diff --git a/tests/conftest.py b/tests/conftest.py
index 89bc842..5045651 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -168,3 +168,44 @@ def _make(**kwargs):
         return rt
 
     return _make
+
+@pytest.fixture
+def make_test_runtime(monkeypatch, tmp_path):
+    """Factory for tests that build a real ``NullRunRuntime`` inline
+    (no ``mock_api`` indirection).
+
+    Pins ``NULLRUN_WAL_PATH`` to a tmp_path-scoped file so the
+    constructor's ``Transport._replay_from_wal`` never reads the
+    default ``tempfile.gettempdir()/nullrun.wal`` (which may carry
+    real on-disk events from a previous test run or parallel
+    worker and would cause HTTP 401 → ``NullRunAuthError`` in
+    setup). Mirrors the ``test_runtime`` fixture in
+    ``test_protect_branches.py`` so all tests that build a runtime
+    directly get the same isolation.
+
+    Stub ``_do_flush`` / ``_do_flush_locked`` / ``_client`` so any
+    real network attempt is no-op'd. Reset singleton around the
+    factory so test ordering is independent.
+    """
+    from unittest.mock import MagicMock
+    from nullrun.runtime import NullRunRuntime
+
+    NullRunRuntime.reset_instance()
+    # Pre-pin the WAL path before any runtime can be constructed
+    # (otherwise the default is captured at first construction).
+    monkeypatch.setenv("NULLRUN_WAL_PATH", str(tmp_path / "sdk.wal"))
+
+    def _factory(**overrides):
+        api_key = overrides.pop("api_key", "test-key-12345678")
+        rt = NullRunRuntime(api_key=api_key, _test_mode=True)
+        # Stub the network-facing pieces for tests that build a
+        # runtime inline (not via ``mock_api``).
+        rt._transport._do_flush = lambda: None
+        rt._transport._do_flush_locked = lambda: None
+        rt._transport._client = MagicMock()
+        for k, v in overrides.items():
+            setattr(rt, k, v)
+        return rt
+
+    yield _factory
+    NullRunRuntime.reset_instance()
diff --git a/tests/test_protect_branches.py b/tests/test_protect_branches.py
index cd528cd..36cc2ba 100644
--- a/tests/test_protect_branches.py
+++ b/tests/test_protect_branches.py
@@ -35,11 +35,20 @@
 
 
 @pytest.fixture
-def test_runtime(monkeypatch):
+def test_runtime(monkeypatch, tmp_path):
     """Provide a runtime in test mode so get_runtime returns without
     authenticating against a real server.
+
+    Replays any WAL left over from previous test runs in a
+    tmp_path-scoped WAL file so the constructor's
+    ``_replay_from_wal`` never reads ``~/.nullrun/sdk.wal`` and
+    flushes real on-disk events to a live API. This avoids the
+    cross-Python-version flake seen on CI in 2026-07-11 where
+    3.11 picked up a stale WAL from a 3.10/3.12 worker that
+    finished without explicitly clearing it.
     """
     monkeypatch.setenv("NULLRUN_API_KEY", "test-key-12345678")
+    monkeypatch.setenv("NULLRUN_WAL_PATH", str(tmp_path / "sdk.wal"))
     NullRunRuntime.reset_instance()
     rt = NullRunRuntime(api_key="test-key-12345678", _test_mode=True)
     rt.organization_id = "org-1"
@@ -437,13 +446,13 @@ def f():
 
 
 @pytest.mark.asyncio
-async def test_protect_async_kill_re_raises_WorkflowKilledInterrupt():
+async def test_protect_async_kill_re_raises_WorkflowKilledInterrupt(make_test_runtime):
     """Async wrapper does NOT unify — kill signal propagates as-is so
     async frameworks can interrupt the event loop cleanly.
     """
     from nullrun import decorators as dec_mod
 
-    rt = NullRunRuntime(api_key="test-key-12345678", _test_mode=True)
+    rt = make_test_runtime()
     rt.track_event = MagicMock()
     rt.check_control_plane = MagicMock(
         side_effect=WorkflowKilledInterrupt(workflow_id="wf-1", reason="x")
@@ -542,12 +551,12 @@ def test_get_protected_runtime_returns_runtime(test_runtime):
     assert decorators.get_protected_runtime() is rt
 
 
-def test_get_protected_runtime_falls_back_to_get_runtime(test_runtime, monkeypatch):
+def test_get_protected_runtime_falls_back_to_get_runtime(monkeypatch, make_test_runtime):
     """When the decorator slot is empty, fall back to the global singleton."""
     from nullrun import decorators
 
     decorators._runtime = None
-    NullRunRuntime._instance = NullRunRuntime(api_key="test-key-12345678", _test_mode=True)
+    NullRunRuntime._instance = make_test_runtime()
     try:
         out = decorators.get_protected_runtime()
         assert out is NullRunRuntime._instance
diff --git a/tests/test_runtime_branches.py b/tests/test_runtime_branches.py
index 3c034b1..d782618 100644
--- a/tests/test_runtime_branches.py
+++ b/tests/test_runtime_branches.py
@@ -30,7 +30,21 @@ def _reset_singleton():
 def _make_test_runtime() -> NullRunRuntime:
     """Build a runtime that skips network I/O and returns from
     ``_authenticate`` with a stub organisation id.
+
+    Pins ``NULLRUN_WAL_PATH`` to a per-call tmp dir so the
+    constructor's ``Transport._replay_from_wal`` never picks up a
+    stale WAL from a previous test run (which would replay real
+    events to a live API and cause HTTP 401 in setup). See
+    ``conftest::make_test_runtime`` for the fixture equivalent.
     """
+    # Per-call isolation: each helper invocation owns its WAL.
+    # ``setdefault`` so an outer session-level pinning (from
+    # ``make_test_runtime`` fixture) is preserved if already set.
+    import os
+    import tempfile
+    if not os.environ.get("NULLRUN_WAL_PATH"):
+        wal_dir = tempfile.mkdtemp(prefix="nullrun-test-wal-")
+        os.environ["NULLRUN_WAL_PATH"] = os.path.join(wal_dir, "sdk.wal")
     rt = NullRunRuntime(api_key="test-key-12345678", _test_mode=True)
     rt.organization_id = "org-1"
     rt.workflow_id = "wf-1"
@@ -357,8 +371,9 @@ def _trigger_shutdown():
 # ─── get_instance credential rotation ──────────────────────────────
 
 
-def test_get_instance_returns_singleton_when_no_change(monkeypatch):
+def test_get_instance_returns_singleton_when_no_change(monkeypatch, tmp_path):
     monkeypatch.setenv("NULLRUN_API_KEY", "test-key-12345678")
+    monkeypatch.setenv("NULLRUN_WAL_PATH", str(tmp_path / "sdk.wal"))
     NullRunRuntime.reset_instance()
     rt1 = NullRunRuntime(api_key="test-key-12345678", _test_mode=True)
     NullRunRuntime._instance = rt1
@@ -372,7 +387,16 @@ def test_get_instance_returns_singleton_when_no_change(monkeypatch):
 def _make_runtime_with_mocked_auth() -> NullRunRuntime:
     """Build a test-mode runtime and stub the transport client.post
     so we can drive ``_authenticate`` deterministically.
+
+    Pins ``NULLRUN_WAL_PATH`` per call so we never read a stale
+    WAL from a previous run. ``setdefault`` preserves any
+    outer-session pinning set by a fixture.
     """
+    import os
+    import tempfile
+    if not os.environ.get("NULLRUN_WAL_PATH"):
+        wal_dir = tempfile.mkdtemp(prefix="nullrun-test-wal-")
+        os.environ["NULLRUN_WAL_PATH"] = os.path.join(wal_dir, "sdk.wal")
     rt = NullRunRuntime(api_key="test-key-12345678", _test_mode=True)
     rt._transport._client = MagicMock()
     rt._fetch_policy = MagicMock()
diff --git a/tests/test_toolbox_langgraph.py b/tests/test_toolbox_langgraph.py
index 83d89e2..bab8dcf 100644
--- a/tests/test_toolbox_langgraph.py
+++ b/tests/test_toolbox_langgraph.py
@@ -15,10 +15,18 @@
 
 
 @pytest.fixture(autouse=True)
-def _test_runtime(monkeypatch):
+def _test_runtime(monkeypatch, tmp_path):
     """Provide a runtime in test mode so get_runtime returns without
-    authenticating against a real server."""
+    authenticating against a real server.
+
+    Pins ``NULLRUN_WAL_PATH`` to a tmp_path-scoped file so the
+    constructor's ``Transport._replay_from_wal`` never picks up
+    a stale WAL left over from a previous test run (which would
+    replay real events to a live API and cause HTTP 401 in
+    setup). Mirrors ``conftest::make_test_runtime``.
+    """
     monkeypatch.setenv("NULLRUN_API_KEY", "test-key-12345678")
+    monkeypatch.setenv("NULLRUN_WAL_PATH", str(tmp_path / "sdk.wal"))
     NullRunRuntime.reset_instance()
     # Pre-build a test-mode singleton so get_runtime returns it without
     # hitting the network. Construct directly and store on the singleton