diff --git a/chat/public/builder.js b/chat/public/builder.js
index 510ae6c..a5dc83e 100644
--- a/chat/public/builder.js
+++ b/chat/public/builder.js
@@ -329,8 +329,13 @@ async function redoMessage(msg, session) {
       .slice(0, messageIndex)
       .some(m => m.cli === 'opencode');
 
-    // Only undo if there are previous opencode messages
-    if (hasPreviousOpencodeMessages) {
+    // Check if any previous messages had output (files were changed)
+    const hasPreviousOutput = session.messages
+      .slice(0, messageIndex)
+      .some(m => m.cli === 'opencode' && (m.reply || m.partialOutput));
+
+    // Only undo if there are previous opencode messages AND they produced output
+    if (hasPreviousOpencodeMessages && hasPreviousOutput) {
       try {
         await api(`/api/sessions/${session.id}/messages/${msg.id}/undo`, {
           method: 'POST',
@@ -340,7 +345,7 @@ async function redoMessage(msg, session) {
         console.warn('Undo failed, continuing with redo:', err.message);
       }
     } else {
-      console.log('First build message - skipping undo, proceeding directly to rebuild');
+      console.log('No previous output to undo - skipping undo, proceeding directly to rebuild');
       setStatus('Starting build...');
     }