diff --git a/tests/hermes_cli/test_ssh_ownership_endpoint.py b/tests/hermes_cli/test_ssh_ownership_endpoint.py
index ad8d21d447..ee6154ba80 100644
--- a/tests/hermes_cli/test_ssh_ownership_endpoint.py
+++ b/tests/hermes_cli/test_ssh_ownership_endpoint.py
@@ -1,3 +1,7 @@
+import os
+import types
+
+import pytest
 from fastapi.testclient import TestClient
 
 from hermes_cli import web_server
@@ -25,13 +29,23 @@ def test_ssh_ownership_endpoint_requires_token_and_returns_exact_nonce(monkeypat
     }
 
 
-def test_ssh_ownership_reports_replaced_runtime(monkeypatch):
+def test_ssh_ownership_reports_replaced_runtime(tmp_path, monkeypatch):
     token = "t" * 64
     monkeypatch.setattr(web_server, "_SESSION_TOKEN", token)
     monkeypatch.setattr(web_server, "_SSH_OWNER_NONCE", "0123456789abcdef")
     monkeypatch.setattr(web_server, "_SSH_RUNTIME_MARKER", None)
-    monkeypatch.setattr(web_server, "_SSH_RUNTIME_PURELIB", ("/venv/site-packages", 10, 20))
-    monkeypatch.setattr(web_server.os, "stat", lambda _path: type("Stat", (), {"st_dev": 10, "st_ino": 21})())
+    # A REAL purelib file whose recorded inode deliberately mismatches what
+    # os.stat now reports — never patch os.stat globally here: web_server.os
+    # is the os module itself, and swapping its stat() poisons every other
+    # thread in this worker process (daemon threads from earlier tests crash
+    # in their excepthooks → nondeterministic teardown errors across the
+    # whole suite, the Aug 2026 CI flake).
+    purelib = tmp_path / "site-packages"
+    purelib.mkdir()
+    st = purelib.stat()
+    monkeypatch.setattr(
+        web_server, "_SSH_RUNTIME_PURELIB", (str(purelib), st.st_dev, st.st_ino + 1)
+    )
     client = TestClient(web_server.app)
 
     response = client.get("/api/ssh/ownership", headers={"X-Hermes-Session-Token": token})
@@ -49,10 +63,14 @@ def test_ssh_runtime_marker_detects_recreated_venv_even_with_reused_inode(
     file is the deterministic tier: it dies with the old tree."""
     purelib = tmp_path / "venv" / "lib" / "site-packages"
     purelib.mkdir(parents=True)
+    # Swap the MODULE ATTRIBUTE on web_server, not sysconfig.get_paths itself:
+    # sysconfig is process-global, and mutating it races every other thread
+    # in the worker (same cross-thread poisoning class as the os.stat patch
+    # this file used to have).
     monkeypatch.setattr(
-        web_server.sysconfig,
-        "get_paths",
-        lambda *a, **k: {"purelib": str(purelib)},
+        web_server,
+        "sysconfig",
+        types.SimpleNamespace(get_paths=lambda *a, **k: {"purelib": str(purelib)}),
     )
 
     web_server._apply_ssh_owner_nonce("0123456789abcdef")
@@ -76,10 +94,14 @@ def test_ssh_runtime_marker_survives_in_place_installs(tmp_path, monkeypatch):
     """pip/uv installs INTO the live venv must not read as a replacement."""
     purelib = tmp_path / "venv" / "lib" / "site-packages"
     purelib.mkdir(parents=True)
+    # Swap the MODULE ATTRIBUTE on web_server, not sysconfig.get_paths itself:
+    # sysconfig is process-global, and mutating it races every other thread
+    # in the worker (same cross-thread poisoning class as the os.stat patch
+    # this file used to have).
     monkeypatch.setattr(
-        web_server.sysconfig,
-        "get_paths",
-        lambda *a, **k: {"purelib": str(purelib)},
+        web_server,
+        "sysconfig",
+        types.SimpleNamespace(get_paths=lambda *a, **k: {"purelib": str(purelib)}),
     )
 
     web_server._apply_ssh_owner_nonce("0123456789abcdef")
@@ -95,27 +117,33 @@ def test_ssh_runtime_readonly_purelib_falls_back_to_stat(tmp_path, monkeypatch):
     stat-snapshot fallback still arms (weaker, never a false stale)."""
     purelib = tmp_path / "venv" / "lib" / "site-packages"
     purelib.mkdir(parents=True)
+    # Swap the MODULE ATTRIBUTE on web_server, not sysconfig.get_paths itself:
+    # sysconfig is process-global, and mutating it races every other thread
+    # in the worker (same cross-thread poisoning class as the os.stat patch
+    # this file used to have).
     monkeypatch.setattr(
-        web_server.sysconfig,
-        "get_paths",
-        lambda *a, **k: {"purelib": str(purelib)},
+        web_server,
+        "sysconfig",
+        types.SimpleNamespace(get_paths=lambda *a, **k: {"purelib": str(purelib)}),
     )
-    real_open = open
-
-    def refuse_marker(path, *a, **k):
-        if ".hermes-ssh-runtime-" in str(path):
-            raise OSError(30, "Read-only file system")
-        return real_open(path, *a, **k)
-
-    monkeypatch.setattr("builtins.open", refuse_marker)
-
-    web_server._apply_ssh_owner_nonce("0123456789abcdef")
+    # Make the directory REALLY unwritable instead of patching builtins.open:
+    # a global open() patch races every other thread in the worker process
+    # (daemon threads crash in their excepthooks → nondeterministic teardown
+    # errors file-wide, the Aug 2026 CI flake). chmod is thread-safe and
+    # exercises the genuine OSError path.
+    if os.geteuid() == 0:  # pragma: no cover - root ignores mode bits
+        pytest.skip("directory write bits are not enforced for root")
+    purelib.chmod(0o555)
     try:
-        assert web_server._SSH_RUNTIME_MARKER is None
-        assert web_server._SSH_RUNTIME_PURELIB is not None
-        assert web_server._ssh_runtime_intact() is True
+        web_server._apply_ssh_owner_nonce("0123456789abcdef")
+        try:
+            assert web_server._SSH_RUNTIME_MARKER is None
+            assert web_server._SSH_RUNTIME_PURELIB is not None
+            assert web_server._ssh_runtime_intact() is True
+        finally:
+            web_server._apply_ssh_owner_nonce(None)
     finally:
-        web_server._apply_ssh_owner_nonce(None)
+        purelib.chmod(0o755)  # let tmp_path cleanup succeed
 
 
 def test_ssh_ownership_endpoint_is_absent_without_owner_nonce(monkeypatch):