diff --git a/src/common/test_tools/plugin.py b/src/common/test_tools/plugin.py
index 47e5ee2e..92459631 100644
--- a/src/common/test_tools/plugin.py
+++ b/src/common/test_tools/plugin.py
@@ -31,10 +31,17 @@ def assert_metric_impl() -> Generator[AssertMetricFixture, None, None]:
     registry = prometheus_client.REGISTRY
     collectors = [*registry._collector_to_names]
 
-    # Reset registry state
+    # Reset registry state. `MetricWrapperBase.clear()` is implemented for
+    # parent metrics (those declared with labels) — calling it on an
+    # unlabeled metric raises AttributeError because `_lock` is only set on
+    # parents. Reset unlabeled metrics' observable state via `_metric_init`.
     for collector in collectors:
-        if isinstance(collector, prometheus_client.metrics.MetricWrapperBase):
+        if not isinstance(collector, prometheus_client.metrics.MetricWrapperBase):
+            continue
+        if collector._is_parent():  # type: ignore[no-untyped-call]
             collector.clear()
+        else:
+            collector._metric_init()  # type: ignore[no-untyped-call]
 
     def _assert_metric(
         *,
diff --git a/tests/conftest.py b/tests/conftest.py
index afe35873..ad91ead7 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -92,3 +92,11 @@ def test_metric() -> prometheus_client.Counter:
         "Total number of tests run by pytest.",
         ["test_name"],
     )
+
+
+@pytest.fixture(scope="session")
+def test_unlabeled_metric() -> prometheus_client.Counter:
+    return prometheus_client.Counter(
+        "pytest_unlabeled_total",
+        "Total number of pytest unlabeled-metric assertions.",
+    )
diff --git a/tests/unit/common/test_tools/test_plugin.py b/tests/unit/common/test_tools/test_plugin.py
index 1ac164d4..14afa29b 100644
--- a/tests/unit/common/test_tools/test_plugin.py
+++ b/tests/unit/common/test_tools/test_plugin.py
@@ -25,6 +25,22 @@ def test_assert_metrics__metric_incremented__asserts_expected(
     )
 
 
+def test_assert_metrics__unlabeled_metric_incremented__asserts_expected(
+    assert_metric: AssertMetricFixture,
+    test_unlabeled_metric: prometheus_client.Counter,
+) -> None:
+    # Given an unlabeled counter incremented during the test (the fixture
+    # has already reset the registry). The unlabeled-metric reset previously
+    # raised AttributeError because `MetricWrapperBase.clear()` only works
+    # on labeled parents; the fixture now uses `_metric_init` for unlabeled.
+
+    # When the counter is incremented
+    test_unlabeled_metric.inc()
+
+    # Then the assertion sees a fresh count of 1
+    assert_metric(name="pytest_unlabeled_total", labels={}, value=1)
+
+
 def test_assert_metrics__after_registry_reset__raises_assertion(
     test_metric: prometheus_client.Counter,
 ) -> None: