diff --git a/package.json b/package.json
index daf6116..1a25e25 100755
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "ts-patch",
-  "version": "1.2.5",
+  "version": "1.3.0",
   "description": "Patch typescript to support custom transformers in tsconfig.json",
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",
@@ -65,6 +65,7 @@
     "jest-mock-process": "^1.4.0",
     "mock-fs": "^4.12.0",
     "resolve": "^1.15.1",
+    "rfdc": "^1.1.4",
     "rimraf": "^3.0.2",
     "rollup": "^2.1.0",
     "rollup-plugin-dts": "^1.4.7",
diff --git a/test/lib/mock-utils.ts b/test/lib/mock-utils.ts
index a984ab4..3014c7d 100755
--- a/test/lib/mock-utils.ts
+++ b/test/lib/mock-utils.ts
@@ -7,19 +7,21 @@ import { resourcesDir, testAssetsDir, tsInstallationDirs, tsProjectsDir } from '
 import { patch } from '../../src/installer';
 import { normalizeSlashes } from 'ts-node';
 import resolve from 'resolve';
+import FileSystem from 'mock-fs/lib/filesystem';
 
 
 /* ****************************************************************************************************************** */
 // region: Constants
 /* ****************************************************************************************************************** */
 
-const vanillaFiles = [
-  ...cacheDirectory(tsProjectsDir),
-  ...cacheDirectory(resourcesDir),
-  ...cacheDirectory(testAssetsDir)
-];
+const vanillaFiles = cacheDirectories(
+  tsProjectsDir,
+  resourcesDir,
+  testAssetsDir
+);
 
 let patchedFiles = new Map<string, { ts: any, tscCode: string }>();
+const cachedFiles = new Map<string, Buffer>();
 
 // endregion
 
@@ -28,11 +30,28 @@ let patchedFiles = new Map<string, { ts: any, tscCode: string }>();
 // region: Helpers
 /* ****************************************************************************************************************** */
 
-function cacheDirectory(dir: string): [ string, string | {} ][] {
-  return (shell.ls('-RAl', dir) as unknown as (fs.Stats & { name: string })[]).map(stat => {
-    const statPath = normalizeSlashes(path.join(dir, stat.name));
-    return [ statPath, stat.isFile() ? fs.readFileSync(statPath, 'utf-8') : {} ]
-  });
+/**
+ * Create a DirectoryItems from actual files and directories for mock-fs (uses caching)
+ */
+function cacheDirectories(...directory: string[]): FileSystem.DirectoryItems {
+  const res: FileSystem.DirectoryItems = {};
+  for (const dir of directory)
+    for (const stat of shell.ls('-RAl', dir) as unknown as (fs.Stats & { name: string })[]) {
+      const statPath = normalizeSlashes(path.join(dir, stat.name));
+      res[statPath] =
+        !stat.isFile() ? {} :
+        <ReturnType<typeof mock.file>>(() => {
+          const fileData = fs.readFileSync(statPath);
+          cachedFiles.set(statPath, fileData);
+
+          return Object.defineProperty(mock.file({ content: '' })(), '_content', {
+            get: () => cachedFiles.get(statPath),
+            set: (data: any) => cachedFiles.set(statPath, data)
+          });
+        });
+    }
+
+  return res;
 }
 
 // endregion
@@ -42,12 +61,13 @@ function cacheDirectory(dir: string): [ string, string | {} ][] {
 // region: FileSystem
 /* ****************************************************************************************************************** */
 
-export function mockFs(fileEntries: [ string, string | {} ][] = vanillaFiles) {
-  mock(fileEntries.reduce((p, [key, value]) => { p[key] = value; return p; }, <Record<string, string | {}>>{}));
+export function mockFs() {
+  mock(vanillaFiles);
 }
 
 export function restoreFs() {
   mock.restore();
+  cachedFiles.clear();
 }
 
 export function resetFs() {
diff --git a/test/patch/specified-project.test.ts b/test/patch/specified-project.test.ts
index a611d6f..c1fdaf0 100755
--- a/test/patch/specified-project.test.ts
+++ b/test/patch/specified-project.test.ts
@@ -1,7 +1,7 @@
 /**
  * Test cases for specifying tsconfig as 'project' in config.
  *
- * Note: Unfortunately, because of jest's sandbox, we cannot directly test path mapping. It is not able to resolve
+ * Notes: Unfortunately, because of jest's sandbox, we cannot directly test path mapping. It is not able to resolve
  *       mapped paths. Ideally, we'd be able to execute virtually in a context outside of
  *       jest, but there is not currently a clear way to do this.
  *
@@ -16,14 +16,20 @@ import child_process from 'child_process';
 import { getPatchedTS } from '../lib/mock-utils';
 
 
+/* ****************************************************************************************************************** *
+ * Config
+ * ****************************************************************************************************************** */
+
+const srcFilesPath = path.join(testAssetsDir, 'src-files/transformer-with-project');
+const destDir = path.join(os.tmpdir(), 'tmpTSC');
+const tscPath = path.join(destDir, 'node_modules/typescript/lib/tsc.js');
+
+
 /* ****************************************************************************************************************** *
  * Tests
  * ****************************************************************************************************************** */
 
 describe('Specify Project', () => {
-  const srcFilesPath = path.join(testAssetsDir, 'src-files/transformer-with-project');
-  const destDir = path.join(os.tmpdir(), 'tmpTSC');
-  const tscPath = path.join(destDir, 'node_modules/typescript/lib/tsc.js');
   beforeAll(() => {
     const { tscCode } = getPatchedTS('latest');
 
@@ -39,21 +45,21 @@ describe('Specify Project', () => {
 
   test(`Loads project file & path mapping works`, () => {
     const cmd = `node ${tscPath} --noEmit false -p ${srcFilesPath}`;
-    const res = child_process.execSync(cmd, { stdio: 'pipe' });
-    expect(res.toString()).toMatch(/Path-Mapping Success!/);
+    const res = child_process.spawnSync(cmd, { stdio: 'pipe', shell: true });
+    expect(res.stdout.toString()).toMatch(/Path-Mapping Success!/);
   });
 
   test(`Mapping fails without project specified`, () => {
     const cmd = `node ${tscPath} --noEmit false -p ${path.join(srcFilesPath, 'tsconfig.noproject.json')}`;
-    const fail = () => child_process.execSync(cmd, { stdio: 'pipe' });
-    expect(fail).toThrow(/Cannot find module '#a'/);
+    const res = child_process.spawnSync(cmd, { stdio: 'pipe', shell: true  });
+    expect(res.stderr.toString()).toMatch(/Cannot find module '#a'/);
   });
 
   test(`Logs warning if can't find tsconfig-paths`, () => {
     shell.rm('-r', path.join(destDir, 'node_modules/tsconfig-paths'));
 
     const cmd = `node ${tscPath} --noEmit false -p ${srcFilesPath}`;
-    const fail = () => child_process.execSync(cmd, { stdio: 'pipe' });
-    expect(fail).toThrow(/Try adding 'tsconfig-paths'/);
+    const res = child_process.spawnSync(cmd, { stdio: 'pipe', shell: true  });
+    expect(res.stderr.toString()).toMatch(/Try adding 'tsconfig-paths'/);
   });
 });
diff --git a/tsconfig.base.json b/tsconfig.base.json
index de2bb6b..b6fe486 100755
--- a/tsconfig.base.json
+++ b/tsconfig.base.json
@@ -9,7 +9,7 @@
     "lib": [ "es2019" ],
     "outDir": "dist",
     "target": "es5",
-    "module":"CommonJS",
+    "module": "CommonJS",
     "moduleResolution": "node",
 
     "newLine": "LF",
diff --git a/yarn.lock b/yarn.lock
index f9c3758..921dc83 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -3439,6 +3439,11 @@ ret@~0.1.10:
   resolved "https://registry.yarnpkg.com/ret/-/ret-0.1.15.tgz#b8a4825d5bdb1fc3f6f53c2bc33f81388681c7bc"
   integrity sha512-TTlYpa+OL+vMMNG24xSlQGEJ3B/RzEfUlLct7b5G/ytav+wPrplCpVMFuwzXbkecJrb6IYo1iFb0S9v37754mg==
 
+rfdc@^1.1.4:
+  version "1.1.4"
+  resolved "https://registry.yarnpkg.com/rfdc/-/rfdc-1.1.4.tgz#ba72cc1367a0ccd9cf81a870b3b58bd3ad07f8c2"
+  integrity sha512-5C9HXdzK8EAqN7JDif30jqsBzavB7wLpaubisuQIGHWf2gUXSpzy6ArX/+Da8RjFpagWsCn+pIgxTMAmKw9Zug==
+
 rimraf@^3.0.0, rimraf@^3.0.2:
   version "3.0.2"
   resolved "https://registry.yarnpkg.com/rimraf/-/rimraf-3.0.2.tgz#f1a5402ba6220ad52cc1282bac1ae3aa49fd061a"