diff --git a/Sources/Overlays/_Testing_Foundation/Attachments/Attachment+URL.swift b/Sources/Overlays/_Testing_Foundation/Attachments/Attachment+URL.swift
index d70641c6..815fcfd1 100644
--- a/Sources/Overlays/_Testing_Foundation/Attachments/Attachment+URL.swift
+++ b/Sources/Overlays/_Testing_Foundation/Attachments/Attachment+URL.swift
@@ -16,6 +16,10 @@ public import Foundation
 private import UniformTypeIdentifiers
 #endif
 
+#if !SWT_NO_PROCESS_SPAWNING && os(Windows)
+private import WinSDK
+#endif
+
 #if !SWT_NO_FILE_IO
 extension URL {
   /// The file system path of the URL, equivalent to `path`.
@@ -32,17 +36,13 @@ extension URL {
   }
 }
 
-#if SWT_TARGET_OS_APPLE && canImport(UniformTypeIdentifiers)
-@available(_uttypesAPI, *)
-extension UTType {
-  /// A type that represents a `.tgz` archive, or `nil` if the system does not
-  /// recognize that content type.
-  fileprivate static let tgz = UTType("org.gnu.gnu-zip-tar-archive")
-}
-#endif
-
 @_spi(Experimental)
 extension Attachment where AttachableValue == Data {
+#if SWT_TARGET_OS_APPLE
+  /// An operation queue to use for asynchronously reading data from disk.
+  private static let _operationQueue = OperationQueue()
+#endif
+
   /// Initialize an instance of this type with the contents of the given URL.
   ///
   /// - Parameters:
@@ -65,8 +65,6 @@ extension Attachment where AttachableValue == Data {
       throw CocoaError(.featureUnsupported, userInfo: [NSLocalizedDescriptionKey: "Attaching downloaded files is not supported"])
     }
 
-    // FIXME: use NSFileCoordinator on Darwin?
-
     let url = url.resolvingSymlinksInPath()
     let isDirectory = try url.resourceValues(forKeys: [.isDirectoryKey]).isDirectory!
 
@@ -83,79 +81,178 @@ extension Attachment where AttachableValue == Data {
       // Ensure the preferred name of the archive has an appropriate extension.
       preferredName = {
 #if SWT_TARGET_OS_APPLE && canImport(UniformTypeIdentifiers)
-        if #available(_uttypesAPI, *), let tgz = UTType.tgz {
-          return (preferredName as NSString).appendingPathExtension(for: tgz)
+        if #available(_uttypesAPI, *) {
+          return (preferredName as NSString).appendingPathExtension(for: .zip)
         }
 #endif
-        return (preferredName as NSString).appendingPathExtension("tgz") ?? preferredName
+        return (preferredName as NSString).appendingPathExtension("zip") ?? preferredName
       }()
+    }
 
-      try await self.init(Data(compressedContentsOfDirectoryAt: url), named: preferredName, sourceLocation: sourceLocation)
+#if SWT_TARGET_OS_APPLE
+    let data: Data = try await withCheckedThrowingContinuation { continuation in
+      let fileCoordinator = NSFileCoordinator()
+      let fileAccessIntent = NSFileAccessIntent.readingIntent(with: url, options: [.forUploading])
+
+      fileCoordinator.coordinate(with: [fileAccessIntent], queue: Self._operationQueue) { error in
+        let result = Result {
+          if let error {
+            throw error
+          }
+          return try Data(contentsOf: fileAccessIntent.url, options: [.mappedIfSafe])
+        }
+        continuation.resume(with: result)
+      }
+    }
+#else
+    let data = if isDirectory {
+      try await _compressContentsOfDirectory(at: url)
     } else {
       // Load the file.
-      try self.init(Data(contentsOf: url, options: [.mappedIfSafe]), named: preferredName, sourceLocation: sourceLocation)
+      try Data(contentsOf: url, options: [.mappedIfSafe])
     }
+#endif
+
+    self.init(data, named: preferredName, sourceLocation: sourceLocation)
   }
 }
 
-// MARK: - Attaching directories
+#if !SWT_NO_PROCESS_SPAWNING && os(Windows)
+/// The filename of the archiver tool.
+private let _archiverName = "tar.exe"
 
-extension Data {
-  /// Initialize an instance of this type by compressing the contents of a
-  /// directory.
-  ///
-  /// - Parameters:
-  ///   - directoryURL: A URL referring to the directory to attach.
-  ///
-  /// - Throws: Any error encountered trying to compress the directory, or if
-  ///   directories cannot be compressed on this platform.
-  ///
-  /// This initializer asynchronously compresses the contents of `directoryURL`
-  /// into an archive (currently of `.tgz` format, although this is subject to
-  /// change) and stores a mapped copy of that archive.
-  init(compressedContentsOfDirectoryAt directoryURL: URL) async throws {
-    let temporaryName = "\(UUID().uuidString).tgz"
-    let temporaryURL = FileManager.default.temporaryDirectory.appendingPathComponent(temporaryName)
+/// The path to the archiver tool.
+///
+/// This path refers to a file (named `_archiverName`) within the `"System32"`
+/// folder of the current system, which is not always located in `"C:\Windows."`
+///
+/// If the path cannot be determined, the value of this property is `nil`.
+private let _archiverPath: String? = {
+  let bufferCount = GetSystemDirectoryW(nil, 0)
+  guard bufferCount > 0 else {
+    return nil
+  }
+
+  return withUnsafeTemporaryAllocation(of: wchar_t.self, capacity: Int(bufferCount)) { buffer -> String? in
+    let bufferCount = GetSystemDirectoryW(buffer.baseAddress!, UINT(buffer.count))
+    guard bufferCount > 0 && bufferCount < buffer.count else {
+      return nil
+    }
 
+    return _archiverName.withCString(encodedAs: UTF16.self) { archiverName -> String? in
+      var result: UnsafeMutablePointer<wchar_t>?
+
+      let flags = ULONG(PATHCCH_ALLOW_LONG_PATHS.rawValue)
+      guard S_OK == PathAllocCombine(buffer.baseAddress!, archiverName, flags, &result) else {
+        return nil
+      }
+      defer {
+        LocalFree(result)
+      }
+
+      return result.flatMap { String.decodeCString($0, as: UTF16.self)?.result }
+    }
+  }
+}()
+#endif
+
+/// Compress the contents of a directory to an archive, then map that archive
+/// back into memory.
+///
+/// - Parameters:
+///   - directoryURL: A URL referring to the directory to attach.
+///
+/// - Returns: An instance of `Data` containing the compressed contents of the
+///   given directory.
+///
+/// - Throws: Any error encountered trying to compress the directory, or if
+///   directories cannot be compressed on this platform.
+///
+/// This function asynchronously compresses the contents of `directoryURL` into
+/// an archive (currently of `.zip` format, although this is subject to change.)
+private func _compressContentsOfDirectory(at directoryURL: URL) async throws -> Data {
 #if !SWT_NO_PROCESS_SPAWNING
-#if os(Windows)
-    let tarPath = #"C:\Windows\System32\tar.exe"#
+  let temporaryName = "\(UUID().uuidString).zip"
+  let temporaryURL = FileManager.default.temporaryDirectory.appendingPathComponent(temporaryName)
+  defer {
+    try? FileManager().removeItem(at: temporaryURL)
+  }
+
+  // The standard version of tar(1) does not (appear to) support writing PKZIP
+  // archives. FreeBSD's (AKA bsdtar) was long ago rebased atop libarchive and
+  // knows how to write PKZIP archives, while Windows inherited FreeBSD's tar
+  // tool in Windows 10 Build 17063 (per https://techcommunity.microsoft.com/blog/containers/tar-and-curl-come-to-windows/382409).
+  //
+  // On Linux (which does not have FreeBSD's version of tar(1)), we can use
+  // zip(1) instead.
+#if os(Linux)
+  let archiverPath = "/usr/bin/zip"
+#elseif SWT_TARGET_OS_APPLE || os(FreeBSD)
+  let archiverPath = "/usr/bin/tar"
+#elseif os(Windows)
+  guard let archiverPath = _archiverPath else {
+    throw CocoaError(.fileWriteUnknown, userInfo: [
+      NSLocalizedDescriptionKey: "Could not determine the path to '\(_archiverName)'.",
+    ])
+  }
 #else
-    let tarPath = "/usr/bin/tar"
+#warning("Platform-specific implementation missing: tar or zip tool unavailable")
+  let archiverPath = ""
+  throw CocoaError(.featureUnsupported, userInfo: [NSLocalizedDescriptionKey: "This platform does not support attaching directories to tests."])
 #endif
+
+  try await withCheckedThrowingContinuation { continuation in
+    let process = Process()
+
+    process.executableURL = URL(fileURLWithPath: archiverPath, isDirectory: false)
+
     let sourcePath = directoryURL.fileSystemPath
     let destinationPath = temporaryURL.fileSystemPath
-    defer {
-      try? FileManager().removeItem(at: temporaryURL)
-    }
+#if os(Linux)
+    // The zip command constructs relative paths from the current working
+    // directory rather than from command-line arguments.
+    process.arguments = [destinationPath, "--recurse-paths", "."]
+    process.currentDirectoryURL = directoryURL
+#elseif SWT_TARGET_OS_APPLE || os(FreeBSD)
+    process.arguments = ["--create", "--auto-compress", "--directory", sourcePath, "--file", destinationPath, "."]
+#elseif os(Windows)
+    // The Windows version of bsdtar can handle relative paths for other archive
+    // formats, but produces empty archives when inferring the zip format with
+    // --auto-compress, so archive with absolute paths here.
+    //
+    // An alternative may be to use PowerShell's Compress-Archive command,
+    // however that comes with a security risk as we'd be responsible for two
+    // levels of command-line argument escaping.
+    process.arguments = ["--create", "--auto-compress", "--file", destinationPath, sourcePath]
+#endif
 
-    try await withCheckedThrowingContinuation { continuation in
-      do {
-        _ = try Process.run(
-          URL(fileURLWithPath: tarPath, isDirectory: false),
-          arguments: ["--create", "--gzip", "--directory", sourcePath, "--file", destinationPath, "."]
-        ) { process in
-          let terminationReason = process.terminationReason
-          let terminationStatus = process.terminationStatus
-          if terminationReason == .exit && terminationStatus == EXIT_SUCCESS {
-            continuation.resume()
-          } else {
-            let error = CocoaError(.fileWriteUnknown, userInfo: [
-              NSLocalizedDescriptionKey: "The directory at '\(sourcePath)' could not be compressed.",
-            ])
-            continuation.resume(throwing: error)
-          }
-        }
-      } catch {
+    process.standardOutput = nil
+    process.standardError = nil
+
+    process.terminationHandler = { process in
+      let terminationReason = process.terminationReason
+      let terminationStatus = process.terminationStatus
+      if terminationReason == .exit && terminationStatus == EXIT_SUCCESS {
+        continuation.resume()
+      } else {
+        let error = CocoaError(.fileWriteUnknown, userInfo: [
+          NSLocalizedDescriptionKey: "The directory at '\(sourcePath)' could not be compressed (\(terminationStatus)).",
+        ])
         continuation.resume(throwing: error)
       }
     }
 
-    try self.init(contentsOf: temporaryURL, options: [.mappedIfSafe])
+    do {
+      try process.run()
+    } catch {
+      continuation.resume(throwing: error)
+    }
+  }
+
+  return try Data(contentsOf: temporaryURL, options: [.mappedIfSafe])
 #else
-    throw CocoaError(.featureUnsupported, userInfo: [NSLocalizedDescriptionKey: "This platform does not support attaching directories to tests."])
+  throw CocoaError(.featureUnsupported, userInfo: [NSLocalizedDescriptionKey: "This platform does not support attaching directories to tests."])
 #endif
-  }
 }
 #endif
 #endif
diff --git a/Tests/TestingTests/AttachmentTests.swift b/Tests/TestingTests/AttachmentTests.swift
index 431e08d2..e9a50f0d 100644
--- a/Tests/TestingTests/AttachmentTests.swift
+++ b/Tests/TestingTests/AttachmentTests.swift
@@ -275,7 +275,13 @@ struct AttachmentTests {
           return
         }
 
-        #expect(attachment.preferredName == "\(temporaryDirectoryName).tgz")
+        #expect(attachment.preferredName == "\(temporaryDirectoryName).zip")
+        try! attachment.withUnsafeBufferPointer { buffer in
+          #expect(buffer.count > 32)
+          #expect(buffer[0] == UInt8(ascii: "P"))
+          #expect(buffer[1] == UInt8(ascii: "K"))
+          #expect(buffer.contains("loremipsum.txt".utf8))
+        }
         valueAttached()
       }