diff --git a/tests/test_issue17_workspace_root_isolation.py b/tests/test_issue17_workspace_root_isolation.py
index 50c7a9b..fa2913f 100644
--- a/tests/test_issue17_workspace_root_isolation.py
+++ b/tests/test_issue17_workspace_root_isolation.py
@@ -68,6 +68,7 @@ def test_w1_no_test_writes_into_the_checkout_it_runs_in(tmp_path):
     assert int(re.search(r"(\d+) passed", summary).group(1)) >= len(NODES) and "skipped" not in summary, summary
     written = sorted(str(p.relative_to(copy)) for p in (copy / ".mam").rglob("*")) if (copy / ".mam").exists() else []
     assert written == [], f"tests wrote into the checkout they ran in: {written}"
+    assert not (copy / ".mam").exists(), "tests created the checkout's .mam (e.g. the loop lock's directory)"
 
 
 _ONBOARD_PROBE = r"""
diff --git a/tests/test_loop_marker_isolation.py b/tests/test_loop_marker_isolation.py
new file mode 100644
index 0000000..4b535e9
--- /dev/null
+++ b/tests/test_loop_marker_isolation.py
@@ -0,0 +1,50 @@
+"""Tests never take the loop lock of the checkout they run in.
+
+run_loop.sh puts its control root at its own checkout (MAM_REAL_ROOT =
+REPO_ROOT) and takes `${MAM_LOOP_MARKER:-$MAM_REAL_ROOT/.mam/loop-guard-active}`
+once the arguments parse (mam_acquire_loop_lock creates the directory). In the
+main checkout that is the live loop's lock: a test run would take it for a
+moment - refusing a real loop, or being refused by one - and leave `.mam/`
+behind in a review tree. Every test that reaches the lock through the
+checkout's own run_loop.sh therefore names a marker under its tmp_path.
+
+L1 copies this checkout and runs those tests there: the copy gets no `.mam`.
+"""
+from __future__ import annotations
+
+import os
+import re
+import subprocess
+import sys
+
+from test_issue17_workspace_root_isolation import _copy_checkout
+
+# every test that runs the checkout's run_loop.sh far enough to take the lock
+# (a sweep of the run_loop.sh users on 97c2946; --help and argument errors
+# end before the lock, copies of the tree and mam_sandbox trees are not the
+# checkout). The first two took the checkout's lock at 97c2946.
+NODES = [
+    "tests/test_o3_scoped_guard.py::test_b13_reexec_preserves_original_argv",
+    "tests/test_o3_scoped_guard.py::test_b13_no_freeze_switch_disables_reexec",
+    "tests/test_issue5_phase3_loop.py::test_p3_02_run_loop_wrapper_delegates_to_python_default",
+    "tests/test_issue5_phase3_loop.py::test_p3_02_run_loop_wrapper_rollback_mode",
+    "tests/test_loop_cli.py::test_creator_parses_and_reaches_session_check",
+    "tests/test_o2_race_free_lock.py::test_o2_12_run_loop_exits_on_lock_failure",
+    "tests/test_issue25_plan_reuse.py::test_p7b_the_legacy_shell_loop_refuses_the_option",
+]
+
+
+def test_l1_no_test_takes_the_loop_lock_of_its_checkout(tmp_path):
+    copy = tmp_path / "checkout"
+    _copy_checkout(copy)
+    assert not (copy / ".mam").exists()
+    env = {k: v for k, v in os.environ.items() if not k.startswith(("HERDR_", "MAM_", "MQTT_", "SKS_"))}
+    env.update(WORKSPACE_ROOT=str(copy), AGENT_PYTHON_BIN=sys.executable, PYTHONDONTWRITEBYTECODE="1")
+    env.pop("PYTHONPATH", None)
+    res = subprocess.run([sys.executable, "-m", "pytest", "-p", "no:cacheprovider", "-q",
+                          f"--basetemp={tmp_path / 'bt'}", *NODES],
+                         capture_output=True, text=True, cwd=str(copy), env=env, timeout=900)
+    assert res.returncode == 0, res.stdout[-3000:] + res.stderr[-2000:]
+    summary = res.stdout.strip().splitlines()[-1]
+    assert int(re.search(r"(\d+) passed", summary).group(1)) >= len(NODES) and "skipped" not in summary, summary
+    assert not (copy / ".mam").exists(), "a test took the checkout's loop lock (its .mam was created)"
diff --git a/tests/test_o3_scoped_guard.py b/tests/test_o3_scoped_guard.py
index 8618351..c91c176 100644
--- a/tests/test_o3_scoped_guard.py
+++ b/tests/test_o3_scoped_guard.py
@@ -382,10 +382,13 @@ def test_b13_reexec_preserves_original_argv(tmp_path):
     The arg parser consumes "$@" (shift), so capturing argv beforehand is mandatory.
     """
     run_loop_sh = REPO_ROOT / ".agents" / "skills" / "multi-agent-mux-loop" / "scripts" / "run_loop.sh"
+    # The run reaches the loop lock: a marker of this test's own, never the
+    # checkout's .mam/loop-guard-active (the live loop's, in the main checkout).
     res = subprocess.run(
         ["bash", str(run_loop_sh), "--creator", "nonexistent-agent", "--task", "test goal with spaces"],
         capture_output=True,
         text=True,
+        env=dict(os.environ, MAM_LOOP_MARKER=str(tmp_path / "loop-guard-active")),
         cwd=str(REPO_ROOT),
     )
     combined = res.stdout + res.stderr
@@ -464,6 +467,7 @@ def test_b13_no_freeze_switch_disables_reexec(tmp_path):
     run_loop_sh = REPO_ROOT / ".agents" / "skills" / "multi-agent-mux-loop" / "scripts" / "run_loop.sh"
     env = os.environ.copy()
     env["MAM_LOOP_NO_FREEZE"] = "1"
+    env["MAM_LOOP_MARKER"] = str(tmp_path / "loop-guard-active")  # not the checkout's loop lock
     res = subprocess.run(
         ["bash", str(run_loop_sh), "--creator", "nonexistent-agent", "--task", "test goal"],
         capture_output=True,