diff --git a/Sources/SWBCore/Settings/Settings.swift b/Sources/SWBCore/Settings/Settings.swift
index e2180bb4..0bfabcf0 100644
--- a/Sources/SWBCore/Settings/Settings.swift
+++ b/Sources/SWBCore/Settings/Settings.swift
@@ -4210,7 +4210,7 @@ private class SettingsBuilder {
         if scope.evaluate(BuiltinMacros.ENABLE_TESTING_SEARCH_PATHS) {
             table.push(BuiltinMacros.SYSTEM_FRAMEWORK_SEARCH_PATHS, BuiltinMacros.namespace.parseStringList(["$(inherited)", "$(TEST_FRAMEWORK_SEARCH_PATHS$(TEST_BUILD_STYLE))"]))
             table.push(BuiltinMacros.LIBRARY_SEARCH_PATHS, BuiltinMacros.namespace.parseStringList(["$(inherited)", "$(TEST_LIBRARY_SEARCH_PATHS$(TEST_BUILD_STYLE))"]))
-            table.push(BuiltinMacros.SWIFT_INCLUDE_PATHS, BuiltinMacros.namespace.parseStringList(["$(inherited)", "$(TEST_LIBRARY_SEARCH_PATHS$(TEST_BUILD_STYLE))"]))
+            table.push(BuiltinMacros.SWIFT_SYSTEM_INCLUDE_PATHS, BuiltinMacros.namespace.parseStringList(["$(inherited)", "$(TEST_LIBRARY_SEARCH_PATHS$(TEST_BUILD_STYLE))"]))
 
             // If the toolchain contains a copy of Swift Testing, prefer it.
             let toolchainPath = Path(scope.evaluateAsString(BuiltinMacros.TOOLCHAIN_DIR))
diff --git a/Sources/SWBCore/SpecImplementations/Tools/SwiftCompiler.swift b/Sources/SWBCore/SpecImplementations/Tools/SwiftCompiler.swift
index 6a495ad6..70e9ad3a 100644
--- a/Sources/SWBCore/SpecImplementations/Tools/SwiftCompiler.swift
+++ b/Sources/SWBCore/SpecImplementations/Tools/SwiftCompiler.swift
@@ -1058,6 +1058,7 @@ public struct DiscoveredSwiftCompilerToolSpecInfo: DiscoveredCommandLineToolSpec
         case constExtractCompleteMetadata = "const-extract-complete-metadata"
         case emitPackageModuleInterfacePath = "emit-package-module-interface-path"
         case compilationCaching = "compilation-caching"
+        case Isystem = "Isystem"
     }
     public var toolFeatures: ToolFeatures<FeatureFlag>
     public func hasFeature(_ flag: String) -> Bool {
@@ -1573,7 +1574,11 @@ public final class SwiftCompilerSpec : CompilerSpec, SpecIdentifierType, SwiftDi
             }
 
             for searchPath in cbc.producer.expandedSearchPaths(for: BuiltinMacros.SWIFT_SYSTEM_INCLUDE_PATHS, scope: cbc.scope) {
-                args.append(contentsOf: ["-Isystem", searchPath])
+                if LibSwiftDriver.supportsDriverFlag(spelled: "-Isystem") && toolSpecInfo.hasFeature(DiscoveredSwiftCompilerToolSpecInfo.FeatureFlag.Isystem.rawValue) {
+                    args.append(contentsOf: ["-Isystem", searchPath])
+                } else {
+                    args.append(contentsOf: ["-I", searchPath])
+                }
             }
 
             // Add -F for the effective framework search paths.
diff --git a/Tests/SWBTaskConstructionTests/SwiftTaskConstructionTests.swift b/Tests/SWBTaskConstructionTests/SwiftTaskConstructionTests.swift
index 79189a64..8899422e 100644
--- a/Tests/SWBTaskConstructionTests/SwiftTaskConstructionTests.swift
+++ b/Tests/SWBTaskConstructionTests/SwiftTaskConstructionTests.swift
@@ -214,7 +214,11 @@ fileprivate struct SwiftTaskConstructionTests: CoreBasedTests {
 
                 results.checkTask(.matchTarget(target), .matchRuleType("SwiftDriver Compilation")) { task in
                     task.checkRuleInfo(["SwiftDriver Compilation", target.target.name, "normal", "x86_64", "com.apple.xcode.tools.swift.compiler"])
-                    task.checkCommandLineContains([swiftCompilerPath.str, "-module-name", "AppTarget", "-O", "@\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.SwiftFileList", "-sdk", core.loadSDK(.macOS).path.str, "-target", "x86_64-apple-macos\(MACOSX_DEPLOYMENT_TARGET)", /* options from the xcspec which sometimes change appear here */ "-swift-version", swiftVersion, "-I", "\(SRCROOT)/build/Debug", "-I", "/tmp/include", "-Isystem", "/tmp/system/include", "-F", "\(SRCROOT)/build/Debug", "-c", "-j\(compilerParallelismLevel)", "-incremental", "-output-file-map", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-OutputFileMap.json", "-serialize-diagnostics", "-emit-dependencies", "-emit-module", "-emit-module-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.swiftmodule", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/swift-overrides.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-generated-files.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-own-target-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-all-target-headers.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-project-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/Debug/include", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources-normal/x86_64", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources", "-emit-objc-header", "-emit-objc-header-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-Swift.h", "-working-directory", SRCROOT])
+                    if LibSwiftDriver.supportsDriverFlag(spelled: "-Isystem") && swiftFeatures.has(.Isystem) {
+                        task.checkCommandLineContains([swiftCompilerPath.str, "-module-name", "AppTarget", "-O", "@\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.SwiftFileList", "-sdk", core.loadSDK(.macOS).path.str, "-target", "x86_64-apple-macos\(MACOSX_DEPLOYMENT_TARGET)", /* options from the xcspec which sometimes change appear here */ "-swift-version", swiftVersion, "-I", "\(SRCROOT)/build/Debug", "-I", "/tmp/include", "-Isystem", "/tmp/system/include", "-F", "\(SRCROOT)/build/Debug", "-c", "-j\(compilerParallelismLevel)", "-incremental", "-output-file-map", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-OutputFileMap.json", "-serialize-diagnostics", "-emit-dependencies", "-emit-module", "-emit-module-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.swiftmodule", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/swift-overrides.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-generated-files.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-own-target-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-all-target-headers.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-project-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/Debug/include", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources-normal/x86_64", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources", "-emit-objc-header", "-emit-objc-header-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-Swift.h", "-working-directory", SRCROOT])
+                    } else {
+                        task.checkCommandLineContains([swiftCompilerPath.str, "-module-name", "AppTarget", "-O", "@\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.SwiftFileList", "-sdk", core.loadSDK(.macOS).path.str, "-target", "x86_64-apple-macos\(MACOSX_DEPLOYMENT_TARGET)", /* options from the xcspec which sometimes change appear here */ "-swift-version", swiftVersion, "-I", "\(SRCROOT)/build/Debug", "-I", "/tmp/include", "-I", "/tmp/system/include", "-F", "\(SRCROOT)/build/Debug", "-c", "-j\(compilerParallelismLevel)", "-incremental", "-output-file-map", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-OutputFileMap.json", "-serialize-diagnostics", "-emit-dependencies", "-emit-module", "-emit-module-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.swiftmodule", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/swift-overrides.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-generated-files.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-own-target-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-all-target-headers.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-project-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/Debug/include", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources-normal/x86_64", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources", "-emit-objc-header", "-emit-objc-header-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-Swift.h", "-working-directory", SRCROOT])
+                    }
 
                     task.checkInputs([
                         .path("\(SRCROOT)/main.swift"),
@@ -247,7 +251,11 @@ fileprivate struct SwiftTaskConstructionTests: CoreBasedTests {
 
                 results.checkTask(.matchTarget(target), .matchRuleType("SwiftDriver Compilation Requirements")) { task in
                     task.checkRuleInfo(["SwiftDriver Compilation Requirements", target.target.name, "normal", "x86_64", "com.apple.xcode.tools.swift.compiler"])
-                    task.checkCommandLineContains([swiftCompilerPath.str, "-module-name", "AppTarget", "-O", "@\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.SwiftFileList", "-sdk", core.loadSDK(.macOS).path.str, "-target", "x86_64-apple-macos\(MACOSX_DEPLOYMENT_TARGET)", /* options from the xcspec which sometimes change appear here */ "-swift-version", swiftVersion, "-I", "\(SRCROOT)/build/Debug", "-I", "/tmp/include", "-Isystem", "/tmp/system/include", "-F", "\(SRCROOT)/build/Debug", "-c", "-j\(compilerParallelismLevel)", "-incremental", "-output-file-map", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-OutputFileMap.json", "-serialize-diagnostics", "-emit-dependencies", "-emit-module", "-emit-module-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.swiftmodule", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/swift-overrides.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-generated-files.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-own-target-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-all-target-headers.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-project-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/Debug/include", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources-normal/x86_64", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources", "-emit-objc-header", "-emit-objc-header-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-Swift.h", "-working-directory", SRCROOT])
+                    if LibSwiftDriver.supportsDriverFlag(spelled: "-Isystem") && swiftFeatures.has(.Isystem) {
+                        task.checkCommandLineContains([swiftCompilerPath.str, "-module-name", "AppTarget", "-O", "@\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.SwiftFileList", "-sdk", core.loadSDK(.macOS).path.str, "-target", "x86_64-apple-macos\(MACOSX_DEPLOYMENT_TARGET)", /* options from the xcspec which sometimes change appear here */ "-swift-version", swiftVersion, "-I", "\(SRCROOT)/build/Debug", "-I", "/tmp/include", "-Isystem", "/tmp/system/include", "-F", "\(SRCROOT)/build/Debug", "-c", "-j\(compilerParallelismLevel)", "-incremental", "-output-file-map", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-OutputFileMap.json", "-serialize-diagnostics", "-emit-dependencies", "-emit-module", "-emit-module-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.swiftmodule", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/swift-overrides.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-generated-files.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-own-target-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-all-target-headers.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-project-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/Debug/include", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources-normal/x86_64", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources", "-emit-objc-header", "-emit-objc-header-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-Swift.h", "-working-directory", SRCROOT])
+                    } else {
+                        task.checkCommandLineContains([swiftCompilerPath.str, "-module-name", "AppTarget", "-O", "@\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.SwiftFileList", "-sdk", core.loadSDK(.macOS).path.str, "-target", "x86_64-apple-macos\(MACOSX_DEPLOYMENT_TARGET)", /* options from the xcspec which sometimes change appear here */ "-swift-version", swiftVersion, "-I", "\(SRCROOT)/build/Debug", "-I", "/tmp/include", "-I", "/tmp/system/include", "-F", "\(SRCROOT)/build/Debug", "-c", "-j\(compilerParallelismLevel)", "-incremental", "-output-file-map", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-OutputFileMap.json", "-serialize-diagnostics", "-emit-dependencies", "-emit-module", "-emit-module-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget.swiftmodule", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/swift-overrides.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-generated-files.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-own-target-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-all-target-headers.hmap", "-Xcc", "-iquote", "-Xcc", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/AppTarget-project-headers.hmap", "-Xcc", "-I\(SRCROOT)/build/Debug/include", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources-normal/x86_64", "-Xcc", "-I\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/DerivedSources", "-emit-objc-header", "-emit-objc-header-path", "\(SRCROOT)/build/aProject.build/Debug/AppTarget.build/Objects-normal/x86_64/AppTarget-Swift.h", "-working-directory", SRCROOT])
+                    }
 
                     task.checkInputs([
                         .path("\(SRCROOT)/main.swift"),