diff --git a/bundler/bundle.js b/bundler/bundle.js
index 6c0c9b6..7140872 100644
--- a/bundler/bundle.js
+++ b/bundler/bundle.js
@@ -1,6 +1,6 @@
 import assert from 'node:assert/strict'
 import { readFile } from 'node:fs/promises'
-import { existsSync } from 'node:fs'
+import { existsSync, readFileSync } from 'node:fs'
 import { fileURLToPath, pathToFileURL } from 'node:url'
 import { basename, dirname, extname, resolve, join } from 'node:path'
 import { createRequire } from 'node:module'
@@ -165,6 +165,26 @@ export const build = async (...files) => {
   }
 
   const fsfiles = await getPackageFiles(filename ? dirname(resolve(filename)) : process.cwd())
+  const fsFilesContents = new Map()
+  loadPipeline.push(async (source) => {
+    const cwd = process.cwd()
+    for (const re of [/readFileSync\('([^'\\]+)'[),]/gu, /readFileSync\("([^"\\]+)"[),]/gu]) {
+      for (const match of source.matchAll(re)) {
+        const file = match[1]
+        if (file && /^[a-z0-9@_./-]+$/iu.test(file)) {
+          if (!resolve(file).startsWith(`${cwd}/`)) continue
+          const data = readFileSync(file, 'base64')
+          if (fsFilesContents.has(file)) {
+            assert(fsFilesContents.get(file) === data)
+          } else {
+            fsFilesContents.set(file, data)
+          }
+        }
+      }
+    }
+
+    return source
+  })
 
   const hasBuffer = ['node', 'bun'].includes(options.platform)
   const api = (f) => resolveRequire(`./modules/${f}`)
@@ -191,10 +211,7 @@ export const build = async (...files) => {
     zlib: resolveRequire('browserify-zlib'),
   }
 
-  const defines = {}
-  if (files.length === 1) defines['process.argv'] = stringify(['exodus-test', resolve(files[0])])
-
-  const res = await buildWrap({
+  const config = {
     logLevel: 'silent',
     stdin: {
       contents: `(async function () {\n${main}\n})()`,
@@ -206,7 +223,6 @@ export const build = async (...files) => {
     platform: 'neutral',
     mainFields: ['browser', 'module', 'main'],
     define: {
-      ...defines,
       'process.env.FORCE_COLOR': stringify('0'),
       'process.env.NO_COLOR': stringify('1'),
       'process.env.NODE_ENV': stringify(process.env.NODE_ENV),
@@ -233,6 +249,7 @@ export const build = async (...files) => {
       EXODUS_TEST_SNAPSHOTS: stringify(EXODUS_TEST_SNAPSHOTS),
       EXODUS_TEST_RECORDINGS: stringify(EXODUS_TEST_RECORDINGS),
       EXODUS_TEST_FSFILES: stringify(fsfiles), // TODO: can we safely use relative paths?
+      EXODUS_TEST_FSFILES_CONTENTS: stringify([...fsFilesContents.entries()]),
     },
     alias: {
       // Jest, tape and node:test
@@ -286,9 +303,21 @@ export const build = async (...files) => {
         },
       },
     ],
-  })
+  }
+
+  if (files.length === 1)
+    config.define['process.argv'] = stringify(['exodus-test', resolve(files[0])])
+
+  let res = await buildWrap(config)
   assert.equal(res instanceof Error, res.errors.length > 0)
 
+  if (fsFilesContents.size > 0) {
+    // re-run as we detected that tests depend on fsReadFileSync contents
+    config.define.EXODUS_TEST_FSFILES_CONTENTS = stringify([...fsFilesContents.entries()])
+    res = await buildWrap(config)
+    assert.equal(res instanceof Error, res.errors.length > 0)
+  }
+
   // if (res.errors.length === 0) require('fs').copyFileSync(outfile, 'tempout.cjs') // DEBUG
 
   // We treat warnings as errors, so just merge all them
diff --git a/bundler/modules/fs.cjs b/bundler/modules/fs.cjs
index a6b3a75..362937c 100644
--- a/bundler/modules/fs.cjs
+++ b/bundler/modules/fs.cjs
@@ -77,11 +77,33 @@ const promises = { ...stubsPromises, constants }
 // eslint-disable-next-line no-undef
 const fsFiles = typeof EXODUS_TEST_FSFILES === 'undefined' ? null : new Set(EXODUS_TEST_FSFILES)
 const existsSync = (file) => {
-  if (fsFiles.has(file)) return true
+  if (fsFiles?.has(file)) return true
   err('existsSync', file)
 }
 
-const readFileSync = (file /*, options */) => {
+const fsFilesContents =
+  // eslint-disable-next-line no-undef
+  typeof EXODUS_TEST_FSFILES_CONTENTS === 'undefined' ? null : new Map(EXODUS_TEST_FSFILES_CONTENTS)
+const readFileSync = (file, options) => {
+  let encoding
+  if (typeof options === 'string') {
+    encoding = options
+  } else if (options !== undefined) {
+    if (typeof options !== 'object') throw new Error('Unexpected readFileSync options')
+    const { encoding: enc, ...rest } = options
+    if (enc !== undefined && typeof enc !== 'string') throw new Error('encoding should be a string')
+    encoding = enc
+    if (Object.keys(rest).length > 0) throw new Error('Unsupported readFileSync options')
+  }
+
+  if (typeof file !== 'string') throw new Error('file argument should be string')
+  if (fsFilesContents?.has(file)) {
+    const data = Buffer.from(fsFilesContents.get(file), 'base64')
+    if (encoding?.toLowerCase().replace('-', '') === 'utf8') return data.toString('utf8')
+    if (encoding === undefined) return data
+    throw new Error('Unsupported encoding')
+  }
+
   err('readFileSync', file)
 }