diff --git a/nanobot/agent/tools/shell.py b/nanobot/agent/tools/shell.py
index 279e2e77..849dcf6e 100644
--- a/nanobot/agent/tools/shell.py
+++ b/nanobot/agent/tools/shell.py
@@ -541,8 +541,9 @@ class ExecTool(Tool):
     def _build_env(self) -> dict[str, str]:
         """Build a minimal environment for subprocess execution.
 
-        On Unix, only HOME/LANG/TERM are passed; ``bash -l`` sources the
-        user's profile which sets PATH and other essentials.
+        On Unix, only HOME/LANG/TERM are passed by default. If callers request
+        ``login=True``, bash/zsh may source the user's profile and add PATH or
+        other variables.
 
         On Windows, ``cmd.exe`` has no login-profile mechanism, so a curated
         set of system variables (including PATH) is forwarded.  API keys and
diff --git a/tests/tools/test_exec_platform.py b/tests/tools/test_exec_platform.py
index 039888ff..0a5337f6 100644
--- a/tests/tools/test_exec_platform.py
+++ b/tests/tools/test_exec_platform.py
@@ -400,6 +400,29 @@ class TestExecuteEndToEnd:
         assert "hello world" in result
         assert "Exit code: 0" in result
 
+    @pytest.mark.asyncio
+    async def test_execute_defaults_to_non_login_shell(self):
+        """The public execute path must not silently request a login shell."""
+        mock_proc = AsyncMock()
+        mock_proc.communicate.return_value = (b"ok\n", b"")
+        mock_proc.returncode = 0
+        captured_login = []
+
+        async def capture_spawn(cmd, cwd, env, shell_program=None, login=None, *, stdin=None):
+            captured_login.append(login)
+            return mock_proc
+
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", False),
+            patch.object(ExecTool, "_spawn", side_effect=capture_spawn),
+            patch.object(ExecTool, "_guard_command", return_value=None),
+        ):
+            tool = ExecTool()
+            await tool.execute(command="echo ok")
+            await tool.execute(command="echo ok", login=True)
+
+        assert captured_login == [False, True]
+
 
 # ---------------------------------------------------------------------------
 # _extract_absolute_paths - UNC path support