fix(tools): allow whole-file reads of empty files

src/google/adk/tools/environment/_read_file_tool.py

@@ -121,6 +121,8 @@ async def run_async(
       total = len(lines_bytes)
       start = max(1, start_line or 1)
       end = min(total, end_line or total)
+      if total == 0 and start == 1 and end == 0:
+        return {'status': 'ok', 'content': ''}
       if start > total:
         return {
             'status': 'error',

tests/unittests/tools/environment/test_read_file_tool.py

@@ -112,6 +112,68 @@ async def _env(tmp_path: Path):
 class TestReadFileTool:
   """Tests for ReadFileTool behavior."""
 
+  @pytest.mark.parametrize('content', [b'', b'a\nb\n'])
+  @pytest.mark.parametrize(
+      'line_range',
+      [
+          {},
+          {'start_line': 1},
+          {'start_line': 0},
+          {'start_line': -1},
+          {'end_line': 0},
+          {'end_line': 5},
+          {'start_line': 1, 'end_line': 100},
+      ],
+  )
+  async def test_read_whole_file_with_equivalent_line_ranges(
+      self, env: LocalEnvironment, content: bytes, line_range: dict[str, int]
+  ):
+    """Equivalent whole-file ranges succeed for empty and nonempty files."""
+    await env.write_file('sample.txt', content)
+
+    result = await ReadFileTool(env).run_async(
+        args={'path': 'sample.txt', **line_range}, tool_context=None
+    )
+
+    expected = '' if not content else '     1\ta\n     2\tb\n'
+    assert result == {'status': 'ok', 'content': expected}
+
+  @pytest.mark.parametrize(
+      'line_range', [{'end_line': 1}, {'start_line': 1, 'end_line': 1}]
+  )
+  async def test_read_empty_file_through_first_line(
+      self, env: LocalEnvironment, line_range: dict[str, int]
+  ):
+    """An explicit first-line boundary also succeeds for an empty file."""
+    await env.write_file('empty.txt', b'')
+
+    result = await ReadFileTool(env).run_async(
+        args={'path': 'empty.txt', **line_range}, tool_context=None
+    )
+
+    assert result == {'status': 'ok', 'content': ''}
+
+  @pytest.mark.parametrize(
+      'line_range',
+      [{'start_line': 2}, {'start_line': 2, 'end_line': 100}, {'end_line': -1}],
+  )
+  async def test_read_empty_file_with_line_range_returns_error(
+      self, env: LocalEnvironment, line_range: dict[str, int]
+  ):
+    """Invalid ranges still report that an empty file has no lines."""
+    await env.write_file('empty.txt', b'')
+    start = line_range.get('start_line', 1)
+
+    result = await ReadFileTool(env).run_async(
+        args={'path': 'empty.txt', **line_range}, tool_context=None
+    )
+
+    assert result == {
+        'status': 'error',
+        'error': f'`start_line` {start} exceeds file length (0 lines).',
+        'total_lines': 0,
+    }
+
   @pytest.mark.asyncio
   async def test_read_file_with_line_range_returns_selected_lines(
       self, env: LocalEnvironment