diff --git a/tests/conftest.py b/tests/conftest.py
index 23448b80a9..e12d56ad0b 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1227,8 +1227,40 @@ _OS_MARKS = {
 }
 
 
+def _relocate_basetemp_outside_operator_home(config) -> None:
+    """Move pytest's basetemp out of the operator's platform-native Hermes home.
+
+    Every per-test sandbox is ``<basetemp>/.../hermes_test``. ``get_default_hermes_root()``
+    prefers the platform-native home whenever ``HERMES_HOME`` sits *under* it, so a basetemp
+    inside ``~/.hermes`` (or ``%LOCALAPPDATA%\\hermes``, where ``TEMP`` commonly lives on
+    Windows) turns the sandbox back into the live install and ``get_profile_dir("default")``
+    writes fixtures over the operator's config.yaml / .env / MEMORY.md (#111101).
+    """
+    from hermes_constants import _get_platform_default_hermes_home
+
+    native = _get_platform_default_hermes_home().resolve()
+    factory = config._tmp_path_factory
+    given = factory._given_basetemp
+    candidate = given if given is not None else Path(
+        os.environ.get("PYTEST_DEBUG_TEMPROOT") or tempfile.gettempdir()
+    )
+    if not candidate.resolve().is_relative_to(native):
+        return
+    # The system temp dir may itself be inside the home (Windows TEMP under the
+    # Hermes home); the repo's ignored cache dir is always outside it.
+    fallback = PROJECT_ROOT / ".pytest_cache"
+    safe_root = None if not Path(tempfile.gettempdir()).resolve().is_relative_to(native) else fallback
+    if safe_root is not None:
+        safe_root.mkdir(exist_ok=True)
+    safe = Path(tempfile.mkdtemp(prefix="hermes-pytest-basetemp-", dir=safe_root))
+    factory._given_basetemp = safe
+    config.option.basetemp = str(safe)
+
+
+@pytest.hookimpl(trylast=True)  # after _pytest.tmpdir has built config._tmp_path_factory
 def pytest_configure(config):  # noqa: D401 — pytest hook
     """Register markers used by hermetic conftest."""
+    _relocate_basetemp_outside_operator_home(config)
     config.addinivalue_line(
         "markers",
         f"{_LIVE_SYSTEM_GUARD_BYPASS_MARK}: bypass the live-system guard "
diff --git a/tests/test_basetemp_isolation.py b/tests/test_basetemp_isolation.py
new file mode 100644
index 0000000000..3819b4bf8a
--- /dev/null
+++ b/tests/test_basetemp_isolation.py
@@ -0,0 +1,51 @@
+"""pytest's basetemp must never sit inside the operator's platform-native Hermes home.
+
+Every per-test sandbox is ``<basetemp>/.../hermes_test`` and ``get_default_hermes_root()``
+prefers the platform-native home whenever ``HERMES_HOME`` sits *under* it — so a basetemp
+inside the home silently turns the sandbox back into the live install (#111101).
+"""
+from __future__ import annotations
+
+from pathlib import Path
+from types import SimpleNamespace
+
+import pytest
+
+import hermes_constants
+from tests import conftest as suite_conftest
+
+
+def _config_with_basetemp(given: Path | None) -> SimpleNamespace:
+    return SimpleNamespace(
+        _tmp_path_factory=SimpleNamespace(_given_basetemp=given),
+        option=SimpleNamespace(basetemp=str(given) if given else None),
+    )
+
+
+def test_basetemp_inside_the_native_home_is_relocated_outside_it(tmp_path, monkeypatch):
+    native = tmp_path / "native-home"
+    native.mkdir()
+    monkeypatch.setattr(hermes_constants, "_get_platform_default_hermes_home", lambda: native)
+    config = _config_with_basetemp(native / ".repro")
+
+    suite_conftest._relocate_basetemp_outside_operator_home(config)
+
+    relocated = config._tmp_path_factory._given_basetemp
+    assert relocated is not None and not relocated.resolve().is_relative_to(native.resolve())
+    assert config.option.basetemp == str(relocated)
+    # The sandbox derived from it no longer resolves to the native root.
+    monkeypatch.setenv("HERMES_HOME", str(relocated / "t0" / "hermes_test"))
+    assert hermes_constants.get_default_hermes_root() == relocated / "t0" / "hermes_test"
+
+
+def test_basetemp_outside_the_native_home_is_left_alone(tmp_path, monkeypatch):
+    native = tmp_path / "native-home"
+    native.mkdir()
+    monkeypatch.setattr(hermes_constants, "_get_platform_default_hermes_home", lambda: native)
+    given = tmp_path / "elsewhere"
+    config = _config_with_basetemp(given)
+
+    suite_conftest._relocate_basetemp_outside_operator_home(config)
+
+    assert config._tmp_path_factory._given_basetemp == given
+    assert config.option.basetemp == str(given)