diff --git a/scripts/bootstrap_local_tests.sh b/scripts/bootstrap_local_tests.sh
index f3770c6..4082f24 100755
--- a/scripts/bootstrap_local_tests.sh
+++ b/scripts/bootstrap_local_tests.sh
@@ -14,6 +14,14 @@ VENV_DIR="${MLX_METALLIB_VENV:-/tmp/swiftlm_mlx_venv}"
 echo "=> Building test harness (swift build --build-tests)..."
 swift build --build-tests
 
+if [ -d .build/out ]; then
+    # Swift 6.4+ layout: the metallib is embedded in each test bundle (see
+    # scripts/install-test-metallib.sh); copying one in breaks the next incremental build.
+    bash scripts/check-metal-toolchain.sh
+    echo "=> Done. Run tests with: swift test --skip-build"
+    exit 0
+fi
+
 echo "=> Installing MLX Metal library..."
 if [ ! -d "$VENV_DIR" ]; then
     python3 -m venv "$VENV_DIR"
diff --git a/scripts/check-metal-toolchain.sh b/scripts/check-metal-toolchain.sh
new file mode 100755
index 0000000..74a851c
--- /dev/null
+++ b/scripts/check-metal-toolchain.sh
@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# check-metal-toolchain.sh — fail early, with the fix, when the Metal compiler is missing.
+#
+# SwiftPM compiles the Cmlx kernels itself and embeds `default.metallib` in
+# mlx-swift_Cmlx.bundle inside every .xctest, so `swift build --build-tests` followed by
+# `swift test --skip-build` needs no hand-copied metallib. Copying one into
+# <bundle>.xctest/Contents/MacOS invalidates the bundle signature and the next incremental
+# build then fails at CodeSign. The one thing the build does need is the Metal compiler,
+# which Xcode 26+ ships as a separate download. See #128 Tier 3.
+set -euo pipefail
+
+if xcrun -find metal >/dev/null 2>&1; then
+    exit 0
+fi
+
+cat >&2 <<'MSG'
+error: the Metal Toolchain is not installed, so the MLX kernels cannot be compiled.
+Install it with:
+
+    xcodebuild -downloadComponent MetalToolchain
+MSG
+exit 1
diff --git a/scripts/install-test-metallib.sh b/scripts/install-test-metallib.sh
index 10d1ed6..8bd2a40 100755
--- a/scripts/install-test-metallib.sh
+++ b/scripts/install-test-metallib.sh
@@ -11,11 +11,22 @@
 set -euo pipefail
 cd "$(dirname "$0")/.."
 
+# Swift 6.4+ (Xcode 27) builds into .build/out/ and embeds default.metallib in
+# mlx-swift_Cmlx.bundle inside every .xctest, so there is nothing to install. Copying a
+# metallib into <bundle>.xctest/Contents/MacOS there invalidates the bundle signature and
+# the next incremental build fails at CodeSign. Older toolchains (Swift 6.2 / Xcode 26, the
+# layout CI uses: .build/arm64-apple-macosx/) do not embed one and still need the copy below.
+if [ -d .build/out ]; then
+    bash scripts/check-metal-toolchain.sh
+    echo "Swift 6.4+ build layout: metallib is embedded in the test bundles, nothing to install."
+    exit 0
+fi
+
 CONFIGS=("${1:-debug release}")
 
 find_metallib() {
     # 1. Already built by build.sh
-    for candidate in .build/*/release/mlx.metallib .build/*/release/default.metallib default.metallib; do
+    for candidate in .build/release/mlx.metallib .build/*/release/mlx.metallib .build/*/release/default.metallib default.metallib; do
         [ -f "$candidate" ] && { echo "$candidate"; return 0; }
     done
     # 2. From the mlx Python wheel, as CI does