diff --git a/mk/prefix.mk b/mk/prefix.mk
index 37c211eb05..3efcb091c0 100644
--- a/mk/prefix.mk
+++ b/mk/prefix.mk
@@ -103,6 +103,22 @@ ifeq ($(PODMAN_BUILD),1)
 else
 	rm -rf "$@"
 	cp -r "$(PREFIX)/relibc-install/" "$@"
+# Patch relibc sysroot and redoxer toolchain GCC <cstdlib> for strtold.
+# relibc declares strtold in stdlib.h (cbindgen trailer) but GCC's
+# cstdlib does 'using ::strtold' before the trailer is processed.
+# Add explicit #include <stdlib.h> after #include_next <stdlib.h>
+# so ::strtold is visible when the C++ wrapper runs its using.
+	for _cstdlib in "$(COOKBOOK_ROOT)/prefix/${TARGET}/sysroot/${GNU_TARGET}/include/c++"/*/cstdlib; do \
+		[ -f "$$_cstdlib" ] || continue; \
+		sed -i '/^#include_next <stdlib.h>$$/a\\n// Red Bear: relibc declares strtold via cbindgen trailer.\n// The explicit include ensures ::strtold is visible when the\n// C++ wrapper processes its using-directives below.\n#include <stdlib.h>' "$$_cstdlib"; \
+	done
+	@REDOXER_TC=$(HOME)/.redoxer/$(TARGET)/toolchain/$(GNU_TARGET); \
+	if [ -d "$$REDOXER_TC/include/c++" ]; then \
+		for _cstdlib in "$$REDOXER_TC/include/c++"/*/cstdlib; do \
+			[ -f "$$_cstdlib" ] || continue; \
+			sed -i '/^#include_next <stdlib.h>$$/a\\n// Red Bear: relibc declares strtold via cbindgen trailer.\n// The explicit include ensures ::strtold is visible when the\n// C++ wrapper processes its using-directives below.\n#include <stdlib.h>' "$$_cstdlib"; \
+		done; \
+	fi
 # Propagate fresh relibc artifacts (libc.a, crt*.o, headers) from the
 # rebuilt sysroot back into gcc-install.  The GCC driver's built-in
 # library search path always looks inside gcc-install/ first; if that