diff --git a/components/cxx/project_include.cmake b/components/cxx/project_include.cmake
index 01bd8a5013f..8214c4fa302 100644
--- a/components/cxx/project_include.cmake
+++ b/components/cxx/project_include.cmake
@@ -1,3 +1,9 @@
+# Sub-projects with a custom toolchain (e.g. ULP) do not use the IDF
+# toolchain response file machinery. Skip flag manipulation.
+if(IDF_CUSTOM_TOOLCHAIN)
+    return()
+endif()
+
 if(NOT CONFIG_IDF_TARGET_LINUX)
     if(CONFIG_COMPILER_CXX_ATOMIC_LOCK_POLICY)
         idf_toolchain_add_flags(CXX_COMPILE_OPTIONS "-D_GLIBCXX_HAVE_ATOMIC_LOCK_POLICY=1")