version, meaning the most recent "prerelease" tag will
     // always be used.
     .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "604.0.0-latest"),
+
+    // Add a dependency on Swift Argument Parser. Note that the rest of the
+    // toolchain (as of this writing) uses 1.5.x, so we match that; if the
+    // toolchain updates its dependency, please update the version number here
+    // and in Sources/Harness/CMakeLists.txt.
+    .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMajor(from: "1.5.0")),
   ],
 
   targets: [
@@ -289,6 +299,19 @@ let package = Package(
       swiftSettings: .packageSettings() + .enableLibraryEvolution() + .moduleABIName("_Testing_WinSDK")
     ),
 
+    // Testing harness: a process that runs in between a host like SwiftPM and
+    // the actual test process(es) and which manages interactions between them.
+    .executableTarget(
+      name: "swift-testing-harness",
+      dependencies: [
+        "Testing",
+        .product(name: "ArgumentParser", package: "swift-argument-parser"),
+      ],
+      path: "Sources/Harness",
+      exclude: ["CMakeLists.txt"],
+      swiftSettings: .packageSettings()
+    ),
+
     // Utility targets: These are utilities intended for use when developing
     // this package, not for distribution.
     .executableTarget(
@@ -510,6 +533,7 @@ extension Array where Element: _LanguageBuildSetting {
       "SWT_NO_ABI_JSON_SCHEMA": (platforms: .none, embedded: true),
       "SWT_NO_CODABLE": (platforms: .none, embedded: true),
       "SWT_NO_INTEROP": (platforms: .none, embedded: true),
+      "SWT_NO_HARNESS": (platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android], embedded: true),
       "SWT_NO_UNSTRUCTURED_TASKS": (platforms: .none, embedded: true),
       "SWT_NO_GLOBAL_ACTORS": (platforms: .none, embedded: true),
       "SWT_NO_SUSPENDING_CLOCK": (platforms: .none, embedded: true),

Sources/CMakeLists.txt

@@ -107,4 +107,5 @@ add_subdirectory(_TestDiscovery)
 add_subdirectory(_TestingInterop)
 add_subdirectory(_TestingInternals)
 add_subdirectory(Overlays)
+add_subdirectory(Harness)
 add_subdirectory(Testing)

Sources/Harness/CMakeLists.txt

@@ -0,0 +1,29 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2026 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https://swift.org/LICENSE.txt for license information
+# See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+
+if(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
+  include(ModuleABIName)
+
+  find_package(ArgumentParser CONFIG)
+  if(NOT ArgumentParser_FOUND)
+    message("-- Vending swift-argument-parser")
+    FetchContent_Declare(ArgumentParser
+      GIT_REPOSITORY https://github.com/apple/swift-argument-parser
+      GIT_TAG 1.5.1)
+    FetchContent_MakeAvailable(ArgumentParser)
+  endif()
+
+  add_executable(swift-testing-harness
+    Harness.swift)
+  target_link_libraries(swift-testing-harness PRIVATE
+    ArgumentParser
+    Testing)
+
+  install(TARGETS swift-testing-harness
+    RUNTIME DESTINATION "libexec/swift/testing")
+endif()

Sources/Harness/Harness.swift

@@ -0,0 +1,23 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+@_spi(ForToolsIntegrationOnly) private import Testing
+private import ArgumentParser
+
+/// The harness' main command (i.e. its entry point).
+@main struct Harness: Sendable, AsyncParsableCommand {
+  fileprivate static let configuration = CommandConfiguration(
+    commandName: "swift-testing-harness"
+  )
+
+  mutating func run() async throws {
+    throw ValidationError("This tool is currently unimplemented.")
+  }
+}

cmake/modules/shared/CompilerSettings.cmake

@@ -64,3 +64,8 @@ if((NOT BUILD_SHARED_LIBS) AND (NOT CMAKE_SYSTEM_NAME STREQUAL "WASI"))
   # When building a static library, Interop is not supported at this time
   add_compile_definitions("SWT_NO_INTEROP")
 endif()
+
+set(SWT_NO_HARNESS_LIST "iOS" "watchOS" "tvOS" "visionOS" "WASI" "Android")
+if(CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST)
+  add_compile_definitions("SWT_NO_HARNESS")
+endif()