diff --git a/src/google/adk/tools/environment/_tools.py b/src/google/adk/tools/environment/_tools.py
index 662947de58..648d0d182b 100644
--- a/src/google/adk/tools/environment/_tools.py
+++ b/src/google/adk/tools/environment/_tools.py
@@ -180,18 +180,6 @@ async def run_async(
     start_line = args.get('start_line')
     end_line = args.get('end_line')
 
-    # Use `sed` to read the file if start_line or end_line are specified.
-    if (start_line and start_line > 1) or end_line:
-      start = start_line or 1
-      if end_line:
-        sed_range = f'{start},{end_line}'
-      else:
-        sed_range = f'{start},$'
-      cmd = f"cat -n '{path}' | sed -n '{sed_range}p'"
-      res = await self._environment.execute(cmd)
-      if res.exit_code == 0:
-        return {'status': 'ok', 'content': _truncate(res.stdout)}
-
     try:
       data_bytes = await self._environment.read_file(path)
       text = data_bytes.decode('utf-8', errors='replace')
diff --git a/tests/unittests/tools/test_environment_tools.py b/tests/unittests/tools/test_environment_tools.py
new file mode 100644
index 0000000000..30dbec2465
--- /dev/null
+++ b/tests/unittests/tools/test_environment_tools.py
@@ -0,0 +1,96 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from pathlib import Path
+from typing import Optional
+
+from google.adk.environment._base_environment import BaseEnvironment
+from google.adk.environment._base_environment import ExecutionResult
+from google.adk.tools.environment._tools import ReadFileTool
+import pytest
+
+
+class _StubEnvironment(BaseEnvironment):
+  """Minimal environment double for ReadFileTool tests."""
+
+  def __init__(self, files: dict[str, bytes]):
+    self._files = files
+    self.execute_calls: list[str] = []
+
+  @property
+  def working_dir(self) -> Path:
+    return Path('/tmp/adk-test')
+
+  async def execute(
+      self,
+      command: str,
+      *,
+      timeout: Optional[float] = None,
+  ) -> ExecutionResult:
+    del timeout
+    self.execute_calls.append(command)
+    raise AssertionError('ReadFileTool should not invoke execute().')
+
+  async def read_file(self, path: Path) -> bytes:
+    key = str(path)
+    if key not in self._files:
+      raise FileNotFoundError(key)
+    return self._files[key]
+
+  async def write_file(self, path: Path, content: str | bytes) -> None:
+    del path, content
+    raise NotImplementedError
+
+
+@pytest.mark.asyncio
+async def test_read_file_with_line_range_uses_direct_file_read():
+  """ReadFileTool slices lines without shelling out."""
+  environment = _StubEnvironment({
+      'notes.txt': b'alpha\nbeta\ngamma\ndelta\n',
+  })
+
+  result = await ReadFileTool(environment).run_async(
+      args={'path': 'notes.txt', 'start_line': 2, 'end_line': 3},
+      tool_context=None,
+  )
+
+  assert result == {
+      'status': 'ok',
+      'content': '     2\tbeta\n     3\tgamma\n',
+      'total_lines': 4,
+  }
+  assert environment.execute_calls == []
+
+
+@pytest.mark.asyncio
+async def test_read_file_with_line_range_treats_shell_payload_as_literal_path():
+  """Shell metacharacters in the path do not trigger command execution."""
+  environment = _StubEnvironment({
+      'safe.txt': b'line1\nline2\n',
+  })
+  payload = (
+      '\'; python3 -c "from pathlib import Path;'
+      " Path('pwned.txt').write_text('owned')\"; echo '"
+  )
+
+  result = await ReadFileTool(environment).run_async(
+      args={'path': payload, 'start_line': 1, 'end_line': 2},
+      tool_context=None,
+  )
+
+  assert result == {
+      'status': 'error',
+      'error': f'File not found: {payload}',
+  }
+  assert environment.execute_calls == []