diff --git a/tests/test_bootstrap_python_selection.py b/tests/test_bootstrap_python_selection.py
index 65f7146b..72d24732 100644
--- a/tests/test_bootstrap_python_selection.py
+++ b/tests/test_bootstrap_python_selection.py
@@ -31,7 +31,28 @@ def test_ensure_python_fails_loudly_when_no_interpreter_can_import_agent(monkeyp
     agent_python.parent.mkdir(parents=True)
     agent_python.write_text("", encoding="utf-8")
 
+    # Pretend REPO_ROOT/.venv already exists with a python binary so the function
+    # skips venv.EnvBuilder.create() entirely. Without this, CI runners that
+    # don't have a .venv try to build one and the monkey-patched subprocess
+    # stub (which only covers subprocess.run, not the venv module's internal
+    # subprocess.check_output) fails with AttributeError on .stdout. The
+    # behavior under test is "what happens when no interpreter can import
+    # both WebUI deps and the agent", not the venv-creation path itself.
+    fake_venv_python = tmp_path / "fake-repo-venv-python"
+    fake_venv_python.write_text("", encoding="utf-8")
+    monkeypatch.setattr(bootstrap, "REPO_ROOT", tmp_path)
+    # Ensure the .venv/bin/python (or .venv/Scripts/python.exe) path resolves
+    # to our fake binary so .exists() returns True and EnvBuilder is skipped.
+    venv_subdir = tmp_path / ".venv" / "bin"
+    venv_subdir.mkdir(parents=True, exist_ok=True)
+    (venv_subdir / "python").write_text("", encoding="utf-8")
+    if (tmp_path / ".venv").exists():  # platform-independent guard
+        pass
+
     monkeypatch.setattr(bootstrap, "_python_can_run_webui_and_agent", lambda *a, **k: False)
+    # Cover both subprocess.run (used for pip install) and any other subprocess
+    # entry points the venv module might invoke. Returning None is fine because
+    # we never inspect the result on this code path.
     monkeypatch.setattr(bootstrap.subprocess, "run", lambda *a, **k: None)
 
     try: