diff --git a/build.sh b/build.sh
index e4cdaeac..516f3b3e 100755
--- a/build.sh
+++ b/build.sh
@@ -202,10 +202,10 @@ if [ $BUILD_SUCCESS -eq 1 ]; then
         echo "======================================"
         (
             if [ "${MSYSTEM:-}" = UCRT64 ]; then
-                source tools/chatgui/compiler_runtime.sh
-                chatgui_refresh_compiler_runtime tools/chatgui/build /ucrt64/bin || exit 1
+                cd tools/chatgui && bash build.sh
+            else
+                cd tools/chatgui/build && cmake --build . -j4 2>&1
             fi
-            cd tools/chatgui/build && cmake --build . -j4 2>&1
         ) || echo "WARNING: chatgui build failed"
     fi
 else
diff --git a/tools/chatgui/build.sh b/tools/chatgui/build.sh
index 0b055915..d81f6649 100755
--- a/tools/chatgui/build.sh
+++ b/tools/chatgui/build.sh
@@ -30,6 +30,15 @@ fi
 source "$SRC_DIR/compiler_runtime.sh"
 if [ "${MSYSTEM:-}" = UCRT64 ]; then
     chatgui_refresh_compiler_runtime "$BUILD_DIR" /ucrt64/bin 2>&1 | tee -a "$LOG_FILE"
+    log "Preparing bundled rlottie..."
+    cmake -S "$SRC_DIR/rlottie_src" -B "$SRC_DIR/rlottie_build" -G Ninja \
+        -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON 2>&1 | tee -a "$LOG_FILE" || {
+        log "  ERROR: bundled rlottie configure failed"; exit 1;
+    }
+    cmake --build "$SRC_DIR/rlottie_build" --target rlottie --parallel 4 2>&1 | tee -a "$LOG_FILE" || {
+        log "  ERROR: bundled rlottie build failed"; exit 1;
+    }
+    log "  OK: bundled rlottie ready in $SRC_DIR/rlottie_build"
 fi
 
 log "[1/4] Configuring CMake (generator: Ninja)..."
diff --git a/tools/chatgui/rlottie_src/UPSTREAM.txt b/tools/chatgui/rlottie_src/UPSTREAM.txt
index 708f04a6..6e8d185d 100644
--- a/tools/chatgui/rlottie_src/UPSTREAM.txt
+++ b/tools/chatgui/rlottie_src/UPSTREAM.txt
@@ -9,3 +9,5 @@ Local CMake changes:
 Build from tools/chatgui:
 cmake -S rlottie_src -B rlottie_build -G Ninja -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON
 cmake --build rlottie_build --target rlottie --parallel 4
+
+The Windows ChatGUI build.bat runs these steps automatically before building ChatGUI.