diff --git a/frontend/vite.config.ts b/frontend/vite.config.ts
index 34032ad4..fb606239 100644
--- a/frontend/vite.config.ts
+++ b/frontend/vite.config.ts
@@ -55,6 +55,34 @@ export default defineConfig(({ command }) => ({
   optimizeDeps: {
     include: ['avr8js', 'rp2040js', '@wokwi/elements', 'littlefs'],
   },
+  build: {
+    // Phase 1d #4 — split heavy long-lived chunks so cache-hits stay
+    // meaningful and the cold-load entry stays small.  The previous
+    // bundle landed the whole app + Monaco + every MCU sim in one
+    // index chunk (>23 MB).  Manual chunks below collapse it into
+    // cacheable groups that match the user's actual flow (editor
+    // load, run simulator, edit code in Monaco).
+    rollupOptions: {
+      output: {
+        manualChunks: {
+          // ngspice WASM client — only loaded when the user opens
+          // a circuit with electrical components.
+          'spice-wasm': [
+            './src/simulation/spice/adapters/NgSpiceWorkerAdapter.ts',
+            './src/simulation/spice/wasm/NgSpiceInteractive.ts',
+          ],
+          // MCU emulators — bulky, infrequent updates.
+          'mcu-emulators': ['avr8js', 'rp2040js'],
+          // Wokwi visual elements — large but cacheable; only loaded
+          // once per session.
+          'wokwi-elements': ['@wokwi/elements'],
+          // React vendor — stable across deploys, near-permanent cache.
+          'react-vendor': ['react', 'react-dom', 'react-router-dom'],
+        },
+      },
+    },
+    chunkSizeWarningLimit: 8000,
+  },
   test: {
     globals: true,
     environment: 'node',