diff --git a/tests/conftest.py b/tests/conftest.py
index 5dfef27..0caf9f4 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -275,3 +275,34 @@ def _fast_sleep(seconds):
     except Exception:
         pass
     yield
+
+
+
+@pytest.fixture(autouse=True)
+def _isolated_wal(monkeypatch, tmp_path):
+    # CI flakefix (Sprint 0 follow-up): every test gets a private
+    # ``NULLRUN_WAL_PATH`` so ``Transport._replay_from_wal`` cannot
+    # replay events from a previous run / parallel xdist worker /
+    # failed teardown against the real backend.
+    #
+    # Root cause (observed on run 29809829695 job 88568154484):
+    # ``NullRunRuntime.__init__`` calls ``self._transport.start()``
+    # which calls ``_replay_from_wal()``. With no monkeypatched
+    # ``NULLRUN_WAL_PATH``, the SDK reads the default
+    # ``tempfile.gettempdir()/nullrun.wal`` and tries to drain any
+    # events found there against the real ``api_url``. The
+    # real-backend httpx call hits ``/api/v1/track/batch`` with a
+    # placeholder test key, the backend returns 401, and
+    # ``NullRunAuthError`` propagates back into the test fixture
+    # setup — failing any test that builds a runtime via
+    # ``NullRunRuntime(api_key=..., _test_mode=True)`` without the
+    # ``make_test_runtime`` fixture. CI 3.12 hits this race more
+    # often than 3.10/3.11 due to thread-scheduling differences
+    # in ``Transport.start()``.
+    #
+    # ``make_test_runtime`` already pins ``NULLRUN_WAL_PATH`` per
+    # factory call; this autouse covers tests that build a runtime
+    # inline (e.g. ``test_state_compare_case_insensitive.py:28``
+    # and ``test_v3_wire_contract.py::TestPingChainScheduler``).
+    monkeypatch.setenv("NULLRUN_WAL_PATH", str(tmp_path / "sdk.wal"))
+    yield
diff --git a/tests/test_webhook_backoff.py b/tests/test_webhook_backoff.py
index fa67c16..efa45dc 100644
--- a/tests/test_webhook_backoff.py
+++ b/tests/test_webhook_backoff.py
@@ -8,6 +8,22 @@
 
 Post-fix the schedule is ``0.5 * 2**attempt`` capped at 30s:
 0.5s, 1.0s, 2.0s, 4.0s, 8.0s, 16.0s, 30.0s (cap).
+
+These tests mock ``nullrun.actions.time.sleep`` directly via
+``unittest.mock.patch``. The conftest autouse ``_fast_sleep``
+fixture caps test-code ``time.sleep`` at 1ms, which does NOT
+interfere with the per-test ``patch`` (the patch goes through
+``unittest.mock`` and replaces the sleep function inside
+``with``; the autouse cap is active outside the ``with`` block).
+However, the singleton ``_action_handler`` module-level
+webhook-delivery thread started by another test in the same
+process may call ``time.sleep(0.5)`` (its idle poll) at exactly
+the moment this test enters the assertion — and on Python 3.11
+under xdist the singleton's ``sleeps`` collection was visible
+on the assertion path in CI run 29814323742. Marking the whole
+module ``@pytest.mark.slow_sleep`` opts out of the autouse
+cap so the sleep calls in the test body and the singleton
+idle poll use real wall-clock sleeps.
 """
 
 import time
@@ -18,6 +34,9 @@
 from nullrun.actions import ActionHandler, WebhookConfig
 
 
+pytestmark = pytest.mark.slow_sleep
+
+
 def _make_handler_with_webhook(retries: int = 7) -> ActionHandler:
     """Build an ActionHandler with one registered webhook.