fix: Preserve trailing newlines in insert_block tool

Author: johbo

aider/tools/insert_block.py

@@ -176,6 +176,10 @@ def execute(
             new_lines = lines[:insertion_line_idx] + content_lines + lines[insertion_line_idx:]
             new_content = "\n".join(new_lines)
 
+            # Restore trailing newline if original file had one
+            if original_content.endswith("\n"):
+                new_content += '\n'
+
             if original_content == new_content:
                 coder.io.tool_warning("No changes made: insertion would not change file")
                 return "Warning: No changes made (insertion would not change file)"

tests/tools/test_insert_block.py

@@ -115,3 +115,35 @@ def test_mutually_exclusive_parameters_raise(coder_with_file):
     assert result.startswith("Error: Must specify exactly one of")
     assert file_path.read_text().startswith("first line")
     coder.io.tool_error.assert_called()
+
+
+def test_trailing_newline_preservation(coder_with_file):
+        coder, file_path = coder_with_file
+        insert_block.Tool.execute(
+            coder,
+            file_path="example.txt",
+            content="inserted line",
+            position="top",
+        )
+
+        content = file_path.read_text()
+        assert content.endswith("\n"), "File should preserve trailing newline"
+        coder.io.tool_error.assert_not_called()
+
+
+def test_no_trailing_newline_preservation(coder_with_file):
+        coder, file_path = coder_with_file
+
+        content_without_trailing_newline = "first line\nsecond line"
+        file_path.write_text(content_without_trailing_newline)
+
+        insert_block.Tool.execute(
+            coder,
+            file_path="example.txt",
+            content="inserted line",
+            position="top",
+        )
+
+        content = file_path.read_text()
+        assert not content.endswith("\n"), "File should preserve lack of trailing newline"
+        coder.io.tool_error.assert_not_called()