diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7dde3090..2633dcfc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -831,6 +831,12 @@ if(VLLM_CPP_METAL)
     endif()
     target_compile_definitions(vllm PUBLIC VLLM_CPP_MLX)
     target_sources(vllm PRIVATE src/vt/metal/metal_mlx_provider.mm)
+    # MLX 0.29.x headers use GNU-folding expressions that AppleClang diagnoses.
+    # Suppress that one external-header diagnostic for this TU. A source-level
+    # -Wno-error is ordered before vllm_cpp_set_warnings' target-level -Werror,
+    # so AppleClang promotes it again.
+    set_source_files_properties(src/vt/metal/metal_mlx_provider.mm PROPERTIES
+      COMPILE_OPTIONS "$<$<COMPILE_LANG_AND_ID:OBJCXX,AppleClang>:-Wno-gnu-folding-constant>")
     # SYSTEM so MLX's own headers cannot break our -Werror build.
     target_include_directories(vllm SYSTEM PRIVATE "${MLX_ROOT}/include")
     target_link_libraries(vllm PUBLIC ${MLX_LIBRARY})