fix(chat): make resend message non-destructive

Keep normal resend from truncating session history while preserving replace-from-here behavior for regenerate flows.

static/js/chat.js

@@ -3876,9 +3876,11 @@ import { wireArrowUpRecall, getLastUserMessageFromChatHistory } from './composer
   }
 
   /**
-   * Resend a user message — truncates history to that point and resubmits.
+   * Resend a user message. Normal resend appends a fresh copy at the end of
+   * the current thread; regenerate flows can opt into replacing from here.
    */
-  export async function resendUserMessage(userMsgElement) {
+  export async function resendUserMessage(userMsgElement, opts = {}) {
+    const replaceFromHere = Boolean(opts && opts.replaceFromHere);
     const box = document.getElementById('chat-history');
     const allMsgs = Array.from(box.querySelectorAll('.msg'));
     const msgIndex = allMsgs.indexOf(userMsgElement);
@@ -3924,25 +3926,28 @@ import { wireArrowUpRecall, getLastUserMessageFromChatHistory } from './composer
     const sessionId = sessionModule.getCurrentSessionId();
     if (!sessionId) return;
 
-    // Truncate backend to keep everything before this user message
-    const keepCount = msgIndex;
     try {
-      await fetch(`${API_BASE}/api/session/${sessionId}/truncate`, {
-        method: 'POST',
-        headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ keep_count: keepCount })
-      });
+      if (replaceFromHere) {
+        // Regenerate flows intentionally trim history to this point before
+        // resubmitting. The plain "Resend message" action must not do this.
+        const keepCount = msgIndex;
+        await fetch(`${API_BASE}/api/session/${sessionId}/truncate`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({ keep_count: keepCount })
+        });
 
-      // Drop the AI replies after the user message but KEEP the user bubble
-      // itself (so its photo stays visible). Then suppress the new user
-      // bubble that send would otherwise add — same pattern as regenerate.
-      let sibling = userMsgElement.nextSibling;
-      while (sibling) {
-        const next = sibling.nextSibling;
-        sibling.remove();
-        sibling = next;
+        // Drop the AI replies after the user message but KEEP the user bubble
+        // itself (so its photo stays visible). Then suppress the new user
+        // bubble that send would otherwise add — same pattern as regenerate.
+        let sibling = userMsgElement.nextSibling;
+        while (sibling) {
+          const next = sibling.nextSibling;
+          sibling.remove();
+          sibling = next;
+        }
+        _hideUserBubble = true;
       }
-      _hideUserBubble = true;
       _pendingRegenAttachments = _ids;
 
       // Resubmit

static/js/chatRenderer.js

@@ -362,7 +362,7 @@ function _openVisionEditor(att, userMsgEl) {
       await _saveVisionText();
       _closeVisionEditor();
       if (userMsgEl && window.chatModule?.resendUserMessage) {
-        window.chatModule.resendUserMessage(userMsgEl);
+        window.chatModule.resendUserMessage(userMsgEl, { replaceFromHere: true });
       } else if (uiModule?.showToast) {
         uiModule.showToast('Saved');
       }

tests/test_resend_message_nondestructive.py

@@ -0,0 +1,43 @@
+"""Regression guard for #4149: normal Resend must not delete chat history.
+
+chat.js is browser-heavy, so this pins the source-level contract: the footer's
+plain "Resend message" path appends a fresh send, while regenerate-only paths
+must opt into truncating/replacing from the selected message.
+"""
+
+from pathlib import Path
+
+
+_REPO = Path(__file__).resolve().parent.parent
+_CHAT_JS = _REPO / "static" / "js" / "chat.js"
+_CHAT_RENDERER_JS = _REPO / "static" / "js" / "chatRenderer.js"
+
+
+def _resend_body() -> str:
+    src = _CHAT_JS.read_text(encoding="utf-8")
+    start = src.index("export async function resendUserMessage(")
+    end = src.index("export async function regenerateFrom(", start)
+    return src[start:end]
+
+
+def test_resend_message_does_not_truncate_by_default():
+    body = _resend_body()
+
+    assert "opts = {}" in body
+    assert "const replaceFromHere = Boolean(opts && opts.replaceFromHere);" in body
+
+    guard_idx = body.index("if (replaceFromHere)")
+    truncate_idx = body.index("/api/session/${sessionId}/truncate")
+    hide_idx = body.index("_hideUserBubble = true;")
+
+    assert guard_idx < truncate_idx
+    assert guard_idx < hide_idx
+    assert "/truncate" not in body[:guard_idx]
+    assert "_hideUserBubble = true;" not in body[:guard_idx]
+
+
+def test_only_regenerate_callers_opt_into_replace_from_here():
+    renderer = _CHAT_RENDERER_JS.read_text(encoding="utf-8")
+
+    assert "window.chatModule.resendUserMessage(msgElement);" in renderer
+    assert "window.chatModule.resendUserMessage(userMsgEl, { replaceFromHere: true });" in renderer