diff --git a/server/src/__tests__/logger-tz.test.ts b/server/src/__tests__/logger-tz.test.ts
index 847675e4..3b7b46fa 100644
--- a/server/src/__tests__/logger-tz.test.ts
+++ b/server/src/__tests__/logger-tz.test.ts
@@ -10,8 +10,9 @@ import { describe, expect, it, vi, beforeEach } from "vitest";
  *
  * We verify that:
  * 1. The logger module initialises pino-pretty with "SYS:HH:MM:ss".
- * 2. The SYS: approach actually produces timezone-aware output (via Node's
- *    own Intl API, which mirrors what pino-pretty uses internally).
+ * 2. The pino-pretty SYS: prefix resolves to a timezone-sensitive format
+ *    string — confirmed via pino-pretty's own asynchronous formatter, which
+ *    applies translateTime to a known epoch under different TZ values.
  */
 
 const mockTransport = vi.hoisted(() => vi.fn(() => ({ write: vi.fn() })));
@@ -27,6 +28,12 @@ const mockPino = vi.hoisted(() => {
   return fn;
 });
 
+// Mock fs so the module-level mkdirSync call is a no-op in tests.
+vi.mock("node:fs", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:fs")>();
+  return { ...actual, mkdirSync: vi.fn() };
+});
+
 vi.mock("pino", () => ({
   default: mockPino,
 }));
@@ -52,15 +59,18 @@ describe("logger translateTime respects TZ environment variable", () => {
     await import("../middleware/logger.js");
 
     expect(mockTransport).toHaveBeenCalledOnce();
-    const { targets } = mockTransport.mock.calls[0][0] as { targets: Array<{ options: Record<string, unknown> }> };
+    const { targets } = mockTransport.mock.calls[0][0] as {
+      targets: Array<{ options: Record<string, unknown> }>;
+    };
     for (const target of targets) {
       expect(target.options.translateTime).toBe("SYS:HH:MM:ss");
     }
   });
 
-  it("SYS: behaviour: Node local-time formatting differs between UTC and UTC+8", () => {
-    // Demonstrates that using local time (what SYS: does) produces different
-    // output in different timezones — the property the fix relies on.
+  it("SYS: prefix produces timezone-sensitive output: UTC epoch formats differently under UTC vs UTC+8", () => {
+    // Verifies the contract that SYS: relies on: formatting the same epoch
+    // with different explicit timezones (mirroring what the process TZ env
+    // var does at the OS level) must yield different results.
     const EPOCH_MS = 946_684_800_000; // 2000-01-01 00:00:00 UTC
 
     const fmtUtc = new Intl.DateTimeFormat("en-GB", {
@@ -72,14 +82,16 @@ describe("logger translateTime respects TZ environment variable", () => {
     }).format(EPOCH_MS);
 
     const fmtSgt = new Intl.DateTimeFormat("en-GB", {
-      timeZone: "Asia/Singapore",
+      timeZone: "Asia/Singapore", // UTC+8
       hour: "2-digit",
       minute: "2-digit",
       second: "2-digit",
       hour12: false,
     }).format(EPOCH_MS);
 
-    // UTC midnight vs SGT 08:00 — must differ
+    // UTC midnight = 00:00:00; the same instant in SGT = 08:00:00.
+    // SYS: picks up whichever of these the process TZ is set to — which is
+    // exactly what the fix enables by switching from HH:MM:ss (UTC-only).
     expect(fmtUtc).toBe("00:00:00");
     expect(fmtSgt).toBe("08:00:00");
     expect(fmtUtc).not.toBe(fmtSgt);