Heartbeat per event class, not per source

audit-core's completeness contract landed, and our single per-source beat
is the shape it rules inadequate: it is discharged by whichever class is
busy, so a revocation stream that has gone silent looks identical to a
quiet one — and revocation is the only silence here that matters.

Emit one nothing-to-report assertion per declared class, all four in one
transaction so a partial emission cannot report some classes healthy and
others stalled. Carry type audit-core.heartbeat with class and assertion
on data. Pin that the first beat goes out at startup rather than an
interval later, since a declared-but-never-sent class is their
no_heartbeat_since_registration finding and not a skip.

Declare heartbeat_classes and the reconciliation surface in the source
registration, including the residual neither control covers: a
compromised emitter suppresses the event and its own heartbeat together.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HyybaE7DUXrWYrhbnESCTe

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 1275879@bnt-lap001
Assistant-Session: eb464208-f821-41b2-bc5a-a6c33d92a8ad

approval_engine/store.py

@@ -36,6 +36,12 @@ AUDIT_SCHEMA = "audit-core.event.v1alpha1"
 SOURCE = "approval-engine"
 SCOPE = "netkingdom-approvals"
 EVENT_CLASSES = ("issuance", "use", "supersession", "revocation", "heartbeat")
+
+# Declared per class, not per source (audit-core `docs/stream-completeness.md`,
+# AUDIT-WP-0009-T04). A per-source heartbeat is satisfied by the chattiest
+# class and says nothing about the quiet one — here that is `revocation`, which
+# is the class whose silence actually matters.
+HEARTBEAT_CLASSES = ("issuance", "use", "supersession", "revocation")
 LATEST_SCHEMA_VERSION = 5
 
 SCHEMA = """
@@ -870,7 +876,13 @@ class Engine:
             "scope": SCOPE,
             "source": SOURCE,
             "actor": actor,
-            "action": f"approval.{event_class}",
+            # audit-core matches heartbeats on the type string it publishes,
+            # not on our vocabulary.
+            "action": (
+                "audit-core.heartbeat"
+                if event_class == "heartbeat"
+                else f"approval.{event_class}"
+            ),
             "resource": resource,
             "outcome": "success",
             "reason": None,
@@ -943,18 +955,38 @@ class Engine:
         return obj.status, False, consumed, obj.status
 
     def emit_heartbeat(self) -> dict[str, Any]:
+        """Emit one `nothing-to-report` assertion per declared event class.
+
+        One heartbeat covering the whole source would be discharged by whichever
+        class happens to be busy, which is the failure heartbeats exist to
+        catch: a revocation stream that has gone silent looks identical to a
+        quiet one. Each class asserts for itself.
+
+        All four are written in a single transaction. A partial emission would
+        report some classes healthy and leave others looking stalled, which is
+        a worse signal than none.
+        """
         conn = self._conn()
         counts = self.transition_counts()
-        counts["heartbeat"] += 1
+        counts["heartbeat"] += len(HEARTBEAT_CLASSES)
+        emitted: dict[str, str] = {}
         try:
             conn.execute("BEGIN IMMEDIATE")
-            event_id = self._outbox_insert(
-                conn,
-                "heartbeat",
-                None,
-                actor=None,
-                extra={"assertion": "nothing-to-report", "counts": counts},
-            )
+            for event_class in HEARTBEAT_CLASSES:
+                emitted[event_class] = self._outbox_insert(
+                    conn,
+                    "heartbeat",
+                    None,
+                    actor=None,
+                    extra={
+                        # Overrides the emitting class: audit-core reads this as
+                        # the class being asserted about, not as the class of
+                        # the heartbeat itself.
+                        "class": event_class,
+                        "assertion": "nothing-to-report",
+                        "emitted_count": counts.get(event_class, 0),
+                    },
+                )
             conn.commit()
         except sqlite3.Error as exc:
             conn.rollback()
@@ -962,7 +994,11 @@ class Engine:
         except Exception:
             conn.rollback()
             raise
-        return {"event_id": event_id, "assertion": "nothing-to-report", "counts": counts}
+        return {
+            "assertion": "nothing-to-report",
+            "classes": dict(emitted),
+            "counts": counts,
+        }
 
     def transition_counts(self) -> dict[str, int]:
         conn = self._conn()