diff --git a/desktop/package.json b/desktop/package.json
index 2f87369c..e7a3d0b9 100644
--- a/desktop/package.json
+++ b/desktop/package.json
@@ -5,7 +5,8 @@
   "type": "module",
   "scripts": {
     "dev": "vite",
-    "build": "tsc -b && vite build",
+    "build": "bun run build:preview-agent && tsc -b && vite build",
+    "build:preview-agent": "bun run ./scripts/build-preview-agent.ts",
     "build:sidecars": "bun run ./scripts/build-sidecars.ts",
     "build:macos-arm64": "bash ./scripts/build-macos-arm64.sh",
     "build:windows-x64": "powershell -ExecutionPolicy Bypass -File ./scripts/build-windows-x64.ps1",
diff --git a/desktop/scripts/build-preview-agent.ts b/desktop/scripts/build-preview-agent.ts
new file mode 100644
index 00000000..2240f5c5
--- /dev/null
+++ b/desktop/scripts/build-preview-agent.ts
@@ -0,0 +1,3 @@
+import { $ } from 'bun'
+await $`bun build ./src/preview-agent/index.ts --outfile=./src-tauri/resources/preview-agent.js --format=iife --minify`
+console.log('preview-agent.js built')
diff --git a/desktop/src-tauri/resources/preview-agent.js b/desktop/src-tauri/resources/preview-agent.js
new file mode 100644
index 00000000..5fa73bcc
--- /dev/null
+++ b/desktop/src-tauri/resources/preview-agent.js
@@ -0,0 +1 @@
+(()=>{(()=>{window.__PREVIEW_AGENT__=!0})();})();
diff --git a/desktop/src-tauri/src/webview_panel.rs b/desktop/src-tauri/src/webview_panel.rs
index 79c44648..b674e111 100644
--- a/desktop/src-tauri/src/webview_panel.rs
+++ b/desktop/src-tauri/src/webview_panel.rs
@@ -3,8 +3,7 @@ use tauri::{AppHandle, LogicalPosition, LogicalSize, Manager, Runtime, WebviewBu
 
 const PREVIEW_LABEL: &str = "preview";
 
-/// M1 placeholder init script; replaced by the real preview-agent bundle in a later milestone.
-const PREVIEW_INIT_SCRIPT: &str = "window.__PREVIEW_AGENT_READY__ = true;";
+const PREVIEW_INIT_SCRIPT: &str = include_str!("../resources/preview-agent.js");
 
 #[derive(Default)]
 pub struct PreviewState(Mutex<PreviewInner>);
@@ -63,7 +62,7 @@ pub async fn preview_open<R: Runtime>(
         .get_window(crate::MAIN_WINDOW_LABEL)
         .ok_or_else(|| "main window not found".to_string())?;
     let builder = WebviewBuilder::new(PREVIEW_LABEL, WebviewUrl::External(target))
-        .initialization_script(PREVIEW_INIT_SCRIPT);
+        .initialization_script_for_all_frames(PREVIEW_INIT_SCRIPT);
     main.add_child(
         builder,
         LogicalPosition::new(bounds.x, bounds.y),
diff --git a/desktop/src/preview-agent/index.ts b/desktop/src/preview-agent/index.ts
new file mode 100644
index 00000000..46ed1d3e
--- /dev/null
+++ b/desktop/src/preview-agent/index.ts
@@ -0,0 +1,5 @@
+// 注入到被预览页面的 IIFE 入口（M3 逐步填实）
+;(() => {
+  // 标记注入成功，供宿主探测
+  ;(window as unknown as { __PREVIEW_AGENT__?: boolean }).__PREVIEW_AGENT__ = true
+})()