diff --git a/src/github/utils/image-downloader.ts b/src/github/utils/image-downloader.ts
index 63922f9..924b009 100644
--- a/src/github/utils/image-downloader.ts
+++ b/src/github/utils/image-downloader.ts
@@ -2,11 +2,12 @@ import fs from "fs/promises";
 import path from "path";
 import type { Octokits } from "../api/client";
 
-// Mutable fs reference for cross-platform test compatibility.
-// On Linux, Bun gives test and source files different fs/promises objects,
-// so spyOn(fs, "mkdir") in a test file won't intercept calls here.
-// Tests override these methods directly instead.
-export const _fsForTest = {
+export type FsOps = {
+  mkdir: typeof fs.mkdir;
+  writeFile: typeof fs.writeFile;
+};
+
+const defaultFsOps: FsOps = {
   mkdir: fs.mkdir.bind(fs),
   writeFile: fs.writeFile.bind(fs),
 };
@@ -72,11 +73,12 @@ export async function downloadCommentImages(
   owner: string,
   repo: string,
   comments: CommentWithImages[],
+  fsOps: FsOps = defaultFsOps,
 ): Promise<Map<string, string>> {
   const urlToPathMap = new Map<string, string>();
   const downloadsDir = "/tmp/github-images";
 
-  await _fsForTest.mkdir(downloadsDir, { recursive: true });
+  await fsOps.mkdir(downloadsDir, { recursive: true });
 
   const { imageRegex, htmlImgRegex } = buildImageRegexes();
 
@@ -226,7 +228,7 @@ export async function downloadCommentImages(
           const arrayBuffer = await imageResponse.arrayBuffer();
           const buffer = Buffer.from(arrayBuffer);
 
-          await _fsForTest.writeFile(localPath, buffer);
+          await fsOps.writeFile(localPath, buffer);
           console.log(`✓ Saved: ${localPath}`);
 
           urlToPathMap.set(originalUrl, localPath);
diff --git a/test/image-downloader.test.ts b/test/image-downloader.test.ts
index d68b63f..36656df 100644
--- a/test/image-downloader.test.ts
+++ b/test/image-downloader.test.ts
@@ -10,8 +10,8 @@ import {
 } from "bun:test";
 import {
   downloadCommentImages,
-  _fsForTest,
   type CommentWithImages,
+  type FsOps,
 } from "../src/github/utils/image-downloader";
 import type { Octokits } from "../src/github/api/client";
 
@@ -22,13 +22,13 @@ describe("downloadCommentImages", () => {
   let fetchSpy: any;
   let savedServerUrl: string | undefined;
 
-  // Store originals for restoration
-  const origMkdir = _fsForTest.mkdir;
-  const origWriteFile = _fsForTest.writeFile;
-
-  // Mock functions for fs operations
+  // Mock functions for fs operations — injected via parameter, no module mutation needed
   const mockMkdir = jest.fn().mockResolvedValue(undefined);
   const mockWriteFile = jest.fn().mockResolvedValue(undefined);
+  const mockFsOps: FsOps = {
+    mkdir: mockMkdir as any,
+    writeFile: mockWriteFile as any,
+  };
 
   beforeEach(() => {
     // Pin GITHUB_SERVER_URL so tests pass regardless of runner environment.
@@ -41,11 +41,8 @@ describe("downloadCommentImages", () => {
     consoleWarnSpy = spyOn(console, "warn").mockImplementation(() => {});
     consoleErrorSpy = spyOn(console, "error").mockImplementation(() => {});
 
-    // Override _fsForTest methods directly — works cross-platform
     mockMkdir.mockClear();
     mockWriteFile.mockClear();
-    _fsForTest.mkdir = mockMkdir as any;
-    _fsForTest.writeFile = mockWriteFile as any;
 
     // Set fake system time for consistent filenames
     setSystemTime(new Date("2024-01-01T00:00:00.000Z")); // 1704067200000
@@ -59,10 +56,6 @@ describe("downloadCommentImages", () => {
       delete process.env.GITHUB_SERVER_URL;
     }
 
-    // Restore original fs methods
-    _fsForTest.mkdir = origMkdir;
-    _fsForTest.writeFile = origWriteFile;
-
     consoleLogSpy.mockRestore();
     consoleWarnSpy.mockRestore();
     consoleErrorSpy.mockRestore();
@@ -90,7 +83,13 @@ describe("downloadCommentImages", () => {
     const mockOctokit = createMockOctokit();
     const comments: CommentWithImages[] = [];
 
-    await downloadCommentImages(mockOctokit, "owner", "repo", comments);
+    await downloadCommentImages(
+      mockOctokit,
+      "owner",
+      "repo",
+      comments,
+      mockFsOps,
+    );
 
     expect(mockMkdir).toHaveBeenCalledWith("/tmp/github-images", {
       recursive: true,
@@ -112,6 +111,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(result.size).toBe(0);
@@ -155,6 +155,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(mockOctokit.rest.issues.getComment).toHaveBeenCalledWith({
@@ -215,6 +216,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(mockOctokit.rest.pulls.getReviewComment).toHaveBeenCalledWith({
@@ -262,6 +264,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(mockOctokit.rest.pulls.getReview).toHaveBeenCalledWith({
@@ -309,6 +312,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(mockOctokit.rest.issues.get).toHaveBeenCalledWith({
@@ -357,6 +361,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(mockOctokit.rest.pulls.get).toHaveBeenCalledWith({
@@ -408,6 +413,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(2);
@@ -459,6 +465,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1); // Only downloaded once
@@ -492,6 +499,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(result.size).toBe(0);
@@ -532,6 +540,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(result.size).toBe(0);
@@ -563,6 +572,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(result.size).toBe(0);
@@ -634,6 +644,7 @@ describe("downloadCommentImages", () => {
         "owner",
         "repo",
         comments,
+        mockFsOps,
       );
       expect(result.get(url)).toBe(
         `/tmp/github-images/image-${1704067200000 + callIndex}-0${ext}`,
@@ -678,6 +689,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1);
@@ -723,6 +735,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(mockOctokit.rest.issues.getComment).toHaveBeenCalledWith({
@@ -787,6 +800,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(2);
@@ -837,6 +851,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(2);
@@ -883,6 +898,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1); // Only downloaded once
@@ -927,6 +943,7 @@ describe("downloadCommentImages", () => {
       "owner",
       "repo",
       comments,
+      mockFsOps,
     );
 
     expect(fetchSpy).toHaveBeenCalledTimes(1);