diff --git a/hermes_cli/banner.py b/hermes_cli/banner.py
index eb5be5addc..3f38577138 100644
--- a/hermes_cli/banner.py
+++ b/hermes_cli/banner.py
@@ -4,6 +4,7 @@ import logging
 import os
 import shutil
 import subprocess
+import sys
 import threading
 import time
 from pathlib import Path
@@ -505,8 +506,36 @@ def _daemon(name: Optional[str], target) -> None:
     threading.Thread(target=lambda: _quiet(target), name=name, daemon=True).start()
 
 
+def _skip_background_prefetch() -> bool:
+    """True when the banner's background prefetch threads must not start.
+
+    Under pytest the prefetch daemon threads shell out to git
+    (``fetch``/``rev-parse``/``rev-list``) at an arbitrary point after import,
+    and any test that patches the process-wide ``subprocess`` singleton
+    (``patch("subprocess.run")`` / ``patch("subprocess.Popen")`` — note
+    ``subprocess.run`` calls ``subprocess.Popen`` internally, so a Popen patch
+    captures run() spawns too) can record that stray git spawn instead of —
+    or in addition to — the call it meant to pin.  That cross-talk
+    manufactured CI flakes in tests/tui_gateway/test_subprocess_encoding.py
+    (``encoding=None`` from the update thread's un-encoded ``git fetch``) and
+    tests/tui_gateway/test_bot_relay_methods.py (``argv == ['rev-parse',
+    'FETCH_HEAD']`` from the shallow-checkout path), both of which import
+    ``tui_gateway.server`` — which starts this prefetch at import time.
+    Nothing under pytest needs a live update check; tests that exercise the
+    prefetch itself monkeypatch this predicate to False.
+    """
+    return "PYTEST_CURRENT_TEST" in os.environ or "pytest" in sys.modules
+
+
 def prefetch_update_check():
-    """Kick off update check in a background daemon thread."""
+    """Kick off update check in a background daemon thread.
+
+    No-op under pytest — see ``_skip_background_prefetch``.
+    """
+    if _skip_background_prefetch():
+        _update_check_done.set()
+        return
+
     def _run():
         global _update_result
         _update_result = check_for_updates(passive=True)
@@ -527,6 +556,13 @@ def prefetch_banner_data():
     global _banner_data_prefetch_started
     if _banner_data_prefetch_started:
         return
+    if _skip_background_prefetch():
+        # Same stray-git-spawn cross-talk class as prefetch_update_check:
+        # get_git_banner_state() shells out via the shared subprocess
+        # singleton from a daemon thread, poisoning process-wide subprocess
+        # mocks in unrelated tests.
+        _banner_data_prefetch_started = True
+        return
     _banner_data_prefetch_started = True
     _daemon("banner-data-prefetch", lambda: [_quiet(warm) for warm in (
         get_git_banner_state, get_latest_release_tag, get_available_skills)])
diff --git a/tests/hermes_cli/test_update_check.py b/tests/hermes_cli/test_update_check.py
index e872988dd7..46b4eee937 100644
--- a/tests/hermes_cli/test_update_check.py
+++ b/tests/hermes_cli/test_update_check.py
@@ -98,10 +98,12 @@ def test_cache_is_daily_but_invalidated_when_head_moves(git_repo, monkeypatch):
     tip.assert_called_once()
 
 
-def test_prefetch_non_blocking():
+def test_prefetch_non_blocking(monkeypatch):
     """prefetch_update_check() should return immediately without blocking."""
+    # Reset module state; force the real (non-pytest) thread path.
     banner._update_result = None
     banner._update_check_done = threading.Event()
+    monkeypatch.setattr(banner, "_skip_background_prefetch", lambda: False)
 
     with patch.object(banner, "check_for_updates", return_value=5):
         start = time.monotonic()
@@ -111,6 +113,39 @@ def test_prefetch_non_blocking():
         assert banner._update_result == 5
 
 
+def test_prefetch_update_check_is_noop_under_pytest():
+    """Under pytest the prefetch must NOT start the git-spawning daemon
+    thread: a process-wide ``patch("subprocess.run")`` in an unrelated test
+    can capture the thread's ``git fetch``/``rev-parse`` spawns, flaking the
+    unrelated test's call_args assertions (seen in
+    tests/tui_gateway/test_subprocess_encoding.py and
+    test_bot_relay_methods.py on CI, 2026-08-28)."""
+    banner._update_result = None
+    banner._update_check_done = threading.Event()
+
+    before = {t.ident for t in threading.enumerate()}
+    with patch.object(banner, "check_for_updates") as mock_check:
+        banner.prefetch_update_check()
+        # The done event is set synchronously so get_update_result() callers
+        # don't burn their timeout waiting on a check that will never run.
+        assert banner._update_check_done.is_set()
+        mock_check.assert_not_called()
+    after = {t.ident for t in threading.enumerate()}
+    assert after <= before, "prefetch_update_check spawned a thread under pytest"
+
+
+def test_prefetch_banner_data_is_noop_under_pytest(monkeypatch):
+    """Same stray-git-spawn class: prefetch_banner_data must not start its
+    daemon thread under pytest."""
+    monkeypatch.setattr(banner, "_banner_data_prefetch_started", False)
+    with patch.object(banner, "get_git_banner_state") as mock_state:
+        banner.prefetch_banner_data()
+        # Give a hypothetical stray thread a beat to run — nothing should.
+        time.sleep(0.05)
+        mock_state.assert_not_called()
+    assert banner._banner_data_prefetch_started is True
+
+
 def test_upstream_main_sha_ls_remote_fallback_disables_git_prompts(monkeypatch):
     """When the API is unreachable the HTTPS ls-remote fallback must never inherit the terminal."""
     monkeypatch.setattr(banner, "_github_branch_tip", lambda slug, branch: None)