diff --git a/esbuild.config.js b/esbuild.config.js
index c7aafb463..e46ec882c 100644
--- a/esbuild.config.js
+++ b/esbuild.config.js
@@ -74,9 +74,12 @@ const external = [
 
 esbuild
   .build({
-    entryPoints: ['packages/cli/index.ts'],
+    entryPoints: { cli: 'packages/cli/index.ts' },
     bundle: true,
-    outfile: 'dist/cli.js',
+    outdir: 'dist',
+    entryNames: '[name]',
+    chunkNames: 'chunks/[name]-[hash]',
+    splitting: true,
     platform: 'node',
     format: 'esm',
     target: 'node22',
@@ -103,6 +106,11 @@ esbuild
       'process.env.CLI_VERSION': JSON.stringify(pkg.version),
       // Make global available for compatibility
       global: 'globalThis',
+      // Redirect free __dirname/__filename references to the shim so that
+      // vendored libraries that emit their own `var __dirname` locals don't
+      // collide with our injected bindings when code-splitting is enabled.
+      __dirname: '__qwen_dirname',
+      __filename: '__qwen_filename',
     },
     loader: { '.node': 'file' },
     plugins: [wasmBinaryPlugin, wasmLoader({ mode: 'embedded' })],
diff --git a/packages/cli/src/ui/utils/CodeColorizer.tsx b/packages/cli/src/ui/utils/CodeColorizer.tsx
index da0d99132..dce845301 100644
--- a/packages/cli/src/ui/utils/CodeColorizer.tsx
+++ b/packages/cli/src/ui/utils/CodeColorizer.tsx
@@ -6,7 +6,6 @@
 
 import React from 'react';
 import { Text, Box } from 'ink';
-import { common, createLowlight } from 'lowlight';
 import type {
   Root,
   Element,
@@ -22,11 +21,17 @@ import {
 } from '../components/shared/MaxSizedBox.js';
 import type { LoadedSettings } from '../../config/settings.js';
 import { createDebugLogger } from '@qwen-code/qwen-code-core';
+import { getLowlightInstance, loadLowlight } from './lowlightLoader.js';
 
-// Configure theming and parsing utilities.
-const lowlight = createLowlight(common);
 const debugLogger = createDebugLogger('CODE_COLORIZER');
 
+// Lowlight is heavy (~1.5 MB bundled, ~36–60 ms V8 parse). It's loaded lazily
+// from `./lowlightLoader.js` via dynamic import so it lives in a separate
+// esbuild chunk that's only parsed once a code block actually needs
+// highlighting. Callers see plain text for the very first render and the
+// highlighted version once React next re-renders the surrounding subtree
+// (typically on the next user keystroke or message).
+
 function renderHastNode(
   node: Root | Element | HastText | RootContent,
   theme: Theme,
@@ -97,11 +102,21 @@ function highlightAndRenderLine(
   language: string | null,
   theme: Theme,
 ): React.ReactNode {
+  // Trigger the lazy load on first use; until it resolves, fall back to a
+  // plain-text rendering of the line. The next React render of the
+  // surrounding subtree will pick up the highlighted version.
+  const ll = getLowlightInstance();
+  if (!ll) {
+    void loadLowlight().catch((err) => {
+      debugLogger.error('[CodeColorizer] failed to load lowlight:', err);
+    });
+    return line;
+  }
   try {
     const getHighlightedLine = () =>
-      !language || !lowlight.registered(language)
-        ? lowlight.highlightAuto(line)
-        : lowlight.highlight(language, line);
+      !language || !ll.registered(language)
+        ? ll.highlightAuto(line)
+        : ll.highlight(language, line);
 
     const renderedNode = renderHastNode(getHighlightedLine(), theme, undefined);
 
diff --git a/packages/cli/src/ui/utils/lowlightLoader.ts b/packages/cli/src/ui/utils/lowlightLoader.ts
new file mode 100644
index 000000000..1b81872a8
--- /dev/null
+++ b/packages/cli/src/ui/utils/lowlightLoader.ts
@@ -0,0 +1,53 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Standalone loader for the lowlight syntax-highlight engine.
+ *
+ * Kept in its own module — with zero imports beyond `lowlight` itself — so
+ * that priming the cache from `test-setup.ts` does not transitively pull
+ * `themeManager`, settings, or `@qwen-code/qwen-code-core` into every test
+ * file's module graph. That cascade was observed to alter theme/config test
+ * outcomes (e.g. theme-manager auto-detection and QWEN_HOME env tests).
+ */
+
+import type { Root } from 'hast';
+
+export type Lowlight = {
+  registered(language: string): boolean;
+  highlight(language: string, value: string): Root;
+  highlightAuto(value: string): Root;
+};
+
+let lowlightInstance: Lowlight | null = null;
+let lowlightLoad: Promise<Lowlight> | null = null;
+
+export function getLowlightInstance(): Lowlight | null {
+  return lowlightInstance;
+}
+
+/**
+ * Kicks off (or returns the in-flight) load of the lowlight chunk. Exported
+ * for two callers:
+ *   1. `CodeColorizer.tsx` — fires the load on first colorize call so the
+ *      next React commit picks up the highlighted output.
+ *   2. `test-setup.ts` — awaits this once to keep snapshot tests
+ *      deterministic without dragging more modules into the test graph.
+ */
+export function loadLowlight(): Promise<Lowlight> {
+  if (lowlightInstance) return Promise.resolve(lowlightInstance);
+  if (lowlightLoad) return lowlightLoad;
+  lowlightLoad = import('lowlight')
+    .then((mod) => {
+      lowlightInstance = mod.createLowlight(mod.common) as Lowlight;
+      return lowlightInstance;
+    })
+    .catch((err) => {
+      lowlightLoad = null;
+      throw err;
+    });
+  return lowlightLoad;
+}
diff --git a/packages/cli/test-setup.ts b/packages/cli/test-setup.ts
index c26e57fa5..1a9725a59 100644
--- a/packages/cli/test-setup.ts
+++ b/packages/cli/test-setup.ts
@@ -16,3 +16,13 @@ if (process.env['QWEN_DEBUG_LOG_FILE'] === undefined) {
 }
 
 import './src/test-utils/customMatchers.js';
+
+// Lowlight is loaded asynchronously in production to keep it out of the
+// startup-critical bundle chunk. Snapshot tests render synchronously via
+// `lastFrame()` and would otherwise capture the plain-text fallback before
+// the dynamic import resolves. Prime the cache once here so every test sees
+// the fully-highlighted output. The loader is intentionally a tiny standalone
+// module (no transitive imports of themeManager / settings / core) so this
+// prime does not perturb any other test's module graph.
+import { loadLowlight } from './src/ui/utils/lowlightLoader.js';
+await loadLowlight();
diff --git a/scripts/create-standalone-package.js b/scripts/create-standalone-package.js
index 968d6da45..3ed860533 100644
--- a/scripts/create-standalone-package.js
+++ b/scripts/create-standalone-package.js
@@ -39,6 +39,7 @@ const TARGETS = new Map([
 const DIST_REQUIRED_PATHS = ['cli.js', 'vendor', 'bundled/qc-helper/docs'];
 const DIST_ALLOWED_ENTRIES = new Set([
   'cli.js',
+  'chunks',
   'vendor',
   'bundled',
   'package.json',
diff --git a/scripts/esbuild-shims.js b/scripts/esbuild-shims.js
index 9b71a3d4b..fbdfa6776 100644
--- a/scripts/esbuild-shims.js
+++ b/scripts/esbuild-shims.js
@@ -5,25 +5,27 @@
  */
 
 /**
- * Shims for esbuild ESM bundles to support require() calls
- * This file is injected into the bundle via esbuild's inject option
+ * Shims for esbuild ESM bundles.
+ *
+ * With code-splitting enabled, the inject is applied per-chunk and the
+ * exported bindings cannot collide with `var __dirname` polyfills that
+ * vendored libraries (e.g. yargs) emit in their own ESM compat layers.
+ * To stay collision-free, this file exposes prefixed names; the build
+ * config uses esbuild `define` to rewrite free `__dirname` / `__filename`
+ * references in source to these prefixed identifiers, while leaving
+ * vendor-declared locals untouched.
  */
 
 import { createRequire } from 'node:module';
 import { fileURLToPath } from 'node:url';
 import { dirname } from 'node:path';
 
-// Create require function for the current module and make it global
 const _require = createRequire(import.meta.url);
 
-// Make require available globally for dynamic requires
 if (typeof globalThis.require === 'undefined') {
   globalThis.require = _require;
 }
 
-// Export for esbuild injection
 export const require = _require;
-
-// Setup __filename and __dirname for compatibility
-export const __filename = fileURLToPath(import.meta.url);
-export const __dirname = dirname(__filename);
+export const __qwen_filename = fileURLToPath(import.meta.url);
+export const __qwen_dirname = dirname(__qwen_filename);
diff --git a/scripts/prepare-package.js b/scripts/prepare-package.js
index 28811c0fb..b9235a489 100644
--- a/scripts/prepare-package.js
+++ b/scripts/prepare-package.js
@@ -159,6 +159,7 @@ const distPackageJson = {
   },
   files: [
     'cli.js',
+    'chunks',
     'vendor',
     '*.sb',
     'README.md',