diff --git a/conftest.py b/conftest.py
index a5c85373..2e04c864 100644
--- a/conftest.py
+++ b/conftest.py
@@ -28,8 +28,11 @@ import pytest
 import engine_state
 
 
+_USES_DEFAULT_ENGINE_STATE_MARKER = "uses_default_engine_state"
+
+
 @pytest.fixture(autouse=True)
-def _isolate_engine_state_default(tmp_path_factory, monkeypatch):
+def _isolate_engine_state_default(request, tmp_path_factory, monkeypatch):
     """Isolate the default engine-state checkpoint dir per test.
 
     A bare ``ChatRuntime()`` (no ``engine_state_path``) falls back to
@@ -41,9 +44,27 @@ def _isolate_engine_state_default(tmp_path_factory, monkeypatch):
     boots under a different identity over the same dir). Point the default at a
     fresh per-test temp dir. Tests passing an explicit ``engine_state_path`` are
     unaffected; within one test, repeated ``ChatRuntime()`` share this dir.
+
+    Two redirections, both pointed at the same per-test dir:
+
+    1. ``engine_state._DEFAULT_DIR`` is monkeypatched directly. It is bound at
+       import (``engine_state/__init__.py``), so an env var alone would NOT
+       redirect an already-imported in-process runtime.
+    2. ``CORE_ENGINE_STATE_DIR`` is set in the environment so subprocess / CLI
+       tests that re-import ``engine_state`` in a child process inherit the same
+       isolation. A child that sets its own ``CORE_ENGINE_STATE_DIR`` (e.g.
+       ``tests/test_l10_always_on_daemon.py::test_real_sigterm_stops_the_daemon_cleanly``)
+       still overrides this and wins.
+
+    A test that intentionally exercises the real process-default dir (default-dir
+    semantics, CLI fallback, legacy-flat migration) can opt out with
+    ``@pytest.mark.uses_default_engine_state``; it then sees neither redirection.
     """
+    if request.node.get_closest_marker(_USES_DEFAULT_ENGINE_STATE_MARKER):
+        return
     isolated = tmp_path_factory.mktemp("engine_state_default")
     monkeypatch.setattr(engine_state, "_DEFAULT_DIR", isolated)
+    monkeypatch.setenv("CORE_ENGINE_STATE_DIR", str(isolated))
 
 
 QUARANTINE: frozenset[str] = frozenset()
diff --git a/pyproject.toml b/pyproject.toml
index 1ce0b7b5..c3907707 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -61,4 +61,5 @@ asyncio_mode = "auto"
 testpaths = ["tests"]
 markers = [
     "quarantine: pre-existing failure tracked in conftest.py QUARANTINE registry; excluded from full-pytest CI gate. See docs/test-debt-quarantine.md.",
+    "uses_default_engine_state: test intentionally exercises the process default engine-state directory; opt out of per-test default isolation. See docs/issues/default-engine-state-test-hygiene.md.",
 ]
diff --git a/tests/test_conftest_engine_state_isolation.py b/tests/test_conftest_engine_state_isolation.py
new file mode 100644
index 00000000..5f675e83
--- /dev/null
+++ b/tests/test_conftest_engine_state_isolation.py
@@ -0,0 +1,54 @@
+"""Proof tests for the root-conftest engine-state isolation fixture.
+
+Covers ``conftest.py::_isolate_engine_state_default`` and the recommended fix in
+``docs/issues/default-engine-state-test-hygiene.md``.  Each assertion fails if the
+corresponding half of the fixture is removed, so these are load-bearing, not
+decoration (CLAUDE.md "Schema-Defined Proof Obligations"):
+
+- drop the ``monkeypatch.setattr`` → ``test_default_dir_is_redirected_*`` fails,
+- drop the ``monkeypatch.setenv`` → ``test_env_var_matches_*`` fails,
+- drop the marker opt-out check → ``test_opt_out_marker_*`` fails.
+"""
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+import pytest
+
+import engine_state
+
+
+def test_default_dir_is_redirected_to_isolated_tmp() -> None:
+    """(req #1) the import-time-bound module attribute is redirected per test."""
+    default_dir = Path(engine_state._DEFAULT_DIR)
+    assert "engine_state_default" in str(default_dir), (
+        "fixture did not monkeypatch engine_state._DEFAULT_DIR to an isolated dir"
+    )
+    repo_default = Path(engine_state.__file__).resolve().parents[1] / "engine_state"
+    assert default_dir.resolve() != repo_default.resolve(), (
+        "isolated default dir must not be the in-repo engine_state/ dir"
+    )
+
+
+def test_env_var_matches_the_isolated_default_dir() -> None:
+    """(req #2) CORE_ENGINE_STATE_DIR is set for child processes, to the SAME dir."""
+    env_dir = os.environ.get("CORE_ENGINE_STATE_DIR")
+    assert env_dir is not None, "fixture did not set CORE_ENGINE_STATE_DIR"
+    assert env_dir == str(engine_state._DEFAULT_DIR), (
+        "CORE_ENGINE_STATE_DIR and engine_state._DEFAULT_DIR must agree so an "
+        "in-process runtime and a subprocess child resolve the same isolated dir"
+    )
+
+
+@pytest.mark.uses_default_engine_state
+def test_opt_out_marker_skips_isolation() -> None:
+    """(req #3) the marker opts a test out of both redirections.
+
+    Asserted as a contrast (not an absolute path) so it is robust whether or not
+    CORE_ENGINE_STATE_DIR is set in the ambient environment: an opted-out test must
+    NOT receive the per-test ``mktemp("engine_state_default")`` dir.
+    """
+    assert "engine_state_default" not in str(engine_state._DEFAULT_DIR), (
+        "uses_default_engine_state marker did not opt out of the isolation fixture"
+    )