diff --git a/internal/driver/ioscompanion/companionassets/embed_stub_test.go b/internal/driver/ioscompanion/companionassets/embed_stub_test.go
new file mode 100644
index 0000000..b0319f7
--- /dev/null
+++ b/internal/driver/ioscompanion/companionassets/embed_stub_test.go
@@ -0,0 +1,27 @@
+//go:build !withcompanion
+
+package companionassets
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestStubBuild_IsPlaceholder(t *testing.T) {
+	if !IsPlaceholder() {
+		t.Error("default build (no -tags withcompanion) must report a placeholder")
+	}
+	if EmbeddedSize() != 0 {
+		t.Errorf("placeholder build must embed no archive, got %d bytes", EmbeddedSize())
+	}
+}
+
+func TestStubBuild_ExtractErrors(t *testing.T) {
+	_, err := Extract(t.TempDir())
+	if err == nil {
+		t.Fatal("Extract must fail when no archive is embedded")
+	}
+	if !strings.Contains(err.Error(), "withcompanion") {
+		t.Errorf("error should tell the user to rebuild with -tags withcompanion, got %v", err)
+	}
+}
diff --git a/internal/driver/ioscompanion/companionassets/embed_withcompanion_test.go b/internal/driver/ioscompanion/companionassets/embed_withcompanion_test.go
new file mode 100644
index 0000000..51aa9b3
--- /dev/null
+++ b/internal/driver/ioscompanion/companionassets/embed_withcompanion_test.go
@@ -0,0 +1,146 @@
+//go:build withcompanion
+
+package companionassets
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestEmbeddedNonZero(t *testing.T) {
+	if EmbeddedSize() == 0 {
+		t.Errorf("expected embedded companion archive to be non-empty")
+	}
+	if IsPlaceholder() {
+		t.Errorf("withcompanion build should not be a placeholder")
+	}
+}
+
+func TestEmbeddedSHA256Matches(t *testing.T) {
+	sum := sha256.Sum256(embeddedArchive)
+	if hex.EncodeToString(sum[:]) != EmbeddedSHA256() {
+		t.Errorf("EmbeddedSHA256 does not match a fresh hash of the archive")
+	}
+}
+
+func TestExtract_WritesBinaryAndChecksum(t *testing.T) {
+	directory := t.TempDir()
+	path, err := Extract(directory)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected, err := filepath.Abs(filepath.Join(directory, "bin", companionBinaryName))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if path != expected {
+		t.Errorf("unexpected binary path: got %s want %s", path, expected)
+	}
+	checksum, err := os.ReadFile(filepath.Join(directory, "companion.sha256"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(checksum) != EmbeddedSHA256() {
+		t.Errorf("checksum file content wrong: %q", checksum)
+	}
+}
+
+func TestExtract_LayoutAndSymlinks(t *testing.T) {
+	directory := t.TempDir()
+	binaryPath, err := Extract(directory)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	info, err := os.Stat(binaryPath)
+	if err != nil {
+		t.Fatalf("companion binary missing: %v", err)
+	}
+	if info.Mode().Perm()&0o111 == 0 {
+		t.Errorf("companion binary is not executable, mode=%v", info.Mode())
+	}
+
+	frameworksDir := filepath.Join(directory, "Frameworks")
+	if stat, err := os.Stat(frameworksDir); err != nil || !stat.IsDir() {
+		t.Fatalf("Frameworks directory missing: %v", err)
+	}
+
+	expectedFrameworks := []string{
+		"FBControlCore.framework",
+		"FBDeviceControl.framework",
+		"FBSimulatorControl.framework",
+		"IDBCompanionUtilities.framework",
+		"IDBGRPCSwift.framework",
+		"XCTestBootstrap.framework",
+	}
+	for _, name := range expectedFrameworks {
+		if stat, err := os.Stat(filepath.Join(frameworksDir, name)); err != nil || !stat.IsDir() {
+			t.Errorf("expected framework %s missing: %v", name, err)
+		}
+	}
+
+	// The Versions/Current symlink must resolve to a real directory, proving
+	// symlinks survived extraction.
+	current := filepath.Join(frameworksDir, "FBControlCore.framework", "Versions", "Current")
+	linkInfo, err := os.Lstat(current)
+	if err != nil {
+		t.Fatalf("Versions/Current missing: %v", err)
+	}
+	if linkInfo.Mode()&os.ModeSymlink == 0 {
+		t.Errorf("Versions/Current is not a symlink")
+	}
+	resolved, err := os.Stat(current)
+	if err != nil || !resolved.IsDir() {
+		t.Errorf("Versions/Current does not resolve to a directory: %v", err)
+	}
+
+	// A runtime dylib that must be preserved inside FBControlCore Resources.
+	dylib := filepath.Join(frameworksDir, "FBControlCore.framework", "Versions", "A", "Resources", "libMaculator.dylib")
+	if _, err := os.Stat(dylib); err != nil {
+		t.Errorf("expected runtime dylib preserved: %v", err)
+	}
+}
+
+func TestExtract_ReusesWhenChecksumMatches(t *testing.T) {
+	directory := t.TempDir()
+	binaryPath, err := Extract(directory)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sentinel := []byte("SENTINEL-do-not-rewrite")
+	if err := os.WriteFile(binaryPath, sentinel, 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := Extract(directory); err != nil {
+		t.Fatal(err)
+	}
+
+	after, err := os.ReadFile(binaryPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(after) != string(sentinel) {
+		t.Errorf("second extract rewrote the binary; reuse branch should have skipped extraction")
+	}
+}
+
+func TestExtract_RewritesIfChecksumMissing(t *testing.T) {
+	directory := t.TempDir()
+	if _, err := Extract(directory); err != nil {
+		t.Fatal(err)
+	}
+	checksumPath := filepath.Join(directory, "companion.sha256")
+	if err := os.Remove(checksumPath); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := Extract(directory); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(checksumPath); err != nil {
+		t.Errorf("checksum should have been rewritten: %v", err)
+	}
+}