diff --git a/nanobot/agent/tools/filesystem.py b/nanobot/agent/tools/filesystem.py
index ac52ca868..ea84fedb1 100644
--- a/nanobot/agent/tools/filesystem.py
+++ b/nanobot/agent/tools/filesystem.py
@@ -827,7 +827,8 @@ class EditFileTool(_FsTool):
     def description(self) -> str:
         return (
             "Perform a small, exact replacement in one file by replacing "
-            "old_text with new_text. Use this for narrow text substitutions "
+            "old_text with new_text. When replacing text in an existing file, "
+            "old_text and new_text must be different. Use this for narrow text substitutions "
             "with old_text copied from read_file. For multi-file, structural, "
             "or generated code edits, prefer apply_patch. If old_text matches "
             "multiple times, provide more context or set occurrence, line_hint, "
@@ -862,9 +863,12 @@ class EditFileTool(_FsTool):
                 return ToolResult.error("Error: expected_replacements must be >= 1.")
 
             fp = self._resolve_write(path)
+            file_exists = fp.exists()
+            if file_exists and old_text == new_text:
+                return ToolResult.error("Error: new_text must be different from old_text.")
 
             # Create-file semantics: old_text='' + file doesn't exist → create
-            if not fp.exists():
+            if not file_exists:
                 if old_text == "":
                     fp.parent.mkdir(parents=True, exist_ok=True)
                     fp.write_text(new_text, encoding="utf-8")
diff --git a/tests/tools/test_filesystem_tools.py b/tests/tools/test_filesystem_tools.py
index b221bfcc1..a1e152c59 100644
--- a/tests/tools/test_filesystem_tools.py
+++ b/tests/tools/test_filesystem_tools.py
@@ -133,6 +133,16 @@ class TestEditFileTool:
         assert "Successfully" in result
         assert f.read_text() == "hello earth"
 
+    @pytest.mark.asyncio
+    async def test_identical_replacement_returns_clear_error(self, tool, tmp_path):
+        f = tmp_path / "a.py"
+        f.write_text("hello world", encoding="utf-8")
+
+        result = await tool.execute(path=str(f), old_text="world", new_text="world")
+
+        assert result == "Error: new_text must be different from old_text."
+        assert f.read_text(encoding="utf-8") == "hello world"
+
     @pytest.mark.asyncio
     async def test_crlf_normalisation(self, tool, tmp_path):
         f = tmp_path / "crlf.py"