diff --git a/server/test/integration/checksRepositoryGroupedChecks.test.ts b/server/test/integration/checksRepositoryGroupedChecks.test.ts
new file mode 100644
index 000000000..cf5384d0c
--- /dev/null
+++ b/server/test/integration/checksRepositoryGroupedChecks.test.ts
@@ -0,0 +1,103 @@
+import { describe, expect, it, beforeAll, afterAll, beforeEach } from "@jest/globals";
+import mongoose from "mongoose";
+import { MongoMemoryServer } from "mongodb-memory-server";
+import MongoChecksRepository from "../../src/domain/checks/check.repository.mongo.ts";
+import { CheckModel } from "../../src/domain/checks/check.model.ts";
+import type { UptimeChecksResult } from "../../src/domain/checks/check.type.ts";
+import type { ILogger } from "../../src/utils/logger.ts";
+import { createMockLogger } from "../helpers/createMockLogger.ts";
+
+// ── Real-Mongo harness ─────────────────────────────────────────────────────────
+// The guarantee under test — every phase average uses the full bucket as its
+// denominator — IS the $ifNull-inside-$avg behavior of the groupedChecks facet, so it
+// can only be exercised against a live aggregation engine. Reused keep-alive sockets
+// produce checks with no timings subdocument at all; without the coercion, $avg would
+// skip them and average dns/tcp/tls over fresh-connection checks only.
+
+let mongod: MongoMemoryServer;
+
+beforeAll(async () => {
+	mongod = await MongoMemoryServer.create();
+	await mongoose.connect(mongod.getUri());
+	await CheckModel.createCollection(); // timeseries collections must exist before insert
+}, 120_000);
+
+afterAll(async () => {
+	await mongoose.disconnect();
+	await mongod.stop();
+});
+
+beforeEach(async () => {
+	await CheckModel.deleteMany({});
+});
+
+// ── Helpers ──────────────────────────────────────────────────────────────────
+
+const MONITOR_ID = new mongoose.Types.ObjectId();
+const TEAM_ID = new mongoose.Types.ObjectId();
+
+// One shared createdAt puts every seeded check in the same hourly bucket of the "day"
+// range, keeping the facet's group stage deterministic.
+const BUCKET_TIME = new Date(Date.now() - 10 * 60 * 1000);
+
+const seedCheck = (overrides: Record<string, unknown> = {}) =>
+	CheckModel.create({
+		metadata: { monitorId: MONITOR_ID, teamId: TEAM_ID, type: "http" },
+		status: true,
+		responseTime: 100,
+		createdAt: BUCKET_TIME,
+		...overrides,
+	});
+
+const findGrouped = async (repo: MongoChecksRepository) =>
+	(await repo.findByDateRangeAndMonitorId(MONITOR_ID.toString(), "day", { type: "http" })) as UptimeChecksResult;
+
+describe("MongoChecksRepository groupedChecks facet", () => {
+	let repo: MongoChecksRepository;
+
+	beforeEach(() => {
+		repo = new MongoChecksRepository(createMockLogger() as unknown as ILogger);
+	});
+
+	it("averages each phase over the full bucket, counting checks without timings as 0", async () => {
+		await seedCheck({
+			responseTime: 100,
+			timings: { phases: { dns: 10, tcp: 20, tls: 30, request: 8, firstByte: 40, download: 6 } },
+		});
+		await seedCheck({ responseTime: 200 }); // reused socket: no timings subdocument at all
+
+		const result = await findGrouped(repo);
+
+		expect(result.groupedChecks).toHaveLength(1);
+		// Plain $avg would skip the timings-less check and report 10/20/30/8/40/6 here.
+		expect(result.groupedChecks[0]).toMatchObject({
+			totalChecks: 2,
+			avgDns: 5,
+			avgTcp: 10,
+			avgTls: 15,
+			avgRequest: 4,
+			avgFirstByte: 20,
+			avgDownload: 3,
+			avgResponseTime: 150,
+		});
+	});
+
+	it("yields 0 (not null) for every phase in a bucket of only timings-less checks", async () => {
+		await seedCheck();
+		await seedCheck();
+
+		const result = await findGrouped(repo);
+
+		expect(result.groupedChecks).toHaveLength(1);
+		// The client sums these fields for the stacked chart; null would poison the math.
+		expect(result.groupedChecks[0]).toMatchObject({
+			totalChecks: 2,
+			avgDns: 0,
+			avgTcp: 0,
+			avgTls: 0,
+			avgRequest: 0,
+			avgFirstByte: 0,
+			avgDownload: 0,
+		});
+	});
+});