diff --git a/extensions/taskplane/engine.ts b/extensions/taskplane/engine.ts
index 43db3812..c54252cb 100644
--- a/extensions/taskplane/engine.ts
+++ b/extensions/taskplane/engine.ts
@@ -4225,14 +4225,23 @@ export async function executeOrchBatch(
 						"info",
 					);
 
-					// TP-040: Emit merge_success event
+					// TP-040: Emit merge_success event.
+					//
+					// `waveIndex` is the segment-round index (0-based), and the supervisor
+					// formatter renders the (N/M) counter using `waveIndex + 1` for N.
+					// For unit consistency we therefore pair it with the segment-level
+					// `batchState.totalWaves` (segment-expanded round count), not
+					// `taskLevelWaveCount` (pre-expansion). Using the task-level count as
+					// the denominator while the numerator counts segment rounds produced
+					// `(4/3)`, `(5/3)`, `(6/3)` style overflow once segments expanded —
+					// see issue #562.
 					emitEvent(
 						stateRoot,
 						{
 							...buildEngineEventBase("merge_success", batchState.batchId, waveIdx, batchState.phase),
 							laneCount: mergedCount,
 							durationMs: mergeResult.totalDurationMs,
-							totalWaves: taskLevelWaveCount,
+							totalWaves: batchState.totalWaves,
 						},
 						onEngineEvent,
 					);
diff --git a/extensions/tests/supervisor.test.ts b/extensions/tests/supervisor.test.ts
index d00ce6c3..701328d6 100644
--- a/extensions/tests/supervisor.test.ts
+++ b/extensions/tests/supervisor.test.ts
@@ -924,6 +924,33 @@ describe("5.x — formatEventNotification", () => {
 		expect(text).toContain("42");
 	});
 
+	it("5.8a: merge_success counter denominator stays >= numerator across segment-expanded waves (regression for #562)", () => {
+		// Polyrepo scenario from #562: 3 task-level waves expand into 6
+		// segment-level merge rounds. The engine emits one merge_success
+		// per segment round, with waveIndex = segment-round index (0..5)
+		// and totalWaves = segment-expanded round count (6). The formatter
+		// must therefore render (1/6) through (6/6), never (4/3), (5/3),
+		// or (6/3) as observed in the bug report.
+		const denominator = 6;
+		for (let waveIdx = 0; waveIdx < denominator; waveIdx++) {
+			const event = {
+				timestamp: "t",
+				type: "merge_success" as any,
+				batchId: "b",
+				waveIndex: waveIdx,
+				totalWaves: denominator,
+			};
+			const text = formatEventNotification(event, "supervised");
+			const expectedNumerator = waveIdx + 1;
+			expect(text).toContain(`(${expectedNumerator}/${denominator})`);
+			// Belt-and-suspenders: the bug surfaced as a numerator that
+			// exceeded the denominator. Pin that the rendered text never
+			// contains `(N/3)` for any N — the only valid denominator in
+			// the segment-expanded case is the segment count.
+			expect(text).not.toMatch(/\(\d+\/3\)/);
+		}
+	});
+
 	it("5.9: formats merge_failed differently for autonomous vs interactive", () => {
 		const event = {
 			timestamp: "t",