diff --git a/tests/test_hermes_hung_builds.py b/tests/test_hermes_hung_builds.py
new file mode 100644
index 0000000..7c128ec
--- /dev/null
+++ b/tests/test_hermes_hung_builds.py
@@ -0,0 +1,96 @@
+"""Tests for detecting Jenkins builds that never reach a terminal result."""
+
+from __future__ import annotations
+
+from ariadne.services import hermes_hung_builds as module
+
+
+NOW = 1_800_000_000.0
+_MINUTE_MS = 60_000
+
+
+def _build(minutes_ago: float, building: bool = True, **overrides):  # type: ignore[no-untyped-def]
+    payload = {
+        "number": 272,
+        "building": building,
+        "timestamp": int((NOW - minutes_ago * 60) * 1000),
+        "url": "https://ci.example/job/metis/272/",
+    }
+    payload.update(overrides)
+    return payload
+
+
+def test_elapsed_minutes_measures_from_the_start_timestamp() -> None:
+    assert module.elapsed_minutes(_build(75), now=NOW) == 75.0
+
+
+def test_elapsed_minutes_treats_an_unreadable_start_as_zero() -> None:
+    """An unreadable timestamp must never look like a hung build."""
+
+    assert module.elapsed_minutes({}, now=NOW) == 0.0
+    assert module.elapsed_minutes({"timestamp": None}, now=NOW) == 0.0
+    assert module.elapsed_minutes({"timestamp": "not-a-number"}, now=NOW) == 0.0
+    assert module.elapsed_minutes({"timestamp": 0}, now=NOW) == 0.0
+    assert module.elapsed_minutes({"timestamp": -5}, now=NOW) == 0.0
+
+
+def test_elapsed_minutes_never_goes_negative() -> None:
+    """A clock skewed into the future must not produce a negative age."""
+
+    assert module.elapsed_minutes(_build(-30), now=NOW) == 0.0
+
+
+def test_elapsed_minutes_defaults_to_the_wall_clock() -> None:
+    assert module.elapsed_minutes(_build(0.0, timestamp=1)) > 0.0
+
+
+def test_a_running_build_past_the_cap_is_hung() -> None:
+    assert module.is_hung(_build(75), 45.0, now=NOW) is True
+
+
+def test_a_running_build_within_the_cap_is_not_hung() -> None:
+    assert module.is_hung(_build(10), 45.0, now=NOW) is False
+    assert module.is_hung(_build(45), 45.0, now=NOW) is False
+
+
+def test_a_finished_build_is_never_hung() -> None:
+    """Terminal builds are the normal path's business, not this one."""
+
+    assert module.is_hung(_build(600, building=False), 45.0, now=NOW) is False
+    assert module.is_hung({"timestamp": 1}, 45.0, now=NOW) is False
+
+
+def test_a_non_positive_cap_disables_the_check() -> None:
+    assert module.is_hung(_build(6000), 0.0, now=NOW) is False
+    assert module.is_hung(_build(6000), -1.0, now=NOW) is False
+
+
+def test_hung_bundle_carries_only_observed_facts() -> None:
+    bundle = module.hung_bundle("metis", _build(75), 45.0)
+    jenkins = bundle["jenkins"]
+
+    assert jenkins["job"] == "metis"
+    assert jenkins["build_number"] == 272
+    assert jenkins["building"] is True
+    assert jenkins["cap_minutes"] == 45.0
+    assert jenkins["console_failures"] == []
+    assert bundle["log_evidence"] == {"records": []}
+    assert "has been running for" in jenkins["console_tail"]
+    assert "holding a Jenkins agent slot" in jenkins["console_tail"]
+
+
+def test_hung_bundle_survives_a_build_with_nothing_in_it() -> None:
+    """Bundle construction must not raise on a sparse Jenkins payload."""
+
+    bundle = module.hung_bundle("metis", {}, 45.0)
+
+    assert bundle["jenkins"]["build_number"] is None
+    assert bundle["jenkins"]["url"] == ""
+    assert bundle["jenkins"]["elapsed_minutes"] == 0.0
+
+
+def test_reason_and_classification_are_stable_labels() -> None:
+    """These land in issue titles and event details, so they are frozen."""
+
+    assert module.HUNG_REASON == "hung_build"
+    assert module.HUNG_CLASSIFICATION == "build_exceeded_time_cap"