diff --git a/src/backend/core/tests/conftest.py b/src/backend/core/tests/conftest.py
index b39af92a7..45e0a4dbb 100644
--- a/src/backend/core/tests/conftest.py
+++ b/src/backend/core/tests/conftest.py
@@ -10,7 +10,7 @@ import responses
 
 from core import factories
 from core.services.yhub_services import YHubService
-from core.tests.utils.urls import reload_urls
+from core.tests.utils.urls import reload_urls, restore_urls
 
 USER = "user"
 TEAM = "team"
@@ -23,6 +23,25 @@ def clear_cache():
     cache.clear()
 
 
+@pytest.fixture(autouse=True)
+def restore_urlconf():
+    """
+    Put the URLs back after a test that reloaded them.
+
+    Reloading is how a test makes the resource server routes appear or checks
+    that they are absent, but the URLconf belongs to the process: without this,
+    a test asserting a 404 on `/external_api/` and one asserting a 401 pass or
+    fail depending on which ran first in their worker.
+
+    Autouse and asking for nothing, so it is set up before the `settings`
+    fixture and torn down after it: the reload then sees the settings of the
+    project, not the ones of the test.
+    """
+    yield
+
+    restore_urls()
+
+
 @pytest.fixture
 def mock_user_teams():
     """Mock for the "teams" property on the User model."""
diff --git a/src/backend/core/tests/external_api/test_external_api_documents.py b/src/backend/core/tests/external_api/test_external_api_documents.py
index b4bb8f1eb..183b092ed 100644
--- a/src/backend/core/tests/external_api/test_external_api_documents.py
+++ b/src/backend/core/tests/external_api/test_external_api_documents.py
@@ -581,11 +581,17 @@ def test_external_api_documents_trashbin_not_allowed(
     assert response.status_code == 403
 
 
-def test_external_api_documents_create_for_owner_not_allowed():
+def test_external_api_documents_create_for_owner_not_allowed(
+    resource_server_backend_conf,
+):
     """
     Authenticated users SHOULD NOT be allowed to call create documents
     on behalf of other users.
     This API endpoint is reserved for server-to-server calls.
+
+    The route only exists when the resource server is enabled, hence the
+    fixture: the endpoint answering 401 is what this asserts, not the
+    `/external_api/` prefix being routed at all.
     """
     user = factories.UserFactory()
 
diff --git a/src/backend/core/tests/utils/urls.py b/src/backend/core/tests/utils/urls.py
index 78455de1e..2da23f7ee 100644
--- a/src/backend/core/tests/utils/urls.py
+++ b/src/backend/core/tests/utils/urls.py
@@ -5,16 +5,47 @@ import importlib
 from django.urls import clear_url_caches
 
 
+class _URLConf:
+    """
+    Whether a test reloaded the URLs of this process.
+
+    The URLconf is module-level state: a reload outlives the test that did it
+    and every test running after it in the same worker sees its routes — which
+    ones share a worker changes from one run to the next. `restore_urls` puts
+    the default back, and this flag keeps it to the tests that need it.
+    """
+
+    reloaded = False
+
+
+def _reload():
+    """Reload the URL modules and drop the resolver caches."""
+    import core.urls  # pylint:disable=import-outside-toplevel # noqa: PLC0415
+
+    import impress.urls  # pylint:disable=import-outside-toplevel # noqa: PLC0415
+
+    importlib.reload(core.urls)
+    importlib.reload(impress.urls)
+    clear_url_caches()
+
+
 def reload_urls():
     """
     Reload the URLs. Since the URLs are loaded based on a
     settings value, we need to reload them to make the
     URL settings based condition effective.
     """
-    import core.urls  # pylint:disable=import-outside-toplevel # noqa: PLC0415
+    _URLConf.reloaded = True
+    _reload()
 
-    import impress.urls  # pylint:disable=import-outside-toplevel # noqa: PLC0415
 
-    importlib.reload(core.urls)
-    importlib.reload(impress.urls)
-    clear_url_caches()
+def restore_urls():
+    """
+    Reload the URLs of a test that changed them, so the next one starts clean.
+
+    Called once the settings of the test are restored, so the routes are the
+    ones the settings of the project declare.
+    """
+    if _URLConf.reloaded:
+        _URLConf.reloaded = False
+        _reload()