diff --git a/src/mudlib/editor.py b/src/mudlib/editor.py
index 833593d..8258cd1 100644
--- a/src/mudlib/editor.py
+++ b/src/mudlib/editor.py
@@ -134,8 +134,10 @@ class Editor:
         # Insert at end if line_num is beyond buffer
         if line_num > len(self.buffer):
             self.buffer.append(text)
+            self.cursor = len(self.buffer) - 1
         else:
             self.buffer.insert(line_num - 1, text)
+            self.cursor = line_num - 1
 
         self._mark_dirty()
         return f"Inserted at line {line_num}"
@@ -150,6 +152,7 @@ class Editor:
         if end is None:
             # Delete single line
             self.buffer.pop(start - 1)
+            self.cursor = min(start - 1, len(self.buffer) - 1)
             self._mark_dirty()
             return f"Deleted line {start}"
         else:
@@ -159,6 +162,7 @@ class Editor:
 
             # Delete from start to end (inclusive)
             del self.buffer[start - 1 : end]
+            self.cursor = min(start - 1, len(self.buffer) - 1)
             self._mark_dirty()
             return f"Deleted lines {start}-{end}"
 
@@ -202,8 +206,11 @@ class Editor:
             return "Nothing to undo"
 
         self.buffer = self.undo_stack.pop()
-        # Undo might restore clean state, but we'll keep dirty flag
-        # (conservative approach - user can save to clear it)
+        # Conservative approach: if we undid something, the buffer is now different
+        # from what was saved (even if we undo past a save point)
+        self.dirty = True
+        # Reset cursor to end of buffer after undo
+        self.cursor = len(self.buffer) - 1 if self.buffer else 0
         return "Undone last change"
 
     async def _save(self) -> str:
@@ -254,6 +261,7 @@ Any line not starting with : is appended to the buffer."""
             # Append to buffer
             self._save_undo_state()
             self.buffer.append(line)
+            self.cursor = len(self.buffer) - 1
             self._mark_dirty()
             return EditorResponse(output="", done=False)
 
@@ -338,24 +346,13 @@ Any line not starting with : is appended to the buffer."""
 
         # Search and replace
         if command == "s":
-            parts = args.split(None, 1)
-            if len(parts) < 2:
+            split_args = args.split(None, 1)
+            if len(split_args) < 2:
                 return EditorResponse(
                     output="Error: :s requires old and new text", done=False
                 )
-            old_new = parts[1].split(None, 1) if len(parts) > 1 else []
-            if len(old_new) < 2:
-                # Try parsing the whole args as "old new"
-                split_args = args.split(None, 1)
-                if len(split_args) < 2:
-                    return EditorResponse(
-                        output="Error: :s requires old and new text", done=False
-                    )
-                old = split_args[0]
-                new = split_args[1]
-            else:
-                old = parts[0]
-                new = old_new[1] if len(old_new) > 1 else old_new[0]
+            old = split_args[0]
+            new = split_args[1]
             output = self._search_replace(old, new, replace_all=False)
             return EditorResponse(output=output, done=False)
 
diff --git a/tests/test_editor.py b/tests/test_editor.py
index a07e6b6..82084d6 100644
--- a/tests/test_editor.py
+++ b/tests/test_editor.py
@@ -418,3 +418,19 @@ async def test_empty_buffer_with_python_content_type():
     response = await editor.handle_input(":")
     # Should show empty buffer message
     assert "empty" in response.output.lower()
+
+
+@pytest.mark.asyncio
+async def test_undo_past_save_marks_dirty():
+    """Undoing after save should mark buffer dirty again."""
+    saved = []
+
+    async def cb(content: str):
+        saved.append(content)
+
+    editor = Editor(initial_content="line 1", save_callback=cb)
+    await editor.handle_input("line 2")  # dirty
+    await editor.handle_input(":w")  # clean
+    assert not editor.dirty
+    await editor.handle_input(":u")  # undo the append - should be dirty
+    assert editor.dirty
diff --git a/tests/test_editor_integration.py b/tests/test_editor_integration.py
index e9c3e6c..8a47fed 100644
--- a/tests/test_editor_integration.py
+++ b/tests/test_editor_integration.py
@@ -161,7 +161,7 @@ async def test_editor_prompt_uses_cursor(player):
     await player.editor.handle_input("line 1")
     await player.editor.handle_input("line 2")
 
-    # Cursor starts at 0. The prompt shows "cursor + 1" for 1-indexed display
+    # After appending, cursor should be at the last line (1 in 0-indexed)
     # This test verifies the cursor field exists and can be used for prompts
-    assert player.editor.cursor == 0
-    # Shell loop prompt: f"  {player.editor.cursor + 1}> " = "  1> "
+    assert player.editor.cursor == 1
+    # Shell loop prompt: f"  {player.editor.cursor + 1}> " = "  2> "