diff --git a/swh/loader/mercurial/tests/conftest.py b/swh/loader/mercurial/tests/conftest.py
--- a/swh/loader/mercurial/tests/conftest.py
+++ b/swh/loader/mercurial/tests/conftest.py
@@ -37,3 +37,12 @@
         "max_content_size": 104857600,
         "temp_directory": str(tmp_path),
     }
+
+
+@pytest.fixture(autouse=True)
+def swh_mercurial_set_plain(monkeypatch):
+    """Mercurial is customizable by users, so we use built-in environment
+    variables to turn off all customization in tests.
+    """
+    monkeypatch.setenv("HGPLAIN", "")
+    monkeypatch.setenv("HGRCPATH", "")