diff --git a/scripts/execution_report_heartbeat.py b/scripts/execution_report_heartbeat.py
index 8f784cb..377df6c 100644
--- a/scripts/execution_report_heartbeat.py
+++ b/scripts/execution_report_heartbeat.py
@@ -91,9 +91,12 @@ def _base_report_uris() -> list[str]:
 
 
 def _load_required_services() -> list[str]:
+    explicit_services = _split_values(os.environ.get("RUNTIME_HEARTBEAT_REQUIRED_SERVICES"))
+    if explicit_services:
+        return _unique_values(explicit_services)
+
     services = []
     for name in (
-        "RUNTIME_HEARTBEAT_REQUIRED_SERVICES",
         "CLOUD_RUN_SERVICES",
         "CLOUD_RUN_SERVICE",
     ):
@@ -124,12 +127,16 @@ def _load_required_services() -> list[str]:
         except json.JSONDecodeError:
             pass
 
+    return _unique_values(services)
+
+
+def _unique_values(values: list[str]) -> list[str]:
     seen = set()
     unique = []
-    for service in services:
-        if service not in seen:
-            seen.add(service)
-            unique.append(service)
+    for value in values:
+        if value not in seen:
+            seen.add(value)
+            unique.append(value)
     return unique
 
 
diff --git a/tests/test_execution_report_heartbeat.py b/tests/test_execution_report_heartbeat.py
new file mode 100644
index 0000000..046b5c8
--- /dev/null
+++ b/tests/test_execution_report_heartbeat.py
@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+import json
+
+from scripts import execution_report_heartbeat as heartbeat
+
+
+def test_explicit_required_services_override_target_derived_services(monkeypatch):
+    monkeypatch.setenv("RUNTIME_HEARTBEAT_REQUIRED_SERVICES", "svc-daily-a,svc-daily-b")
+    monkeypatch.setenv(
+        "CLOUD_RUN_SERVICE_TARGETS_JSON",
+        json.dumps(
+            {
+                "targets": [
+                    {"service": "svc-daily-a"},
+                    {"service": "svc-monthly"},
+                ]
+            }
+        ),
+    )
+
+    assert heartbeat._load_required_services() == ["svc-daily-a", "svc-daily-b"]
+
+
+def test_required_services_fall_back_to_cloud_run_targets(monkeypatch):
+    monkeypatch.delenv("RUNTIME_HEARTBEAT_REQUIRED_SERVICES", raising=False)
+    monkeypatch.setenv(
+        "CLOUD_RUN_SERVICE_TARGETS_JSON",
+        json.dumps(
+            {
+                "targets": [
+                    {"service": "svc-a"},
+                    {"runtime_target": {"service_name": "svc-b"}},
+                    {"service": "svc-a"},
+                ]
+            }
+        ),
+    )
+
+    assert heartbeat._load_required_services() == ["svc-a", "svc-b"]