diff --git a/packages/vscode-ide-companion/src/acp/acpConnection.ts b/packages/vscode-ide-companion/src/acp/acpConnection.ts
index 2f75f60a..204ccf7a 100644
--- a/packages/vscode-ide-companion/src/acp/acpConnection.ts
+++ b/packages/vscode-ide-companion/src/acp/acpConnection.ts
@@ -184,9 +184,17 @@ export class AcpConnection {
         if (line.trim()) {
           try {
             const message = JSON.parse(line) as AcpMessage;
+            console.log(
+              '[ACP] <<< Received message:',
+              JSON.stringify(message).substring(0, 500),
+            );
             this.handleMessage(message);
           } catch (_error) {
             // 忽略非JSON行
+            console.log(
+              '[ACP] <<< Non-JSON line (ignored):',
+              line.substring(0, 200),
+            );
           }
         }
       }
diff --git a/packages/vscode-ide-companion/src/acp/acpMessageHandler.ts b/packages/vscode-ide-companion/src/acp/acpMessageHandler.ts
index 08b59346..967c6e8f 100644
--- a/packages/vscode-ide-companion/src/acp/acpMessageHandler.ts
+++ b/packages/vscode-ide-companion/src/acp/acpMessageHandler.ts
@@ -154,6 +154,10 @@ export class AcpMessageHandler {
 
     switch (method) {
       case CLIENT_METHODS.session_update:
+        console.log(
+          '[ACP] >>> Processing session_update:',
+          JSON.stringify(params).substring(0, 300),
+        );
         callbacks.onSessionUpdate(params as AcpSessionUpdate);
         break;
       case CLIENT_METHODS.session_request_permission:
diff --git a/packages/vscode-ide-companion/src/agents/qwenSessionUpdateHandler.ts b/packages/vscode-ide-companion/src/agents/qwenSessionUpdateHandler.ts
index 3ed27d2d..61fde052 100644
--- a/packages/vscode-ide-companion/src/agents/qwenSessionUpdateHandler.ts
+++ b/packages/vscode-ide-companion/src/agents/qwenSessionUpdateHandler.ts
@@ -40,6 +40,10 @@ export class QwenSessionUpdateHandler {
    */
   handleSessionUpdate(data: AcpSessionUpdate): void {
     const update = data.update;
+    console.log(
+      '[SessionUpdateHandler] Processing update type:',
+      update.sessionUpdate,
+    );
 
     switch (update.sessionUpdate) {
       case 'user_message_chunk':
diff --git a/packages/vscode-ide-companion/src/diff-manager.ts b/packages/vscode-ide-companion/src/diff-manager.ts
index fea9edc4..83636c56 100644
--- a/packages/vscode-ide-companion/src/diff-manager.ts
+++ b/packages/vscode-ide-companion/src/diff-manager.ts
@@ -76,15 +76,26 @@ export class DiffManager {
 
   /**
    * Creates and shows a new diff view.
+   * @param filePath Path to the file being diffed
+   * @param oldContent The original content (left side)
+   * @param newContent The modified content (right side)
    */
-  async showDiff(filePath: string, newContent: string) {
-    const fileUri = vscode.Uri.file(filePath);
+  async showDiff(filePath: string, oldContent: string, newContent: string) {
+    const _fileUri = vscode.Uri.file(filePath);
 
+    // Left side: old content using qwen-diff scheme
+    const leftDocUri = vscode.Uri.from({
+      scheme: DIFF_SCHEME,
+      path: filePath,
+      query: `old&rand=${Math.random()}`,
+    });
+    this.diffContentProvider.setContent(leftDocUri, oldContent);
+
+    // Right side: new content using qwen-diff scheme
     const rightDocUri = vscode.Uri.from({
       scheme: DIFF_SCHEME,
       path: filePath,
-      // cache busting
-      query: `rand=${Math.random()}`,
+      query: `new&rand=${Math.random()}`,
     });
     this.diffContentProvider.setContent(rightDocUri, newContent);
 
@@ -94,26 +105,13 @@ export class DiffManager {
       rightDocUri,
     });
 
-    const diffTitle = `${path.basename(filePath)} ↔ Modified`;
+    const diffTitle = `${path.basename(filePath)} (Before ↔ After)`;
     await vscode.commands.executeCommand(
       'setContext',
       'qwen.diff.isVisible',
       true,
     );
 
-    let leftDocUri;
-    try {
-      await vscode.workspace.fs.stat(fileUri);
-      leftDocUri = fileUri;
-    } catch {
-      // We need to provide an empty document to diff against.
-      // Using the 'untitled' scheme is one way to do this.
-      leftDocUri = vscode.Uri.from({
-        scheme: 'untitled',
-        path: filePath,
-      });
-    }
-
     await vscode.commands.executeCommand(
       'vscode.diff',
       leftDocUri,
diff --git a/packages/vscode-ide-companion/src/extension.ts b/packages/vscode-ide-companion/src/extension.ts
index 1c03ca60..084b3f2a 100644
--- a/packages/vscode-ide-companion/src/extension.ts
+++ b/packages/vscode-ide-companion/src/extension.ts
@@ -177,6 +177,30 @@ export async function activate(context: vscode.ExtensionContext) {
         diffManager.cancelDiff(docUri);
       }
     }),
+    vscode.commands.registerCommand(
+      'qwenCode.showDiff',
+      async (args: { path: string; oldText: string; newText: string }) => {
+        log(`[Command] showDiff called for: ${args.path}`);
+        try {
+          // Convert relative path to absolute if needed
+          let absolutePath = args.path;
+          if (!args.path.startsWith('/') && !args.path.match(/^[a-zA-Z]:/)) {
+            const workspaceFolder = vscode.workspace.workspaceFolders?.[0];
+            if (workspaceFolder) {
+              absolutePath = vscode.Uri.joinPath(
+                workspaceFolder.uri,
+                args.path,
+              ).fsPath;
+            }
+          }
+
+          await diffManager.showDiff(absolutePath, args.oldText, args.newText);
+        } catch (error) {
+          log(`[Command] Error showing diff: ${error}`);
+          vscode.window.showErrorMessage(`Failed to show diff: ${error}`);
+        }
+      },
+    ),
     vscode.commands.registerCommand('qwenCode.openChat', () => {
       // Open or reveal the most recent chat tab
       if (webViewProviders.length > 0) {
diff --git a/packages/vscode-ide-companion/src/ide-server.ts b/packages/vscode-ide-companion/src/ide-server.ts
index e67dfa81..98fafc98 100644
--- a/packages/vscode-ide-companion/src/ide-server.ts
+++ b/packages/vscode-ide-companion/src/ide-server.ts
@@ -437,7 +437,18 @@ const createMcpServer = (diffManager: DiffManager) => {
       inputSchema: OpenDiffRequestSchema.shape,
     },
     async ({ filePath, newContent }: z.infer<typeof OpenDiffRequestSchema>) => {
-      await diffManager.showDiff(filePath, newContent);
+      // Read old content if file exists, otherwise use empty string
+      let oldContent = '';
+      try {
+        const fileUri = vscode.Uri.file(filePath);
+        const document = await vscode.workspace.openTextDocument(fileUri);
+        oldContent = document.getText();
+      } catch (_error) {
+        // File doesn't exist, use empty string (creating new file)
+        oldContent = '';
+      }
+
+      await diffManager.showDiff(filePath, oldContent, newContent);
       return { content: [] };
     },
   );