diff --git a/.github/actions/prepare_environment/action.yml b/.github/actions/prepare_environment/action.yml
index 358070bd..3aea9322 100644
--- a/.github/actions/prepare_environment/action.yml
+++ b/.github/actions/prepare_environment/action.yml
@@ -96,7 +96,14 @@ runs:
           # Try to install warp-channel-config but don't complain if it cannot
           # be installed.
           ./script/install_channel_config || true
-          if ${{ inputs.install_release_deps == 'true' }}; then
+          if ${{ inputs.install_test_deps == 'true' }}; then
+            # Test deps include cargo-nextest (the test harness CI uses).
+            # The linux branch below already handles this; macOS used
+            # to fall through to install_cargo_build_deps, which doesn't
+            # install nextest -- so `cargo nextest run` blew up later
+            # with "no such command: nextest".
+            ./script/install_cargo_test_deps
+          elif ${{ inputs.install_release_deps == 'true' }}; then
             ./script/install_cargo_release_deps
           else
             ./script/install_cargo_build_deps
diff --git a/script/macos/install_build_deps b/script/macos/install_build_deps
index 1a264643..b06668c9 100755
--- a/script/macos/install_build_deps
+++ b/script/macos/install_build_deps
@@ -2,4 +2,20 @@
 #
 # Installs macOS-specific build dependencies required to build Warp.
 
-xcodebuild -downloadComponent MetalToolchain
+# `xcodebuild -downloadComponent` was added in Xcode 15.3. On stock
+# GitHub `macos-latest` runners, this script executes *before*
+# `prepare_environment` runs `setup-xcode` to select Xcode 26, so the
+# active Xcode is still whatever ships with the runner image, which
+# may not support the option. CI's `prepare_environment` action has a
+# dedicated `Install selected Xcode Metal toolchain` step that retries
+# this exact command after `setup-xcode`, so silently skipping here is
+# safe for stock CI. Self-hosted runners hit this script with their
+# own Xcode pre-selected and the call should succeed there.
+if out=$(xcodebuild -downloadComponent MetalToolchain 2>&1); then
+    printf '%s\n' "$out"
+elif printf '%s' "$out" | grep -q "invalid option '-downloadComponent'"; then
+    echo "xcodebuild -downloadComponent unsupported by current Xcode; skipping (CI retries after setup-xcode)."
+else
+    printf '%s\n' "$out" >&2
+    exit 1
+fi