diff --git a/agent/display.py b/agent/display.py
index 3f1341485..474595d76 100644
--- a/agent/display.py
+++ b/agent/display.py
@@ -225,9 +225,11 @@ def build_tool_preview(tool_name: str, args: dict, max_len: int | None = None) -
             content = _oneline(args.get("content", ""))
             return f"+{target}: \"{content[:25]}{'...' if len(content) > 25 else ''}\""
         elif action == "replace":
-            return f"~{target}: \"{_oneline(args.get('old_text', '')[:20])}\""
+            old = _oneline(args.get("old_text") or "") or "<missing old_text>"
+            return f"~{target}: \"{old[:20]}\""
         elif action == "remove":
-            return f"-{target}: \"{_oneline(args.get('old_text', '')[:20])}\""
+            old = _oneline(args.get("old_text") or "") or "<missing old_text>"
+            return f"-{target}: \"{old[:20]}\""
         return action
 
     if tool_name == "send_message":
@@ -939,9 +941,13 @@ def get_cute_tool_message(
         if action == "add":
             return _wrap(f"┊ 🧠 memory    +{target}: \"{_trunc(args.get('content', ''), 30)}\"  {dur}")
         elif action == "replace":
-            return _wrap(f"┊ 🧠 memory    ~{target}: \"{_trunc(args.get('old_text', ''), 20)}\"  {dur}")
+            old = args.get("old_text") or ""
+            old = old if old else "<missing old_text>"
+            return _wrap(f"┊ 🧠 memory    ~{target}: \"{_trunc(old, 20)}\"  {dur}")
         elif action == "remove":
-            return _wrap(f"┊ 🧠 memory    -{target}: \"{_trunc(args.get('old_text', ''), 20)}\"  {dur}")
+            old = args.get("old_text") or ""
+            old = old if old else "<missing old_text>"
+            return _wrap(f"┊ 🧠 memory    -{target}: \"{_trunc(old, 20)}\"  {dur}")
         return _wrap(f"┊ 🧠 memory    {action}  {dur}")
     if tool_name == "skills_list":
         return _wrap(f"┊ 📚 skills    list {args.get('category', 'all')}  {dur}")
diff --git a/tests/agent/test_display.py b/tests/agent/test_display.py
index 5127a930b..4c1309a44 100644
--- a/tests/agent/test_display.py
+++ b/tests/agent/test_display.py
@@ -83,6 +83,13 @@ class TestBuildToolPreview:
         assert result is not None
         assert "user" in result
 
+    def test_memory_replace_missing_old_text_marked(self):
+        # Avoid empty quotes "" in the preview when old_text is missing/None.
+        result = build_tool_preview("memory", {"action": "replace", "target": "memory"})
+        assert result == '~memory: "<missing old_text>"'
+        result = build_tool_preview("memory", {"action": "remove", "target": "memory", "old_text": None})
+        assert result == '-memory: "<missing old_text>"'
+
     def test_session_search_preview(self):
         result = build_tool_preview("session_search", {"query": "find something"})
         assert result is not None