diff --git a/server/test/unit/services/diagnosticService.test.ts b/server/test/unit/services/diagnosticService.test.ts
new file mode 100644
index 000000000..a76f28893
--- /dev/null
+++ b/server/test/unit/services/diagnosticService.test.ts
@@ -0,0 +1,151 @@
+import { describe, expect, it, jest, beforeEach, afterEach } from "@jest/globals";
+import { DiagnosticService } from "../../../src/service/business/diagnosticService.ts";
+
+// ── Tests ────────────────────────────────────────────────────────────────────
+
+describe("DiagnosticService", () => {
+	beforeEach(() => {
+		jest.useFakeTimers();
+	});
+
+	afterEach(() => {
+		jest.useRealTimers();
+	});
+
+	describe("serviceName", () => {
+		it("returns diagnosticService from static property", () => {
+			expect(DiagnosticService.SERVICE_NAME).toBe("diagnosticService");
+		});
+
+		it("returns diagnosticService from instance getter", () => {
+			const service = new DiagnosticService();
+			expect(service.serviceName).toBe("diagnosticService");
+		});
+	});
+
+	// ── constructor ─────────────────────────────────────────────────────────
+
+	describe("constructor", () => {
+		it("sets up PerformanceObserver that clears marks on callback", () => {
+			let capturedCallback: ((list: PerformanceObserverEntryList) => void) | null = null;
+			const originalObserver = globalThis.PerformanceObserver;
+			const mockObserve = jest.fn();
+
+			globalThis.PerformanceObserver = jest.fn().mockImplementation((cb: any) => {
+				capturedCallback = cb;
+				return { observe: mockObserve };
+			}) as any;
+
+			const clearMarksSpy = jest.spyOn(performance, "clearMarks");
+			const service = new DiagnosticService();
+
+			expect(mockObserve).toHaveBeenCalledWith({ entryTypes: ["measure"] });
+			expect(capturedCallback).not.toBeNull();
+
+			// Invoke the callback manually
+			const mockEntryList = { getEntries: jest.fn().mockReturnValue([]) } as unknown as PerformanceObserverEntryList;
+			capturedCallback!(mockEntryList);
+
+			expect(mockEntryList.getEntries).toHaveBeenCalled();
+			expect(clearMarksSpy).toHaveBeenCalled();
+
+			clearMarksSpy.mockRestore();
+			globalThis.PerformanceObserver = originalObserver;
+			expect(service).toBeDefined();
+		});
+	});
+
+	// ── getCPUUsage ─────────────────────────────────────────────────────────
+
+	describe("getCPUUsage", () => {
+		it("returns cpu usage metrics after timing period", async () => {
+			const service = new DiagnosticService();
+			const promise = service.getCPUUsage();
+			jest.advanceTimersByTime(1000);
+			const result = await promise;
+
+			expect(result).toHaveProperty("userUsageMs");
+			expect(result).toHaveProperty("systemUsageMs");
+			expect(result).toHaveProperty("usagePercentage");
+			expect(typeof result.userUsageMs).toBe("number");
+			expect(typeof result.systemUsageMs).toBe("number");
+			expect(typeof result.usagePercentage).toBe("number");
+		});
+	});
+
+	// ── getSystemStats ──────────────────────────────────────────────────────
+
+	describe("getSystemStats", () => {
+		it("returns all diagnostic sections", async () => {
+			const service = new DiagnosticService();
+			const promise = service.getSystemStats();
+			// getCPUUsage has 1s timeout, then event loop delay has 0ms timeout
+			jest.advanceTimersByTime(1000);
+			// Need to flush the microtask queue and advance again for the nested setTimeout(0)
+			await jest.advanceTimersByTimeAsync(10);
+			const result = await promise;
+
+			expect(result).toHaveProperty("osStats");
+			expect(result.osStats).toHaveProperty("freeMemoryBytes");
+			expect(result.osStats).toHaveProperty("totalMemoryBytes");
+
+			expect(result).toHaveProperty("memoryUsage");
+			expect(result.memoryUsage).toHaveProperty("rss");
+			expect(result.memoryUsage).toHaveProperty("heapTotal");
+			expect(result.memoryUsage).toHaveProperty("heapUsed");
+			expect(result.memoryUsage).toHaveProperty("external");
+			expect(result.memoryUsage).toHaveProperty("arrayBuffers");
+
+			expect(result).toHaveProperty("cpuUsage");
+			expect(result.cpuUsage).toHaveProperty("userUsageMs");
+
+			expect(result).toHaveProperty("v8HeapStats");
+			expect(result.v8HeapStats).toHaveProperty("totalHeapSizeBytes");
+			expect(result.v8HeapStats).toHaveProperty("usedHeapSizeBytes");
+			expect(result.v8HeapStats).toHaveProperty("heapSizeLimitBytes");
+
+			expect(typeof result.eventLoopDelayMs).toBe("number");
+			expect(result.eventLoopDelayMs).toBeGreaterThanOrEqual(0);
+			expect(typeof result.uptimeMs).toBe("number");
+			expect(result.uptimeMs).toBeGreaterThan(0);
+		});
+
+		it("reads eventLoopDelay from real performance entries", async () => {
+			jest.useRealTimers();
+			const service = new DiagnosticService();
+
+			// Call getSystemStats with real timers — slow but guarantees real perf entries
+			const result = await service.getSystemStats();
+
+			expect(typeof result.eventLoopDelayMs).toBe("number");
+			expect(result.eventLoopDelayMs).toBeGreaterThanOrEqual(0);
+			jest.useFakeTimers();
+		});
+
+		it.each([
+			{ label: "empty entries array", entries: [] },
+			{ label: "undefined entry in array", entries: [undefined] },
+		])("defaults eventLoopDelayMs to 0 when $label", async ({ entries }) => {
+			const service = new DiagnosticService();
+			const originalGetEntries = performance.getEntriesByName;
+			const originalMeasure = performance.measure;
+			const originalMark = performance.mark;
+
+			performance.mark = jest.fn() as any;
+			performance.measure = jest.fn() as any;
+			performance.getEntriesByName = jest.fn().mockReturnValue(entries) as any;
+
+			const promise = service.getSystemStats();
+			jest.advanceTimersByTime(1000);
+			await jest.advanceTimersByTimeAsync(10);
+			const result = await promise;
+
+			expect(result.eventLoopDelayMs).toBe(0);
+			expect(performance.getEntriesByName).toHaveBeenCalledWith("eventLoopDelay");
+
+			performance.getEntriesByName = originalGetEntries;
+			performance.measure = originalMeasure;
+			performance.mark = originalMark;
+		});
+	});
+});