diff --git a/desktop/src-tauri/src/lib.rs b/desktop/src-tauri/src/lib.rs
index dfefe961..4884c078 100644
--- a/desktop/src-tauri/src/lib.rs
+++ b/desktop/src-tauri/src/lib.rs
@@ -2189,7 +2189,9 @@ pub fn run() {
             webview_panel::preview_navigate,
             webview_panel::preview_set_bounds,
             webview_panel::preview_set_visible,
-            webview_panel::preview_close
+            webview_panel::preview_close,
+            webview_panel::preview_message,
+            webview_panel::preview_eval
         ]);
 
     // macOS: native menu bar (traffic-light overlay style)
diff --git a/desktop/src-tauri/src/webview_panel.rs b/desktop/src-tauri/src/webview_panel.rs
index b674e111..41661673 100644
--- a/desktop/src-tauri/src/webview_panel.rs
+++ b/desktop/src-tauri/src/webview_panel.rs
@@ -1,5 +1,5 @@
 use std::sync::Mutex;
-use tauri::{AppHandle, LogicalPosition, LogicalSize, Manager, Runtime, WebviewBuilder, WebviewUrl};
+use tauri::{AppHandle, Emitter, LogicalPosition, LogicalSize, Manager, Runtime, WebviewBuilder, WebviewUrl};
 
 const PREVIEW_LABEL: &str = "preview";
 
@@ -128,6 +128,21 @@ pub async fn preview_close<R: Runtime>(
     Ok(())
 }
 
+/// 页面 → 宿主：注入脚本经 IPC 调此命令；Rust 校验后转发给前端。
+#[tauri::command]
+pub fn preview_message<R: Runtime>(app: AppHandle<R>, raw: String) -> Result<(), String> {
+    // 仅转发 JSON 字符串；前端做强类型解析（schema 校验在 TS 侧 protocol）
+    serde_json::from_str::<serde_json::Value>(&raw).map_err(|e| e.to_string())?;
+    app.emit("preview://event", raw).map_err(|e| e.to_string())
+}
+
+/// 宿主 → 页面：在子 webview 内 eval 一段 JS。
+#[tauri::command]
+pub async fn preview_eval<R: Runtime>(app: AppHandle<R>, js: String) -> Result<(), String> {
+    let webview = app.get_webview(PREVIEW_LABEL).ok_or_else(|| "preview not open".to_string())?;
+    webview.eval(&js).map_err(|e| e.to_string())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/desktop/src/preview-agent/protocol.test.ts b/desktop/src/preview-agent/protocol.test.ts
new file mode 100644
index 00000000..6fa3a11b
--- /dev/null
+++ b/desktop/src/preview-agent/protocol.test.ts
@@ -0,0 +1,15 @@
+import { describe, expect, it } from 'vitest'
+import { parseHostMessage, serializeAgentMessage } from './protocol'
+
+describe('preview-agent protocol', () => {
+  it('serializes agent→host messages to a stable envelope', () => {
+    expect(JSON.parse(serializeAgentMessage({ type: 'ready' }))).toEqual({ v: 1, type: 'ready' })
+    expect(JSON.parse(serializeAgentMessage({ type: 'navigated', url: 'http://x/', title: 'T' })))
+      .toEqual({ v: 1, type: 'navigated', url: 'http://x/', title: 'T' })
+  })
+  it('parses host→agent messages and rejects unknown/garbage', () => {
+    expect(parseHostMessage('{"v":1,"type":"enter-picker"}')).toEqual({ type: 'enter-picker' })
+    expect(parseHostMessage('not json')).toBeNull()
+    expect(parseHostMessage('{"v":1,"type":"nope"}')).toBeNull()
+  })
+})
diff --git a/desktop/src/preview-agent/protocol.ts b/desktop/src/preview-agent/protocol.ts
new file mode 100644
index 00000000..bd2ac191
--- /dev/null
+++ b/desktop/src/preview-agent/protocol.ts
@@ -0,0 +1,36 @@
+export type AgentMessage =
+  | { type: 'ready' }
+  | { type: 'navigated'; url: string; title: string }
+  | { type: 'error'; message: string }
+  | { type: 'selection'; payload: unknown }   // M5 填充结构
+  | { type: 'screenshot'; dataUrl: string; kind: 'full' | 'viewport' | 'element' } // M4
+
+export type HostMessage =
+  | { type: 'enter-picker' }
+  | { type: 'exit-picker' }
+  | { type: 'capture'; kind: 'full' | 'viewport' | 'element' }
+
+const HOST_TYPES = new Set(['enter-picker', 'exit-picker', 'capture'])
+
+export function serializeAgentMessage(msg: AgentMessage): string {
+  return JSON.stringify({ v: 1, ...msg })
+}
+
+export function parseHostMessage(raw: string): HostMessage | null {
+  try {
+    const obj = JSON.parse(raw) as unknown
+    if (
+      typeof obj !== 'object' ||
+      obj === null ||
+      (obj as Record<string, unknown>)['v'] !== 1 ||
+      typeof (obj as Record<string, unknown>)['type'] !== 'string' ||
+      !HOST_TYPES.has((obj as Record<string, unknown>)['type'] as string)
+    ) {
+      return null
+    }
+    const { v: _v, ...rest } = obj as Record<string, unknown>
+    return rest as unknown as HostMessage
+  } catch {
+    return null
+  }
+}