diff --git a/mk/prefix.mk b/mk/prefix.mk
index 1ce6687588..97c9c33fb7 100644
--- a/mk/prefix.mk
+++ b/mk/prefix.mk
@@ -113,6 +113,10 @@ else
 # libc.a copy in the toolchain is identical to the one just compiled.
 	cp -rf "$@/$(GNU_TARGET)/lib/"* "$(PREFIX)/gcc-install/$(GNU_TARGET)/lib/"
 	cp -rf "$@/$(GNU_TARGET)/include/"* "$(PREFIX)/gcc-install/$(GNU_TARGET)/include/"
+# Fix GCC 13 <cstdlib> — relibc declares but doesn't implement strtold (long double).
+# GCC was built with _GLIBCXX_USE_C99_STDLIB=1 but relibc lacks the symbol.
+# Remove 'using ::strtold' from the C++ stdlib header to prevent compile errors.
+	sed -i '/using ::strtold;/d; /using ::__gnu_cxx::strtold;/d' "$(PREFIX)/gcc-install/$(GNU_TARGET)/include/c++"/*/cstdlib 2>/dev/null || true
 # Propagate fresh relibc artifacts to the redoxer toolchain at
 # ~/.redoxer/<target>/toolchain/.  The redoxer ships its own sysroot
 # snapshot from initial setup; without this sync the GCC driver inside