diff --git a/tests/tools/test_file_tools.py b/tests/tools/test_file_tools.py
index 1de38ec25a8..a6fcf298674 100644
--- a/tests/tools/test_file_tools.py
+++ b/tests/tools/test_file_tools.py
@@ -91,6 +91,33 @@ class TestWriteFileHandler:
         assert any("write_file expected denial" in r.getMessage() for r in caplog.records)
         assert not any(r.levelno >= logging.ERROR for r in caplog.records)
 
+    @patch("tools.file_tools._get_file_ops")
+    def test_rejects_read_file_line_numbered_content(self, mock_get):
+        """#19798 — do not persist read_file's LINE_NUM|CONTENT display format."""
+        from tools.file_tools import write_file_tool
+
+        content = " 1|setting: new_value\n 2|other: thing\n"
+        result = json.loads(write_file_tool("/tmp/config.yaml", content))
+
+        assert "error" in result
+        assert "line-number" in result["error"].lower()
+        mock_get.assert_not_called()
+
+    @patch("tools.file_tools._get_file_ops")
+    def test_allows_sparse_literal_pipe_content(self, mock_get):
+        """A single literal N| line should not be treated as read_file output."""
+        mock_ops = MagicMock()
+        result_obj = MagicMock()
+        result_obj.to_dict.return_value = {"status": "ok", "path": "/tmp/out.txt", "bytes": 21}
+        mock_ops.write_file.return_value = result_obj
+        mock_get.return_value = mock_ops
+
+        from tools.file_tools import write_file_tool
+        result = json.loads(write_file_tool("/tmp/out.txt", "1|literal value\nplain line\n"))
+
+        assert result["status"] == "ok"
+        mock_ops.write_file.assert_called_once()
+
     @patch("tools.file_tools._get_file_ops")
     def test_unexpected_exception_still_logs_error(self, mock_get, caplog):
         mock_get.side_effect = RuntimeError("boom")
diff --git a/tools/file_tools.py b/tools/file_tools.py
index 3f9a9f2ad13..f427132451e 100644
--- a/tools/file_tools.py
+++ b/tools/file_tools.py
@@ -657,6 +657,49 @@ def _is_internal_file_status_text(content: str) -> bool:
     return False
 
 
+def _looks_like_read_file_line_numbered_content(content: str) -> bool:
+    """Return True for content dominated by read_file's ``LINE_NUM|CONTENT`` display.
+
+    ``read_file`` intentionally returns line-numbered text to the model. If
+    that display format is echoed into ``write_file``, config/source files are
+    silently corrupted with prefixes like `` 1|``.  We reject writes where the
+    non-empty lines are mostly consecutive read_file-style numbered lines, while
+    allowing sparse literal pipe content such as a single ``1|value`` line.
+    """
+    if not isinstance(content, str):
+        return False
+
+    lines = [line for line in content.splitlines() if line.strip()]
+    if len(lines) < 2:
+        return False
+
+    numbered: list[int] = []
+    for line in lines:
+        stripped = line.lstrip()
+        prefix, sep, _rest = stripped.partition("|")
+        if sep and prefix.isdigit():
+            numbered.append(int(prefix))
+
+    if len(numbered) < 2:
+        return False
+    if len(numbered) / len(lines) < 0.6:
+        return False
+
+    consecutive_pairs = sum(
+        1 for prev, current in zip(numbered, numbered[1:])
+        if current == prev + 1
+    )
+    return consecutive_pairs >= len(numbered) - 1
+
+
+def _is_internal_file_tool_content(content: str) -> bool:
+    """Return True when content is file-tool display text, not intended file bytes."""
+    return (
+        _is_internal_file_status_text(content)
+        or _looks_like_read_file_line_numbered_content(content)
+    )
+
+
 def _get_file_ops(task_id: str = "default") -> ShellFileOperations:
     """Get or create ShellFileOperations for a terminal environment.
 
@@ -1213,10 +1256,11 @@ def write_file_tool(path: str, content: str, task_id: str = "default",
         cross_warning = _check_cross_profile_path(path, task_id)
         if cross_warning:
             return tool_error(cross_warning)
-    if _is_internal_file_status_text(content):
+    if _is_internal_file_tool_content(content):
         return tool_error(
-            "Refusing to write internal read_file status text as file content. "
-            "Re-read the file or reconstruct the intended file contents before writing."
+            "Refusing to write internal read_file display text as file content. "
+            "Strip read_file line-number prefixes or reconstruct the intended "
+            "file contents before writing."
         )
     try:
         # Resolve once for the registry lock + stale check.  Failures here