diff --git a/testing/conftest.py b/testing/conftest.py
index 8e77fcae5ab..2a3ce5203d9 100644
--- a/testing/conftest.py
+++ b/testing/conftest.py
@@ -21,6 +21,15 @@ def restore_tracing():
             sys.settrace(orig_trace)
 
 
+@pytest.fixture(autouse=True)
+def set_column_width(monkeypatch: pytest.MonkeyPatch) -> None:
+    """
+    Force terminal width to 80: some tests check the formatting of --help, which is sensible
+    to terminal width.
+    """
+    monkeypatch.setenv("COLUMNS", "80")
+
+
 @pytest.hookimpl(hookwrapper=True, tryfirst=True)
 def pytest_collection_modifyitems(items):
     """Prefer faster tests.