diff --git a/Sources/XToolSupport/SDKBuilder.swift b/Sources/XToolSupport/SDKBuilder.swift
index 09af2dd7..59042163 100644
--- a/Sources/XToolSupport/SDKBuilder.swift
+++ b/Sources/XToolSupport/SDKBuilder.swift
@@ -234,14 +234,7 @@ struct SDKBuilder {
     }
 
     // swiftlint:disable:next function_body_length
-    private func installToolset(in output: URL) async throws {
-        let toolsetDir = output.appendingPathComponent("toolset")
-
-        try FileManager.default.createDirectory(
-            at: toolsetDir,
-            withIntermediateDirectories: false
-        )
-
+    private func downloadToolset(into toolsetDir: URL) async throws {
         @Dependency(\.httpClient) var httpClient
         let url = URL(string: """
         https://github.com/xtool-org/darwin-tools-linux-llvm/releases/download/\
@@ -280,6 +273,30 @@ struct SDKBuilder {
             try await execution.standardInputWriter.finish()
         }
         .checkSuccess()
+    }
+
+    private func installToolset(in output: URL) async throws {
+        let toolsetDir = output.appendingPathComponent("toolset")
+
+        try FileManager.default.createDirectory(
+            at: toolsetDir,
+            withIntermediateDirectories: false
+        )
+
+        if let local = ProcessInfo.processInfo.environment["XTOOL_TOOLSET_TARBALL"] {
+            // absolute: tar runs in toolsetDir, where a relative path would point
+            let tarball = URL(fileURLWithPath: local).standardizedFileURL.path
+            try await Subprocess.run(
+                .name("tar"),
+                arguments: ["xzf", tarball],
+                workingDirectory: FilePath(toolsetDir),
+                output: .discarded,
+                error: .discarded,
+            )
+            .checkSuccess()
+        } else {
+            try await downloadToolset(into: toolsetDir)
+        }
 
         // swift-build assumes we have an Apple-flavored librarian for Apple platforms
         // (https://github.com/swiftlang/swift-build/blob/b2433e74e/Sources/SWBCore/SpecImplementations/Tools/LinkerTools.swift#L1735)
@@ -386,6 +403,15 @@ struct SDKBuilder {
     }
 
     private func installMacros(in output: URL) async throws {
+        let outputPath = output.appendingPathComponent("OpenAppleMacrosServer").path
+        if let local = ProcessInfo.processInfo.environment["XTOOL_OAM_SERVER"] {
+            // copyItem copies a symlink as a symlink (it may be relative): copy its target
+            let source = URL(fileURLWithPath: local).resolvingSymlinksInPath().path
+            try FileManager.default.copyItem(atPath: source, toPath: outputPath)
+            // the source may be read-only (e.g. in /nix/store); the copy keeps its mode
+            try FileManager.default.setAttributes([.posixPermissions: 0o755], ofItemAtPath: outputPath)
+            return
+        }
         @Dependency(\.httpClient) var httpClient
         let url = URL(string: """
         https://github.com/xtool-org/OpenAppleMacros/releases/download/\
@@ -400,7 +426,6 @@ struct SDKBuilder {
         case .unknown: nil
         }
 
-        let outputPath = output.appendingPathComponent("OpenAppleMacrosServer").path
         do {
             let handle = try FileDescriptor.open(
                 FilePath(outputPath),