diff --git a/run b/run
index 82d4c3f..348dbd0 100755
--- a/run
+++ b/run
@@ -464,6 +464,26 @@ step19() {
   }
 }
 
+# commit fc9ab4144cc577e5293e685af2d53ac1cb3415ce
+# Author: Andrew Kelley <andrew@ziglang.org>
+# Date:   2023-04-06T09:22:29+03:00
+#
+#     update libcxx to LLVM 16.0.1
+#
+STEP20=fc9ab4144cc577e5293e685af2d53ac1cb3415ce
+STEP20_=0.10.0-2440-gfc9ab4144c
+step20() {
+  rm -fr "../zig-$STEP20_"
+  git archive --prefix=zig-$STEP20_/ $STEP20 | tar -C .. -x
+  {
+    pushd "../zig-$STEP20_"
+    cp ../zig-$STEP19_/stage1/zig1.wasm stage1/zig1.wasm
+    halfbuild
+    popd
+  }
+}
+
+
 if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
   set -xeuo pipefail
   step=${1:-step00}
@@ -490,4 +510,5 @@ if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
   if [[ "$step" -le 17 ]]; then step17; fi
   if [[ "$step" -le 18 ]]; then step18; fi
   if [[ "$step" -le 19 ]]; then step19; fi
+  if [[ "$step" -le 20 ]]; then step20; fi
 fi