diff --git a/Documentation/xtool.docc/Control.md b/Documentation/xtool.docc/Control.md
index babd0c2e..0762a726 100644
--- a/Documentation/xtool.docc/Control.md
+++ b/Documentation/xtool.docc/Control.md
@@ -60,6 +60,20 @@ For example, you might create an `Info.plist` file that looks something like
 - The default `Info.plist` has a `UISupportedInterfaceOrientations` value of portrait. This will replace it with landscape-only.
 - We don't include a `CFBundleDisplayName` by default, so this will add the key and specify a display name of "My App".
 
+## Run a step before packing
+
+When your package depends on files another tool produces, such as a binary target built by a
+different compiler, list the commands that produce them under `prepare` in `xtool.yml`.
+xtool runs them in order, in the package directory, with `/bin/sh -c`, before it plans and
+builds the app; a failing command stops the build.
+
+```yaml
+version: 1
+bundleID: com.example.Hello
+prepare:
+  - make generated-sources
+```
+
 ## Copy resources (SwiftPM)
 
 If you need to include non-code resource files, like images, the easiest way to do so is to use [SwiftPM Resources](https://developer.apple.com/documentation/xcode/bundling-resources-with-a-swift-package). These "just work" with xtool.
diff --git a/Sources/PackLib/PackSchema.swift b/Sources/PackLib/PackSchema.swift
index 297c4241..23c765f5 100644
--- a/Sources/PackLib/PackSchema.swift
+++ b/Sources/PackLib/PackSchema.swift
@@ -20,6 +20,11 @@ public struct PackSchemaBase: Codable, Sendable {
     public var iconPath: String?
     public var resources: [String]?
 
+    /// Shell commands run in the package directory before planning, in order. A command that
+    /// fails stops the build. Use it to generate inputs the package depends on, such as a
+    /// binary target built by another tool.
+    public var prepare: [String]?
+
     public var extensions: [Extension]?
 
     /// Skip auto-creating the LSP support file if it's missing.
diff --git a/Sources/PackLib/Packer.swift b/Sources/PackLib/Packer.swift
index bd2057b7..c550c213 100644
--- a/Sources/PackLib/Packer.swift
+++ b/Sources/PackLib/Packer.swift
@@ -13,6 +13,7 @@ public struct Packer: Sendable {
 
     private func build() async throws {
         let xtoolDir = URL(fileURLWithPath: "xtool")
+        let compilerRT = Self.compilerRTLibrary(for: buildSettings.triple)
         let packageDir = xtoolDir.appendingPathComponent(".xtool-tmp")
         try? FileManager.default.removeItem(at: packageDir)
         try FileManager.default.createDirectory(at: packageDir, withIntermediateDirectories: true)
@@ -38,7 +39,7 @@ public struct Packer: Sendable {
                             dependencies: [
                                 .product(name: "\($0.product)", package: "RootPackage"),
                             ],
-                            linkerSettings: \($0.linkerSettings)
+                            linkerSettings: \($0.linkerSettings(compilerRT: compilerRT))
                         )
                         """
                     }
@@ -211,6 +212,38 @@ public struct Packer: Sendable {
         }
     }
 
+    /// The compiler-rt builtins archive for `triple`, named as Apple's clang resource directory
+    /// names it: `libclang_rt.ios.a`, `libclang_rt.iossim.a`, `libclang_rt.osx.a`.
+    static func compilerRTLibraryName(for triple: String) -> String? {
+        switch cfBundleSupportedPlatforms(for: triple).first {
+        case "iPhoneOS": "libclang_rt.ios.a"
+        case "iPhoneSimulator": "libclang_rt.iossim.a"
+        case "MacOSX": "libclang_rt.osx.a"
+        default: nil
+        }
+    }
+
+    /// Apple's clang driver links the platform's compiler-rt builtins into every binary; the
+    /// Linux clang that drives the link here does not, so an app whose dependencies use
+    /// `__builtin_available` (for example a Kotlin/Native framework) fails with
+    /// `undefined symbol: ___isPlatformVersionAtLeast`. The installed Darwin SDK carries
+    /// Xcode's toolchain, so link its archive explicitly on non-Apple hosts.
+    static func compilerRTLibrary(for triple: String) -> String? {
+        #if os(macOS)
+        return nil
+        #else
+        guard let name = compilerRTLibraryName(for: triple),
+              let sdk = try? DarwinSDK.current() else { return nil }
+        let clang = sdk.bundle.appendingPathComponent(
+            "Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang", isDirectory: true
+        )
+        let versions = (try? FileManager.default.contentsOfDirectory(atPath: clang.path)) ?? []
+        return versions.sorted().lazy
+            .map { version in clang.appendingPathComponent("\(version)/lib/darwin/\(name)").path }
+            .first { FileManager.default.fileExists(atPath: $0) }
+        #endif
+    }
+
     static func cfBundleSupportedPlatforms(for triple: String) -> [String] {
         let components = triple.split(separator: "-").map(String.init)
         guard let appleIndex = components.firstIndex(of: "apple"),
@@ -233,16 +266,18 @@ public struct Packer: Sendable {
 }
 
 extension Plan.Product {
-    fileprivate var linkerSettings: String {
+    fileprivate func linkerSettings(compilerRT: String?) -> String {
+        let compilerRTFlags = compilerRT.map { "\"-Xlinker\", \"\($0)\"," } ?? ""
         switch self.type {
-        case .application: """
+        case .application: return """
         [
             .unsafeFlags([
                 "-Xlinker", "-rpath", "-Xlinker", "@executable_path/Frameworks",
+                \(compilerRTFlags)
             ]),
         ]
         """
-        case .appExtension: """
+        case .appExtension: return """
         [
             // Link to Foundation framework which implements the _NSExtensionMain entrypoint
             .linkedFramework("Foundation"),
@@ -253,6 +288,7 @@ extension Plan.Product {
                 "-Xlinker", "-rpath", "-Xlinker", "@executable_path/../../Frameworks",
                 // ...as well as our own
                 "-Xlinker", "-rpath", "-Xlinker", "@executable_path/Frameworks",
+                \(compilerRTFlags)
             ]),
         ]
         """
diff --git a/Sources/XToolSupport/DevCommand.swift b/Sources/XToolSupport/DevCommand.swift
index 369bfa8d..624ed8d3 100644
--- a/Sources/XToolSupport/DevCommand.swift
+++ b/Sources/XToolSupport/DevCommand.swift
@@ -1,6 +1,7 @@
 import Foundation
 import ArgumentParser
 import PackLib
+import Subprocess
 import XKit
 import Dependencies
 import XUtils
@@ -54,6 +55,21 @@ struct PackOperation {
 
         await LSPConfig.tryEnsure(using: schema)
 
+        for command in schema.prepare ?? [] {
+            print("Preparing: \(command)")
+            try await Subprocess.run(
+                Configuration(
+                    executable: .path("/bin/sh"),
+                    arguments: ["-c", command],
+                    environment: .inherit,
+                    platformOptions: .withGracefulShutDown,
+                ),
+                output: .currentStandardOutput,
+                error: .currentStandardError,
+            )
+            .checkSuccess()
+        }
+
         let buildSettings = try await BuildSettings(
             configuration: buildOptions.configuration,
             triple: triple ?? Self.defaultTriple,
diff --git a/Tests/XToolTests/PackerTests.swift b/Tests/XToolTests/PackerTests.swift
index 5aabda83..09b61158 100644
--- a/Tests/XToolTests/PackerTests.swift
+++ b/Tests/XToolTests/PackerTests.swift
@@ -13,3 +13,9 @@ import Testing
 
     #expect(Packer.cfBundleSupportedPlatforms(for: "unknown-triple") == ["iPhoneOS"])
 }
+
+@Test func compilerRTLibraryNameFollowsBuildTriple() {
+    #expect(Packer.compilerRTLibraryName(for: "arm64-apple-ios") == "libclang_rt.ios.a")
+    #expect(Packer.compilerRTLibraryName(for: "arm64-apple-ios17.0-simulator") == "libclang_rt.iossim.a")
+    #expect(Packer.compilerRTLibraryName(for: "arm64-apple-macosx") == "libclang_rt.osx.a")
+}