diff --git a/lib/internal/modules/cjs/loader.js b/lib/internal/modules/cjs/loader.js
index 801ab9caecc2aa..acec49920eaf61 100644
--- a/lib/internal/modules/cjs/loader.js
+++ b/lib/internal/modules/cjs/loader.js
@@ -113,6 +113,7 @@ const kFormat = Symbol('kFormat');
 
 // Set first due to cycle with ESM loader functions.
 module.exports = {
+  clearStatCache,
   kModuleSource,
   kModuleExport,
   kModuleExportNames,
@@ -155,14 +156,14 @@ const {
 } = internalBinding('contextify');
 
 const assert = require('internal/assert');
-const fs = require('fs');
 const path = require('path');
-const internalFsBinding = internalBinding('fs');
 const { safeGetenv } = internalBinding('credentials');
 const {
   getCjsConditions,
   getCjsConditionsArray,
   initializeCjsConditions,
+  loaderReadFile,
+  loaderStat,
   loadBuiltinModule,
   makeRequireFunction,
   setHasStartedUserCJSExecution,
@@ -272,7 +273,7 @@ function stat(filename) {
     const result = statCache.get(filename);
     if (result !== undefined) { return result; }
   }
-  const result = internalFsBinding.internalModuleStat(filename);
+  const result = loaderStat(filename);
   if (statCache !== null && result >= 0) {
     // Only set cache when `internalModuleStat(filename)` succeeds.
     statCache.set(filename, result);
@@ -280,6 +281,16 @@ function stat(filename) {
   return result;
 }
 
+/**
+ * Clear the stat cache. Called when VFS instances are unmounted
+ * to prevent stale stat results from being returned.
+ */
+function clearStatCache() {
+  if (statCache !== null) {
+    statCache = new SafeMap();
+  }
+}
+
 let _stat = stat;
 ObjectDefineProperty(Module, '_stat', {
   __proto__: null,
@@ -1201,7 +1212,7 @@ function defaultLoadImpl(filename, format) {
     case 'module-typescript':
     case 'commonjs-typescript':
     case 'typescript': {
-      return fs.readFileSync(filename, 'utf8');
+      return loaderReadFile(filename, 'utf8');
     }
     case 'builtin':
       return null;
diff --git a/lib/internal/modules/esm/get_format.js b/lib/internal/modules/esm/get_format.js
index 4f334c7d88c336..507c336d6744fe 100644
--- a/lib/internal/modules/esm/get_format.js
+++ b/lib/internal/modules/esm/get_format.js
@@ -10,7 +10,6 @@ const {
 } = primordials;
 const { getOptionValue } = require('internal/options');
 const { getValidatedPath } = require('internal/fs/utils');
-const fsBindings = internalBinding('fs');
 const { internal: internalConstants } = internalBinding('constants');
 
 const extensionFormatMap = {
@@ -59,7 +58,8 @@ function mimeToFormat(mime) {
  */
 function getFormatOfExtensionlessFile(url) {
   const path = getValidatedPath(url);
-  switch (fsBindings.getFormatOfExtensionlessFile(path)) {
+  const { loaderGetFormatOfExtensionlessFile } = require('internal/modules/helpers');
+  switch (loaderGetFormatOfExtensionlessFile(path)) {
     case internalConstants.EXTENSIONLESS_FORMAT_WASM:
       return 'wasm';
     default:
diff --git a/lib/internal/modules/esm/load.js b/lib/internal/modules/esm/load.js
index 94879761553e02..1a788c0041a752 100644
--- a/lib/internal/modules/esm/load.js
+++ b/lib/internal/modules/esm/load.js
@@ -9,7 +9,7 @@ const {
 
 const { defaultGetFormat } = require('internal/modules/esm/get_format');
 const { validateAttributes, emitImportAssertionWarning } = require('internal/modules/esm/assert');
-const fs = require('fs');
+const { loaderReadFile } = require('internal/modules/helpers');
 
 const { Buffer: { from: BufferFrom } } = require('buffer');
 
@@ -34,11 +34,7 @@ function getSourceSync(url, context) {
   const responseURL = href;
   let source;
   if (protocol === 'file:') {
-    // If you are reading this code to figure out how to patch Node.js module loading
-    // behavior - DO NOT depend on the patchability in new code: Node.js
-    // internals may stop going through the JavaScript fs module entirely.
-    // Prefer module.registerHooks() or other more formal fs hooks released in the future.
-    source = fs.readFileSync(url);
+    source = loaderReadFile(url);
   } else if (protocol === 'data:') {
     const result = dataURLProcessor(url);
     if (result === 'failure') {
diff --git a/lib/internal/modules/esm/resolve.js b/lib/internal/modules/esm/resolve.js
index b028f44f013886..1da2795c117e68 100644
--- a/lib/internal/modules/esm/resolve.js
+++ b/lib/internal/modules/esm/resolve.js
@@ -9,7 +9,6 @@ const {
   ObjectPrototypeHasOwnProperty,
   RegExpPrototypeExec,
   RegExpPrototypeSymbolReplace,
-  SafeMap,
   SafeSet,
   String,
   StringPrototypeEndsWith,
@@ -23,16 +22,13 @@ const {
   encodeURIComponent,
 } = primordials;
 const assert = require('internal/assert');
-const internalFS = require('internal/fs/utils');
 const { BuiltinModule } = require('internal/bootstrap/realm');
-const fs = require('fs');
 const { getOptionValue } = require('internal/options');
 // Do not eagerly grab .manifest, it may be in TDZ
 const { sep, posix: { relative: relativePosixPath }, resolve } = require('path');
 const { URL, pathToFileURL, fileURLToPath, isURL, URLParse } = require('internal/url');
 const { getCWDURL, setOwnProperty } = require('internal/util');
 const { canParse: URLCanParse } = internalBinding('url');
-const { legacyMainResolve: FSLegacyMainResolve } = internalBinding('fs');
 const {
   ERR_INPUT_TYPE_NOT_ALLOWED,
   ERR_INVALID_ARG_TYPE,
@@ -49,7 +45,7 @@ const {
 const { defaultGetFormatWithoutErrors } = require('internal/modules/esm/get_format');
 const { getConditionsSet } = require('internal/modules/esm/utils');
 const packageJsonReader = require('internal/modules/package_json_reader');
-const internalFsBinding = internalBinding('fs');
+const { loaderLegacyMainResolve, loaderStat, toRealPath } = require('internal/modules/helpers');
 
 /**
  * @typedef {import('internal/modules/esm/package_config.js').PackageConfig} PackageConfig
@@ -149,8 +145,6 @@ function emitLegacyIndexDeprecation(url, path, pkgPath, base, main) {
   }
 }
 
-const realpathCache = new SafeMap();
-
 const legacyMainResolveExtensions = [
   '',
   '.js',
@@ -198,7 +192,7 @@ function legacyMainResolve(packageJSONUrl, packageConfig, base) {
 
   const baseStringified = isURL(base) ? base.href : base;
 
-  const resolvedOption = FSLegacyMainResolve(pkgPath, packageConfig.main, baseStringified);
+  const resolvedOption = loaderLegacyMainResolve(pkgPath, packageConfig.main, baseStringified);
 
   const maybeMain = resolvedOption <= legacyMainResolveExtensionsIndexes.kResolvedByMainIndexNode ?
     packageConfig.main || './' : '';
@@ -244,7 +238,7 @@ function finalizeResolution(resolved, base, preserveSymlinks) {
     throw err;
   }
 
-  const stats = internalFsBinding.internalModuleStat(
+  const stats = loaderStat(
     StringPrototypeEndsWith(path, '/') ? StringPrototypeSlice(path, -1) : path,
   );
 
@@ -273,13 +267,7 @@ function finalizeResolution(resolved, base, preserveSymlinks) {
   }
 
   if (!preserveSymlinks) {
-    // If you are reading this code to figure out how to patch Node.js module loading
-    // behavior - DO NOT depend on the patchability in new code: Node.js
-    // internals may stop going through the JavaScript fs module entirely.
-    // Prefer module.registerHooks() or other more formal fs hooks released in the future.
-    const real = fs.realpathSync(path, {
-      [internalFS.realpathCacheKey]: realpathCache,
-    });
+    const real = toRealPath(path);
     const { search, hash } = resolved;
     resolved =
         pathToFileURL(real + (StringPrototypeEndsWith(path, sep) ? '/' : ''));
diff --git a/lib/internal/modules/helpers.js b/lib/internal/modules/helpers.js
index 839ce9af4bb678..9f92496e38ea35 100644
--- a/lib/internal/modules/helpers.js
+++ b/lib/internal/modules/helpers.js
@@ -27,20 +27,22 @@ const { pathToFileURL, fileURLToPath, URL } = require('internal/url');
 const assert = require('internal/assert');
 
 const { getOptionValue } = require('internal/options');
-const { setOwnProperty, getLazy } = require('internal/util');
+const { kEmptyObject, setOwnProperty, getLazy } = require('internal/util');
 const { inspect } = require('internal/util/inspect');
 const { emitWarningSync } = require('internal/process/warning');
 
 const lazyTmpdir = getLazy(() => require('os').tmpdir());
 const { join } = path;
 
+const internalFsBinding = internalBinding('fs');
 const { canParse: URLCanParse } = internalBinding('url');
+const modulesBinding = internalBinding('modules');
 const {
   enableCompileCache: _enableCompileCache,
   getCompileCacheDir: _getCompileCacheDir,
   compileCacheStatus: _compileCacheStatus,
   flushCompileCache,
-} = internalBinding('modules');
+} = modulesBinding;
 
 const lazyCJSLoader = getLazy(() => require('internal/modules/cjs/loader'));
 let debug = require('internal/util/debuglog').debuglog('module', (fn) => {
@@ -56,17 +58,178 @@ let debug = require('internal/util/debuglog').debuglog('module', (fn) => {
  * @type {Map<string, string>}
  */
 const realpathCache = new SafeMap();
+// Toggleable loader fs overrides for VFS support.
+// When null, the fast path (no VFS) is taken with zero overhead.
+let _loaderStat = null;
+let _loaderReadFile = null;
+let _loaderRealpath = null;
+let _loaderLegacyMainResolve = null;
+let _loaderGetFormatOfExtensionlessFile = null;
+
+/**
+ * Set override functions for the module loader's fs operations.
+ * Missing or nullish fields disable that hook (fast-path restored).
+ * @param {{ stat?: Function, readFile?: Function, realpath?: Function,
+ *           legacyMainResolve?: Function, getFormatOfExtensionlessFile?: Function }} overrides
+ */
+function setLoaderFsOverrides(overrides = kEmptyObject) {
+  _loaderStat = overrides.stat ?? null;
+  _loaderReadFile = overrides.readFile ?? null;
+  _loaderRealpath = overrides.realpath ?? null;
+  _loaderLegacyMainResolve = overrides.legacyMainResolve ?? null;
+  _loaderGetFormatOfExtensionlessFile = overrides.getFormatOfExtensionlessFile ?? null;
+}
+
+/**
+ * Wrapper for internalModuleStat that supports VFS toggle.
+ * @param {string} filename Absolute path to stat
+ * @returns {number}
+ */
+function loaderStat(filename) {
+  if (_loaderStat !== null) { return _loaderStat(filename); }
+  return internalFsBinding.internalModuleStat(filename);
+}
+
+/**
+ * Wrapper for fs.readFileSync that supports VFS toggle.
+ * @param {string|URL} filename Path to read
+ * @param {string|object} options Read options
+ * @returns {string|Buffer}
+ */
+function loaderReadFile(filename, options) {
+  if (_loaderReadFile !== null) {
+    const result = _loaderReadFile(filename, options);
+    if (result !== undefined) { return result; }
+  }
+  return fs.readFileSync(filename, options);
+}
+
 /**
  * Resolves the path of a given `require` specifier, following symlinks.
  * @param {string} requestPath The `require` specifier
  * @returns {string}
  */
 function toRealPath(requestPath) {
+  if (_loaderRealpath !== null) {
+    const result = _loaderRealpath(requestPath);
+    if (result !== undefined) { return result; }
+  }
   return fs.realpathSync(requestPath, {
     [internalFS.realpathCacheKey]: realpathCache,
   });
 }
 
+/**
+ * Wrapper for internalBinding('fs').legacyMainResolve that supports VFS toggle.
+ * @param {string} pkgPath The package directory path
+ * @param {string} main The package main field
+ * @param {string} base The base URL string
+ * @returns {number}
+ */
+function loaderLegacyMainResolve(pkgPath, main, base) {
+  if (_loaderLegacyMainResolve !== null) {
+    const result = _loaderLegacyMainResolve(pkgPath, main, base);
+    if (result !== undefined) { return result; }
+  }
+  return internalFsBinding.legacyMainResolve(pkgPath, main, base);
+}
+
+/**
+ * Wrapper for internalBinding('fs').getFormatOfExtensionlessFile that supports VFS toggle.
+ * @param {string} path The file path
+ * @returns {number}
+ */
+function loaderGetFormatOfExtensionlessFile(path) {
+  if (_loaderGetFormatOfExtensionlessFile !== null) {
+    const result = _loaderGetFormatOfExtensionlessFile(path);
+    if (result !== undefined) { return result; }
+  }
+  return internalFsBinding.getFormatOfExtensionlessFile(path);
+}
+
+// Toggleable overrides for package.json C++ methods (VFS support).
+let _loaderReadPackageJSON = null;
+let _loaderGetNearestParentPackageJSON = null;
+let _loaderGetPackageScopeConfig = null;
+let _loaderGetPackageType = null;
+
+/**
+ * Set override functions for the module loader's package.json operations.
+ * Missing or nullish fields disable that hook (fast-path restored).
+ * @param {{
+ *   readPackageJSON?: Function,
+ *   getNearestParentPackageJSON?: Function,
+ *   getPackageScopeConfig?: Function,
+ *   getPackageType?: Function,
+ * }} overrides
+ */
+function setLoaderPackageOverrides(overrides = kEmptyObject) {
+  _loaderReadPackageJSON = overrides.readPackageJSON ?? null;
+  _loaderGetNearestParentPackageJSON = overrides.getNearestParentPackageJSON ?? null;
+  _loaderGetPackageScopeConfig = overrides.getPackageScopeConfig ?? null;
+  _loaderGetPackageType = overrides.getPackageType ?? null;
+}
+
+/**
+ * Clear the realpath cache used by toRealPath's fallback path.
+ * Called when VFS instances are unmounted so that paths that overlap a
+ * removed mount are re-resolved against the real filesystem next time.
+ */
+function clearRealpathCache() {
+  realpathCache.clear();
+}
+
+/**
+ * Wrapper for modulesBinding.readPackageJSON that supports VFS toggle.
+ * @param {string} jsonPath
+ * @param {boolean} isESM
+ * @param {string} base
+ * @param {string} specifier
+ * @returns {object|undefined}
+ */
+function loaderReadPackageJSON(jsonPath, isESM, base, specifier) {
+  if (_loaderReadPackageJSON !== null) {
+    return _loaderReadPackageJSON(jsonPath, isESM, base, specifier);
+  }
+  return modulesBinding.readPackageJSON(jsonPath, isESM, base, specifier);
+}
+
+/**
+ * Wrapper for modulesBinding.getNearestParentPackageJSON that supports VFS toggle.
+ * @param {string} checkPath
+ * @returns {object|undefined}
+ */
+function loaderGetNearestParentPackageJSON(checkPath) {
+  if (_loaderGetNearestParentPackageJSON !== null) {
+    return _loaderGetNearestParentPackageJSON(checkPath);
+  }
+  return modulesBinding.getNearestParentPackageJSON(checkPath);
+}
+
+/**
+ * Wrapper for modulesBinding.getPackageScopeConfig that supports VFS toggle.
+ * @param {string} resolved
+ * @returns {object|string}
+ */
+function loaderGetPackageScopeConfig(resolved) {
+  if (_loaderGetPackageScopeConfig !== null) {
+    return _loaderGetPackageScopeConfig(resolved);
+  }
+  return modulesBinding.getPackageScopeConfig(resolved);
+}
+
+/**
+ * Wrapper for modulesBinding.getPackageType that supports VFS toggle.
+ * @param {string} url
+ * @returns {string|undefined}
+ */
+function loaderGetPackageType(url) {
+  if (_loaderGetPackageType !== null) {
+    return _loaderGetPackageType(url);
+  }
+  return modulesBinding.getPackageType(url);
+}
+
 /** @type {Set<string>} */
 let cjsConditions;
 /** @type {string[]} */
@@ -516,6 +679,7 @@ function getCompileCacheDir() {
 module.exports = {
   addBuiltinLibsToObject,
   assertBufferSource,
+  clearRealpathCache,
   constants,
   enableCompileCache,
   flushCompileCache,
@@ -524,10 +688,20 @@ module.exports = {
   getCjsConditionsArray,
   getCompileCacheDir,
   initializeCjsConditions,
+  loaderGetFormatOfExtensionlessFile,
+  loaderGetNearestParentPackageJSON,
+  loaderGetPackageScopeConfig,
+  loaderGetPackageType,
+  loaderLegacyMainResolve,
+  loaderReadFile,
+  loaderReadPackageJSON,
+  loaderStat,
   loadBuiltinModuleForEmbedder,
   loadBuiltinModule,
   makeRequireFunction,
   normalizeReferrerURL,
+  setLoaderFsOverrides,
+  setLoaderPackageOverrides,
   stringify,
   stripBOM,
   toRealPath,
diff --git a/lib/internal/modules/package_json_reader.js b/lib/internal/modules/package_json_reader.js
index 6c6bf0383bc338..13aa36edb39bd9 100644
--- a/lib/internal/modules/package_json_reader.js
+++ b/lib/internal/modules/package_json_reader.js
@@ -24,10 +24,15 @@ const {
   },
 } = require('internal/errors');
 const { kEmptyObject } = require('internal/util');
-const modulesBinding = internalBinding('modules');
 const path = require('path');
 const { validateString } = require('internal/validators');
-const internalFsBinding = internalBinding('fs');
+const {
+  loaderGetNearestParentPackageJSON,
+  loaderGetPackageScopeConfig,
+  loaderGetPackageType,
+  loaderReadPackageJSON,
+  loaderStat,
+} = require('internal/modules/helpers');
 
 
 /**
@@ -122,7 +127,7 @@ const requiresJSONParse = (value) => (value !== undefined && (value[0] === '[' |
 function read(jsonPath, { base, specifier, isESM } = kEmptyObject) {
   // This function will be called by both CJS and ESM, so we need to make sure
   // non-null attributes are converted to strings.
-  const parsed = modulesBinding.readPackageJSON(
+  const parsed = loaderReadPackageJSON(
     jsonPath,
     isESM,
     base == null ? undefined : `${base}`,
@@ -170,7 +175,7 @@ function getNearestParentPackageJSON(checkPath) {
     return deserializedPackageJSONCache.get(parentPackageJSONPath);
   }
 
-  const result = modulesBinding.getNearestParentPackageJSON(checkPath);
+  const result = loaderGetNearestParentPackageJSON(checkPath);
   const packageConfig = deserializePackageJSON(checkPath, result);
 
   moduleToParentPackageJSONCache.set(checkPath, packageConfig.path);
@@ -190,7 +195,7 @@ function getNearestParentPackageJSON(checkPath) {
  * @returns {import('typings/internalBinding/modules').PackageConfig} - The package configuration.
  */
 function getPackageScopeConfig(resolved) {
-  const result = modulesBinding.getPackageScopeConfig(`${resolved}`);
+  const result = loaderGetPackageScopeConfig(`${resolved}`);
 
   if (ArrayIsArray(result)) {
     const { data, exists, path } = deserializePackageJSON(`${resolved}`, result);
@@ -219,7 +224,7 @@ function getPackageScopeConfig(resolved) {
  * @returns {string}
  */
 function getPackageType(url) {
-  const type = modulesBinding.getPackageType(`${url}`);
+  const type = loaderGetPackageType(`${url}`);
   return type ?? 'none';
 }
 
@@ -280,7 +285,7 @@ function getPackageJSONURL(specifier, base) {
   let packageJSONPath = fileURLToPath(packageJSONUrl);
   let lastPath;
   do {
-    const stat = internalFsBinding.internalModuleStat(
+    const stat = loaderStat(
       StringPrototypeSlice(packageJSONPath, 0, packageJSONPath.length - 13),
     );
     // Check for !stat.isDirectory()
@@ -353,6 +358,15 @@ function findPackageJSON(specifier, base = 'data:') {
   return pkg?.path;
 }
 
+/**
+ * Clears all package.json caches. Called by VFS on unmount to prevent
+ * stale entries from paths that were resolved while a VFS was mounted.
+ */
+function clearPackageJSONCache() {
+  moduleToParentPackageJSONCache.clear();
+  deserializedPackageJSONCache.clear();
+}
+
 module.exports = {
   read,
   getNearestParentPackageJSON,
@@ -360,4 +374,5 @@ module.exports = {
   getPackageType,
   getPackageJSONURL,
   findPackageJSON,
+  clearPackageJSONCache,
 };
diff --git a/lib/internal/vfs/setup.js b/lib/internal/vfs/setup.js
index fe06f578b2f6ca..4443c6f6bd89e4 100644
--- a/lib/internal/vfs/setup.js
+++ b/lib/internal/vfs/setup.js
@@ -1,27 +1,34 @@
 'use strict';
 
 const {
+  ArrayIsArray,
   ArrayPrototypeIndexOf,
   ArrayPrototypePush,
   ArrayPrototypeSplice,
+  JSONParse,
+  JSONStringify,
   PromiseResolve,
+  String,
+  StringPrototypeEndsWith,
   StringPrototypeStartsWith,
 } = primordials;
 
 const { Buffer } = require('buffer');
-const { resolve, sep } = require('path');
+const { dirname, resolve, sep } = require('path');
 const { fileURLToPath, URL } = require('internal/url');
 const { kEmptyObject } = require('internal/util');
 const { validateObject } = require('internal/validators');
 const {
   codes: {
     ERR_INVALID_ARG_TYPE,
+    ERR_INVALID_PACKAGE_CONFIG,
     ERR_INVALID_STATE,
+    ERR_MODULE_NOT_FOUND,
   },
 } = require('internal/errors');
 const { createENOENT, createEXDEV } = require('internal/vfs/errors');
 const { getVirtualFd, closeVirtualFd } = require('internal/vfs/fd');
-const { assertEncoding, vfsState, setVfsHandlers } = require('internal/fs/utils');
+const { assertEncoding, setVfsHandlers } = require('internal/fs/utils');
 const permission = require('internal/process/permission');
 const { getOptionValue } = require('internal/options');
 let debug = require('internal/util/debuglog').debuglog('vfs', (fn) => {
@@ -82,11 +89,7 @@ function registerVFS(vfs) {
   ArrayPrototypePush(activeVFSList, vfs);
   debug('register mount=%s active=%d', newMount, activeVFSList.length);
   if (!hooksInstalled) {
-    vfsHandlerObj = createVfsHandlers();
-    setVfsHandlers(vfsHandlerObj);
-    hooksInstalled = true;
-  } else if (vfsState.handlers === null) {
-    setVfsHandlers(vfsHandlerObj);
+    installHooks();
   }
 }
 
@@ -95,11 +98,192 @@ function deregisterVFS(vfs) {
   if (index === -1) return;
   ArrayPrototypeSplice(activeVFSList, index, 1);
   debug('deregister active=%d', activeVFSList.length);
+  // Loader/path caches are shared across all VFSes and we can't tell
+  // which entries belonged to the one going away, so flush them on
+  // every unmount. The cost is bounded; correctness wins.
+  clearLoaderCaches();
   if (activeVFSList.length === 0) {
-    setVfsHandlers(null);
+    uninstallHooks();
   }
 }
 
+/**
+ * Clear every JS-reachable loader cache that could hold a VFS-resolved
+ * entry. Called from deregisterVFS only when the last VFS unmounts.
+ */
+function clearLoaderCaches() {
+  const cjsLoader = require('internal/modules/cjs/loader');
+  cjsLoader.Module._pathCache = { __proto__: null };
+  cjsLoader.clearStatCache();
+  const helpers = require('internal/modules/helpers');
+  helpers.clearRealpathCache();
+  const { clearPackageJSONCache } = require('internal/modules/package_json_reader');
+  clearPackageJSONCache();
+  // The ESM cascaded loader's loadCache is intentionally NOT cleared here:
+  // clearing it mid-flight (while another import() is awaiting nested
+  // resolution) would let a second ModuleJob be created for the same URL,
+  // breaking module identity. The trade-off is that an ESM module already
+  // loaded from a VFS path remains cached after unmount and across a
+  // re-mount with different content, consistent with how ESM caches
+  // modules everywhere else in Node.js.
+}
+
+/**
+ * Returns the stat result code for a VFS path.
+ * @param {object} vfs The VFS instance
+ * @param {string} filePath The path to check
+ * @returns {number} 0 for file, 1 for directory, -2 for not found
+ */
+function vfsStat(vfs, filePath) {
+  try {
+    const stats = vfs.statSync(filePath);
+    if (stats.isDirectory()) return 1;
+    return 0;
+  } catch {
+    return -2;
+  }
+}
+
+/**
+ * Checks all active VFS instances for a file/directory stat.
+ * @param {string} filename The absolute path to check
+ * @returns {{ vfs: object, result: number }|null}
+ */
+function findVFSForStat(filename) {
+  const normalized = resolve(filename);
+  for (let i = 0; i < activeVFSList.length; i++) {
+    const vfs = activeVFSList[i];
+    if (vfs.shouldHandle(normalized)) {
+      return { vfs, result: vfsStat(vfs, normalized) };
+    }
+  }
+  return null;
+}
+
+/**
+ * Checks all active VFS instances for file content.
+ * @param {string} filename The absolute path to read
+ * @param {string|object} options Read options
+ * @returns {{ vfs: object, content: Buffer|string }|null}
+ */
+function findVFSForRead(filename, options) {
+  const normalized = resolve(filename);
+  for (let i = 0; i < activeVFSList.length; i++) {
+    const vfs = activeVFSList[i];
+    if (vfs.shouldHandle(normalized)) {
+      if (vfs.existsSync(normalized) && vfsStat(vfs, normalized) === 0) {
+        return { vfs, content: vfs.readFileSync(normalized, options) };
+      }
+      // Path inside mount but missing/not-a-file: synthesize ENOENT so the
+      // loader doesn't fall through and read a real-fs file with the same path.
+      throw createENOENT('open', filename);
+    }
+  }
+  return null;
+}
+
+/**
+ * Serialize a parsed package.json object into the C++ tuple format
+ * expected by deserializePackageJSON: [name, main, type, imports, exports, filePath].
+ *
+ * Matches the native binding's validation in src/node_modules.cc so we
+ * neither over- nor under-validate compared to the real-fs path:
+ *   - parsed itself must be a non-null object (throws otherwise)
+ *   - "name" must be a string when present (throws otherwise)
+ *   - "main" non-strings are silently omitted
+ *   - "type" must be a string when present (throws otherwise); only
+ *     "module" / "commonjs" are kept, others default to "none"
+ *   - "imports" / "exports" accept string / object / array; other
+ *     types are silently ignored
+ * @param {object} parsed The parsed package.json content
+ * @param {string} filePath The path to the package.json file
+ * @returns {Array} Serialized package config tuple
+ */
+function serializePackageJSON(parsed, filePath) {
+  if (parsed === null || typeof parsed !== 'object' || ArrayIsArray(parsed)) {
+    throw new ERR_INVALID_PACKAGE_CONFIG(filePath, undefined, '');
+  }
+
+  let name;
+  if (parsed.name !== undefined && parsed.name !== null) {
+    if (typeof parsed.name !== 'string') {
+      throw new ERR_INVALID_PACKAGE_CONFIG(
+        filePath, undefined, '"name" must be a string');
+    }
+    name = parsed.name;
+  }
+
+  let main;
+  if (typeof parsed.main === 'string') {
+    main = parsed.main;
+  }
+
+  let type = 'none';
+  if (parsed.type !== undefined && parsed.type !== null) {
+    if (typeof parsed.type !== 'string') {
+      throw new ERR_INVALID_PACKAGE_CONFIG(
+        filePath, undefined, '"type" must be a string');
+    }
+    if (parsed.type === 'module' || parsed.type === 'commonjs') {
+      type = parsed.type;
+    }
+  }
+
+  let imports;
+  if (typeof parsed.imports === 'string') {
+    imports = parsed.imports;
+  } else if (typeof parsed.imports === 'object' && parsed.imports !== null) {
+    imports = JSONStringify(parsed.imports);
+  }
+
+  let exports;
+  if (typeof parsed.exports === 'string') {
+    exports = parsed.exports;
+  } else if (typeof parsed.exports === 'object' && parsed.exports !== null) {
+    exports = JSONStringify(parsed.exports);
+  }
+
+  return [name, main, type, imports, exports, filePath];
+}
+
+/**
+ * Walk up directories in VFS looking for package.json. Always returns an
+ * object. When a package.json is found `.parsed` is populated; otherwise
+ * `.sentinel` is the last candidate path checked (highest reached before
+ * walking past the mount or hitting node_modules) - used as the "not found"
+ * marker matching the C++ binding's contract for getPackageScopeConfig.
+ * @param {string} startPath Normalized absolute path to start from
+ * @returns {{ vfs?: object, pjsonPath?: string, parsed?: object, sentinel: string }}
+ */
+function findVFSPackageJSON(startPath) {
+  let currentDir = dirname(startPath);
+  let lastDir;
+  let sentinel = resolve(currentDir, 'package.json');
+  while (currentDir !== lastDir) {
+    if (StringPrototypeEndsWith(currentDir, '/node_modules') ||
+        StringPrototypeEndsWith(currentDir, '\\node_modules')) {
+      break;
+    }
+    const pjsonPath = resolve(currentDir, 'package.json');
+    sentinel = pjsonPath;
+    for (let i = 0; i < activeVFSList.length; i++) {
+      const vfs = activeVFSList[i];
+      if (vfs.shouldHandle(pjsonPath) && vfsStat(vfs, pjsonPath) === 0) {
+        try {
+          const content = vfs.readFileSync(pjsonPath, 'utf8');
+          const parsed = JSONParse(content);
+          return { vfs, pjsonPath, parsed, sentinel: pjsonPath };
+        } catch {
+          // SyntaxError or other errors, continue walking
+        }
+      }
+    }
+    lastDir = currentDir;
+    currentDir = dirname(currentDir);
+  }
+  return { sentinel };
+}
+
 function findVFSForExists(filename) {
   const normalized = resolve(filename);
   for (let i = 0; i < activeVFSList.length; i++) {
@@ -655,6 +839,229 @@ function createVfsHandlers() {
   };
 }
 
+/**
+ * Install toggleable loader overrides so that the module loader's
+ * internal fs operations (stat, readFile, realpath) are redirected
+ * to VFS when appropriate.
+ */
+function installModuleLoaderOverrides() {
+  const {
+    setLoaderFsOverrides,
+    setLoaderPackageOverrides,
+  } = require('internal/modules/helpers');
+  const internalFsBinding = internalBinding('fs');
+  const nativeModulesBinding = internalBinding('modules');
+
+  setLoaderFsOverrides({
+    stat(filename) {
+      const result = findVFSForStat(filename);
+      if (result !== null) return result.result;
+      return internalFsBinding.internalModuleStat(filename);
+    },
+    readFile(filename, options) {
+      const pathStr = typeof filename === 'string' ? filename :
+        (filename instanceof URL ? fileURLToPath(filename) : String(filename));
+      const result = findVFSForRead(pathStr, options);
+      return result !== null ? result.content : undefined;
+    },
+    realpath(filename) {
+      return findVFSWith(filename, 'realpath', (vfs, n) => vfs.realpathSync(n));
+    },
+    legacyMainResolve(pkgPath, main, base) {
+      const normalized = resolve(pkgPath);
+      let handled = false;
+      for (let i = 0; i < activeVFSList.length; i++) {
+        if (activeVFSList[i].shouldHandle(normalized)) {
+          handled = true;
+          break;
+        }
+      }
+      if (!handled) return undefined;
+
+      // Extension index mapping (matches C++ legacyMainResolve):
+      // 0-6: try main + extension, then main + /index.ext
+      // 7-9: try pkgPath + ./index.ext
+      const mainExts = ['', '.js', '.json', '.node',
+                        '/index.js', '/index.json', '/index.node'];
+      const indexExts = ['./index.js', './index.json', './index.node'];
+
+      if (main) {
+        for (let i = 0; i < mainExts.length; i++) {
+          const candidate = resolve(pkgPath, main + mainExts[i]);
+          if (findVFSForStat(candidate)?.result === 0) return i;
+        }
+      }
+      for (let i = 0; i < indexExts.length; i++) {
+        const candidate = resolve(pkgPath, indexExts[i]);
+        if (findVFSForStat(candidate)?.result === 0) return 7 + i;
+      }
+
+      // Match the C++ binding's message shape (src/node_file.cc:3927-4005):
+      // when `main` is a non-empty string, the initial candidate path is
+      // pkgPath/main; otherwise the binding sets it to pkgPath/index.js
+      // (the first extensionless fallback + ".js"). The third arg
+      // `exactUrl` must be undefined - not a string - so the message uses
+      // the "package" word and err.url is not overwritten.
+      // ERR_MODULE_NOT_FOUND enforces strict arity in getMessage(), so
+      // the undefined has to be passed explicitly.
+      const initial = main ?
+        resolve(pkgPath, main) :
+        resolve(pkgPath, 'index.js');
+      throw new ERR_MODULE_NOT_FOUND(initial, base, undefined);
+    },
+    getFormatOfExtensionlessFile(filePath) {
+      let result;
+      try {
+        result = findVFSForRead(filePath, null);
+      } catch (e) {
+        // findVFSForRead synthesizes ENOENT for missing paths inside a
+        // mount. Treat that as JS (the caller will surface the real
+        // error when it later tries to load source). Propagate every
+        // other code (EACCES, ELOOP, etc).
+        if (e?.code === 'ENOENT') return 0; // EXTENSIONLESS_FORMAT_JAVASCRIPT
+        throw e;
+      }
+      if (result === null) return undefined;
+      const content = result.content;
+      // Wasm magic bytes: 0x00 0x61 0x73 0x6d
+      if (content && content.length >= 4 &&
+          content[0] === 0x00 && content[1] === 0x61 &&
+          content[2] === 0x73 && content[3] === 0x6d) {
+        return 1; // EXTENSIONLESS_FORMAT_WASM
+      }
+      return 0; // EXTENSIONLESS_FORMAT_JAVASCRIPT
+    },
+  });
+
+  setLoaderPackageOverrides({
+    readPackageJSON(jsonPath, isESM, base, specifier) {
+      const normalized = resolve(jsonPath);
+      for (let i = 0; i < activeVFSList.length; i++) {
+        const vfs = activeVFSList[i];
+        if (!vfs.shouldHandle(normalized)) continue;
+        if (vfsStat(vfs, normalized) !== 0) return undefined;
+        let content;
+        try {
+          content = vfs.readFileSync(normalized, 'utf8');
+        } catch {
+          // Treat read errors as "no package.json" - same as native.
+          return undefined;
+        }
+        let parsed;
+        try {
+          parsed = JSONParse(content);
+        } catch (err) {
+          // ESM raises ERR_INVALID_PACKAGE_CONFIG on malformed JSON;
+          // CJS silently ignores it (legacy behavior).
+          if (isESM) {
+            throw new ERR_INVALID_PACKAGE_CONFIG(normalized, base, err.message);
+          }
+          return undefined;
+        }
+        // serializePackageJSON may throw ERR_INVALID_PACKAGE_CONFIG for
+        // wrong-type fields - intentionally not caught.
+        return serializePackageJSON(parsed, normalized);
+      }
+      return nativeModulesBinding.readPackageJSON(jsonPath, isESM, base, specifier);
+    },
+    getNearestParentPackageJSON(checkPath) {
+      const normalized = resolve(checkPath);
+      for (let i = 0; i < activeVFSList.length; i++) {
+        if (activeVFSList[i].shouldHandle(normalized)) {
+          const found = findVFSPackageJSON(normalized);
+          if (found.parsed !== undefined) {
+            return serializePackageJSON(found.parsed, found.pjsonPath);
+          }
+          return undefined;
+        }
+      }
+      return nativeModulesBinding.getNearestParentPackageJSON(checkPath);
+    },
+    getPackageScopeConfig(resolved) {
+      let filePath;
+      if (StringPrototypeStartsWith(resolved, 'file:')) {
+        try {
+          filePath = fileURLToPath(resolved);
+        } catch {
+          return nativeModulesBinding.getPackageScopeConfig(resolved);
+        }
+      } else {
+        filePath = resolved;
+      }
+      const normalized = resolve(filePath);
+      for (let i = 0; i < activeVFSList.length; i++) {
+        if (activeVFSList[i].shouldHandle(normalized)) {
+          const found = findVFSPackageJSON(normalized);
+          if (found.parsed !== undefined) {
+            return serializePackageJSON(found.parsed, found.pjsonPath);
+          }
+          // No package.json found anywhere up the tree - return the
+          // topmost path that was checked. Matches the C++ binding contract.
+          return found.sentinel;
+        }
+      }
+      return nativeModulesBinding.getPackageScopeConfig(resolved);
+    },
+    getPackageType(url) {
+      let filePath;
+      if (StringPrototypeStartsWith(url, 'file:')) {
+        try {
+          filePath = fileURLToPath(url);
+        } catch {
+          return nativeModulesBinding.getPackageType(url);
+        }
+      } else {
+        filePath = url;
+      }
+      const normalized = resolve(filePath);
+      for (let i = 0; i < activeVFSList.length; i++) {
+        if (activeVFSList[i].shouldHandle(normalized)) {
+          const found = findVFSPackageJSON(normalized);
+          if (found.parsed !== undefined) {
+            // Route through serializePackageJSON so a malformed `type`
+            // (non-string) throws ERR_INVALID_PACKAGE_CONFIG, matching
+            // the native binding and the other two package.json
+            // overrides. The serialized tuple is [name, main, type, ...].
+            const type = serializePackageJSON(found.parsed, found.pjsonPath)[2];
+            if (type === 'module' || type === 'commonjs') return type;
+          }
+          return undefined;
+        }
+      }
+      return nativeModulesBinding.getPackageType(url);
+    },
+  });
+}
+
+/**
+ * Install all VFS hooks: module loader overrides and fs handlers.
+ */
+function installHooks() {
+  if (hooksInstalled) return;
+  debug('install hooks');
+  installModuleLoaderOverrides();
+  vfsHandlerObj = createVfsHandlers();
+  setVfsHandlers(vfsHandlerObj);
+  hooksInstalled = true;
+}
+
+/**
+ * Tear down all VFS hooks when the last instance is deregistered. The
+ * fast path in the loader wrappers is restored so subsequent require/
+ * import calls pay zero overhead until another VFS is mounted.
+ */
+function uninstallHooks() {
+  if (!hooksInstalled) return;
+  debug('uninstall hooks');
+  const { setLoaderFsOverrides, setLoaderPackageOverrides } =
+    require('internal/modules/helpers');
+  setLoaderFsOverrides();
+  setLoaderPackageOverrides();
+  setVfsHandlers(null);
+  vfsHandlerObj = undefined;
+  hooksInstalled = false;
+}
+
 module.exports = {
   registerVFS,
   deregisterVFS,
diff --git a/test/parallel/test-vfs-import.mjs b/test/parallel/test-vfs-import.mjs
new file mode 100644
index 00000000000000..9b5a661d1a4b0d
--- /dev/null
+++ b/test/parallel/test-vfs-import.mjs
@@ -0,0 +1,142 @@
+// Flags: --experimental-vfs
+import '../common/index.mjs';
+import assert from 'assert';
+import vfs from 'node:vfs';
+
+// NOTE: Each test uses a unique mount path because ESM imports are cached
+// by URL — unmounting does not clear the V8 module cache, so reusing a
+// mount path would return stale cached modules from earlier tests.
+
+// Test importing a simple virtual ES module
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/hello.mjs', 'export const message = "hello from vfs";');
+  myVfs.mount('/esm-named');
+
+  const { message } = await import('/esm-named/hello.mjs');
+  assert.strictEqual(message, 'hello from vfs');
+
+  myVfs.unmount();
+}
+
+// Test importing a virtual module with default export
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/default.mjs', 'export default { name: "test", value: 42 };');
+  myVfs.mount('/esm-default');
+
+  const mod = await import('/esm-default/default.mjs');
+  assert.strictEqual(mod.default.name, 'test');
+  assert.strictEqual(mod.default.value, 42);
+
+  myVfs.unmount();
+}
+
+// Test importing a virtual module that imports another virtual module
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/utils.mjs', 'export function add(a, b) { return a + b; }');
+  myVfs.writeFileSync('/main.mjs', `
+    import { add } from '/esm-chain/utils.mjs';
+    export const result = add(10, 20);
+  `);
+  myVfs.mount('/esm-chain');
+
+  const { result } = await import('/esm-chain/main.mjs');
+  assert.strictEqual(result, 30);
+
+  myVfs.unmount();
+}
+
+// Test importing with relative paths
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/lib', { recursive: true });
+  myVfs.writeFileSync('/lib/helper.mjs', 'export const helper = () => "helped";');
+  myVfs.writeFileSync('/lib/index.mjs', `
+    import { helper } from './helper.mjs';
+    export const output = helper();
+  `);
+  myVfs.mount('/esm-relative');
+
+  const { output } = await import('/esm-relative/lib/index.mjs');
+  assert.strictEqual(output, 'helped');
+
+  myVfs.unmount();
+}
+
+// Test importing JSON from VFS (with import assertion)
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/data.json', JSON.stringify({ items: [1, 2, 3], enabled: true }));
+  myVfs.mount('/esm-json');
+
+  const data = await import('/esm-json/data.json', { with: { type: 'json' } });
+  assert.deepStrictEqual(data.default.items, [1, 2, 3]);
+  assert.strictEqual(data.default.enabled, true);
+
+  myVfs.unmount();
+}
+
+// Test that real modules still work when VFS is mounted
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/test.mjs', 'export const x = 1;');
+  myVfs.mount('/esm-builtin');
+
+  // Import from node: should still work
+  const assertMod = await import('node:assert');
+  assert.strictEqual(typeof assertMod.strictEqual, 'function');
+
+  myVfs.unmount();
+}
+
+// Test mixed CJS and ESM - ESM importing from VFS while CJS also works
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/esm-module.mjs', 'export const esmValue = "esm";');
+  myVfs.writeFileSync('/cjs-module.js', 'module.exports = { cjsValue: "cjs" };');
+  myVfs.mount('/esm-mixed');
+
+  const { esmValue } = await import('/esm-mixed/esm-module.mjs');
+  assert.strictEqual(esmValue, 'esm');
+
+  // CJS require should also work (via createRequire)
+  const { createRequire } = await import('module');
+  const require = createRequire(import.meta.url);
+  const { cjsValue } = require('/esm-mixed/cjs-module.js');
+  assert.strictEqual(cjsValue, 'cjs');
+
+  myVfs.unmount();
+}
+
+// Test ESM bare specifier resolution from VFS node_modules.
+// This sets up a proper node_modules structure inside VFS and imports
+// using a bare specifier (e.g., import 'my-vfs-pkg') instead of an
+// absolute path. This exercises the ESM default resolver's
+// internalModuleStat and getPackageJSONURL code paths.
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/my-vfs-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/my-vfs-pkg/package.json', JSON.stringify({
+    name: 'my-vfs-pkg',
+    type: 'module',
+    exports: { '.': './index.mjs' },
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/my-vfs-pkg/index.mjs',
+    'export const fromVfs = true;',
+  );
+  // The importing module must also live inside the VFS mount so that
+  // node_modules resolution walks upward from a VFS path.
+  myVfs.writeFileSync(
+    '/app/entry.mjs',
+    "export { fromVfs } from 'my-vfs-pkg';",
+  );
+  myVfs.mount('/esm-bare');
+
+  const { fromVfs } = await import('/esm-bare/app/entry.mjs');
+  assert.strictEqual(fromVfs, true);
+
+  myVfs.unmount();
+}
diff --git a/test/parallel/test-vfs-invalid-package-json.js b/test/parallel/test-vfs-invalid-package-json.js
new file mode 100644
index 00000000000000..89c9bc23cb71d1
--- /dev/null
+++ b/test/parallel/test-vfs-invalid-package-json.js
@@ -0,0 +1,45 @@
+// Flags: --experimental-vfs --expose-internals
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const vfs = require('node:vfs');
+
+// Test that invalid package.json in VFS falls through to index.js
+// (bad JSON is skipped, like a missing package.json).
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/pkg', { recursive: true });
+  myVfs.writeFileSync('/pkg/package.json', '{ invalid json');
+  myVfs.writeFileSync('/pkg/index.js', 'module.exports = 42;');
+  myVfs.mount('/mnt');
+
+  assert.strictEqual(require('/mnt/pkg'), 42);
+
+  myVfs.unmount();
+}
+
+// Test that valid package.json still works
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/pkg2', { recursive: true });
+  myVfs.writeFileSync('/pkg2/package.json', '{"main": "lib.js"}');
+  myVfs.writeFileSync('/pkg2/lib.js', 'module.exports = 99;');
+  myVfs.mount('/mnt2');
+
+  assert.strictEqual(require('/mnt2/pkg2'), 99);
+
+  myVfs.unmount();
+}
+
+// Test that missing package.json (ENOENT) still falls through to index.js
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/pkg3', { recursive: true });
+  myVfs.writeFileSync('/pkg3/index.js', 'module.exports = 77;');
+  myVfs.mount('/mnt3');
+
+  assert.strictEqual(require('/mnt3/pkg3'), 77);
+
+  myVfs.unmount();
+}
diff --git a/test/parallel/test-vfs-module-hooks-cleanup.js b/test/parallel/test-vfs-module-hooks-cleanup.js
new file mode 100644
index 00000000000000..acf15fbb25e762
--- /dev/null
+++ b/test/parallel/test-vfs-module-hooks-cleanup.js
@@ -0,0 +1,115 @@
+// Flags: --experimental-vfs
+'use strict';
+
+// Regression coverage for VFS-to-module-loader hook cleanup, package.json
+// validation parity with the C++ binding, and cache scoping across
+// register/deregister cycles.
+
+const common = require('../common');
+const assert = require('assert');
+const vfs = require('node:vfs');
+
+// 1) After a full deregister, a fresh register re-installs hooks cleanly:
+//    the second mount must be visible to require().
+{
+  const a = vfs.create();
+  a.writeFileSync('/m1.js', 'module.exports = "first"');
+  a.mount('/mnt-cycle-1');
+  assert.strictEqual(require('/mnt-cycle-1/m1.js'), 'first');
+  a.unmount();
+
+  const b = vfs.create();
+  b.writeFileSync('/m2.js', 'module.exports = "second"');
+  b.mount('/mnt-cycle-2');
+  assert.strictEqual(require('/mnt-cycle-2/m2.js'), 'second');
+  b.unmount();
+}
+
+// 2) After the last VFS is removed, a real-fs require still works (no
+//    stale module-loader override masking real files).
+{
+  const v = vfs.create();
+  v.writeFileSync('/x.js', 'module.exports = 1');
+  v.mount('/mnt-cleanup');
+  require('/mnt-cleanup/x.js');
+  v.unmount();
+  const fs = require('fs');
+  assert.strictEqual(typeof fs.readFileSync, 'function');
+}
+
+// 3) Top-level non-object package.json is rejected with
+//    ERR_INVALID_PACKAGE_CONFIG (matches the native binding's
+//    throw_invalid_package_config path for non-object roots).
+{
+  const v = vfs.create();
+  v.mkdirSync('/pkg');
+  v.writeFileSync('/pkg/package.json', 'null');
+  v.writeFileSync('/pkg/index.js', 'module.exports = 1');
+  v.mount('/mnt-null-pjson');
+  assert.throws(
+    () => require('/mnt-null-pjson/pkg'),
+    { code: 'ERR_INVALID_PACKAGE_CONFIG' },
+  );
+  v.unmount();
+}
+
+// 4) Non-string `main` is silently omitted, matching the native binding
+//    (`USE(value.get_string(...))` in src/node_modules.cc). A package
+//    with `{"main": 42}` and a sibling index.js must still resolve.
+{
+  const v = vfs.create();
+  v.mkdirSync('/pkg');
+  v.writeFileSync('/pkg/package.json', '{"main": 42}');
+  v.writeFileSync('/pkg/index.js', 'module.exports = "via-index"');
+  v.mount('/mnt-lax-main');
+  assert.strictEqual(require('/mnt-lax-main/pkg'), 'via-index');
+  v.unmount();
+}
+
+// 5) Partial deregister of a multi-mount setup leaves the still-mounted
+//    VFS fully functional. Guards against the prior "nuke caches before
+//    checking activeVFSList.length === 0" sledgehammer.
+{
+  const a = vfs.create();
+  a.writeFileSync('/a.js', 'module.exports = "a"');
+  a.mount('/mnt-multi-a');
+  const b = vfs.create();
+  b.writeFileSync('/b.js', 'module.exports = "b"');
+  b.mount('/mnt-multi-b');
+
+  assert.strictEqual(require('/mnt-multi-a/a.js'), 'a');
+  assert.strictEqual(require('/mnt-multi-b/b.js'), 'b');
+
+  // Deregister one; the other must still resolve.
+  a.unmount();
+  assert.strictEqual(require('/mnt-multi-b/b.js'), 'b');
+
+  b.unmount();
+}
+
+// 6) ESM legacyMainResolve override produces ERR_MODULE_NOT_FOUND with
+//    the resolved candidate path (not the bare package directory) when
+//    `main` points at a missing file. Driven through bare-specifier
+//    resolution from a VFS entry file - that is the only code path
+//    that calls loaderLegacyMainResolve. Also asserts err.url is
+//    undefined (not a bogus value) - the previous bug wrote 'package'
+//    into err.url by passing a string as the third constructor arg.
+(async () => {
+  const v = vfs.create();
+  v.mkdirSync('/app/node_modules/badpkg', { recursive: true });
+  v.writeFileSync(
+    '/app/node_modules/badpkg/package.json', '{"main": "./nope.js"}');
+  v.writeFileSync('/app/entry.mjs', "import 'badpkg';");
+  v.mount('/mnt-legacy-err');
+  await assert.rejects(
+    () => import('/mnt-legacy-err/app/entry.mjs'),
+    (err) => {
+      assert.strictEqual(err.code, 'ERR_MODULE_NOT_FOUND');
+      assert.match(err.message, /nope\.js/);
+      assert.match(err.message, /^Cannot find package /);
+      assert.strictEqual(err.url, undefined);
+      return true;
+    },
+  );
+  v.unmount();
+})().then(common.mustCall());
diff --git a/test/parallel/test-vfs-module-hooks.mjs b/test/parallel/test-vfs-module-hooks.mjs
new file mode 100644
index 00000000000000..2e511f9bac8e09
--- /dev/null
+++ b/test/parallel/test-vfs-module-hooks.mjs
@@ -0,0 +1,725 @@
+// Flags: --experimental-vfs
+import '../common/index.mjs';
+import assert from 'assert';
+import { createRequire } from 'module';
+import vfs from 'node:vfs';
+
+const require = createRequire(import.meta.url);
+
+// NOTE: Each test uses a different mount path (/mh1, /mh2, etc.)
+// because ESM imports are cached by URL.
+
+// =================================================================
+// Test: CJS bare specifier resolution with exports string shorthand
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/str-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/str-pkg/package.json', JSON.stringify({
+    name: 'str-pkg',
+    exports: './main.js',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/str-pkg/main.js',
+    'module.exports = { strExport: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry.js',
+    "module.exports = require('str-pkg');",
+  );
+  myVfs.mount('/mh1');
+
+  const result = require('/mh1/app/entry.js');
+  assert.strictEqual(result.strExport, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Conditional exports with import/require/default conditions
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/cond-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/cond-pkg/package.json', JSON.stringify({
+    name: 'cond-pkg',
+    exports: {
+      import: './esm.mjs',
+      require: './cjs.js',
+      default: './default.js',
+    },
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/cond-pkg/esm.mjs',
+    'export const source = "esm";',
+  );
+  myVfs.writeFileSync(
+    '/app/node_modules/cond-pkg/cjs.js',
+    'module.exports = { source: "cjs" };',
+  );
+  myVfs.writeFileSync(
+    '/app/node_modules/cond-pkg/default.js',
+    'module.exports = { source: "default" };',
+  );
+  // ESM entry that imports via bare specifier
+  myVfs.writeFileSync(
+    '/app/esm-entry.mjs',
+    "export { source } from 'cond-pkg';",
+  );
+  // CJS entry that requires via bare specifier
+  myVfs.writeFileSync(
+    '/app/cjs-entry.js',
+    "module.exports = require('cond-pkg');",
+  );
+  myVfs.mount('/mh2');
+
+  // ESM import should get the 'import' condition
+  const esmResult = await import('/mh2/app/esm-entry.mjs');
+  assert.strictEqual(esmResult.source, 'esm');
+
+  // CJS require should get the 'require' condition
+  const cjsResult = require('/mh2/app/cjs-entry.js');
+  assert.strictEqual(cjsResult.source, 'cjs');
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Subpath exports map
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/sub-pkg/lib', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/sub-pkg/package.json', JSON.stringify({
+    name: 'sub-pkg',
+    exports: {
+      '.': './lib/index.mjs',
+      './feature': './lib/feature.mjs',
+    },
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/sub-pkg/lib/index.mjs',
+    'export const main = true;',
+  );
+  myVfs.writeFileSync(
+    '/app/node_modules/sub-pkg/lib/feature.mjs',
+    'export const feature = true;',
+  );
+  myVfs.writeFileSync(
+    '/app/entry.mjs',
+    `
+    import { main } from 'sub-pkg';
+    import { feature } from 'sub-pkg/feature';
+    export { main, feature };
+    `,
+  );
+  myVfs.mount('/mh3');
+
+  const result = await import('/mh3/app/entry.mjs');
+  assert.strictEqual(result.main, true);
+  assert.strictEqual(result.feature, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Subpath exports with conditional object target
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/sub-cond-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/sub-cond-pkg/package.json', JSON.stringify({
+    name: 'sub-cond-pkg',
+    exports: {
+      '.': {
+        import: './esm.mjs',
+        require: './cjs.js',
+      },
+    },
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/sub-cond-pkg/esm.mjs',
+    'export const fromSubCond = "esm";',
+  );
+  myVfs.writeFileSync(
+    '/app/node_modules/sub-cond-pkg/cjs.js',
+    'module.exports = { fromSubCond: "cjs" };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry2.mjs',
+    "export { fromSubCond } from 'sub-cond-pkg';",
+  );
+  myVfs.mount('/mh4');
+
+  const result = await import('/mh4/app/entry2.mjs');
+  assert.strictEqual(result.fromSubCond, 'esm');
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Nested conditional exports (e.g. node → import/require)
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/nested-cond-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/nested-cond-pkg/package.json', JSON.stringify({
+    name: 'nested-cond-pkg',
+    exports: {
+      node: {
+        import: './node-esm.mjs',
+        require: './node-cjs.js',
+      },
+      default: './fallback.js',
+    },
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/nested-cond-pkg/node-esm.mjs',
+    'export const nested = "node-esm";',
+  );
+  myVfs.writeFileSync(
+    '/app/node_modules/nested-cond-pkg/node-cjs.js',
+    'module.exports = { nested: "node-cjs" };',
+  );
+  myVfs.writeFileSync(
+    '/app/node_modules/nested-cond-pkg/fallback.js',
+    'module.exports = { nested: "fallback" };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry3.mjs',
+    "export { nested } from 'nested-cond-pkg';",
+  );
+  myVfs.mount('/mh5');
+
+  const result = await import('/mh5/app/entry3.mjs');
+  assert.strictEqual(result.nested, 'node-esm');
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Package without exports, using main field
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/main-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/main-pkg/package.json', JSON.stringify({
+    name: 'main-pkg',
+    main: './lib/entry.js',
+  }));
+  myVfs.mkdirSync('/app/node_modules/main-pkg/lib', { recursive: true });
+  myVfs.writeFileSync(
+    '/app/node_modules/main-pkg/lib/entry.js',
+    'module.exports = { fromMain: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry4.js',
+    "module.exports = require('main-pkg');",
+  );
+  myVfs.mount('/mh6');
+
+  const result = require('/mh6/app/entry4.js');
+  assert.strictEqual(result.fromMain, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Package without exports/main, fallback to index.js
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/index-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/index-pkg/package.json', JSON.stringify({
+    name: 'index-pkg',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/index-pkg/index.js',
+    'module.exports = { fromIndex: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry5.js',
+    "module.exports = require('index-pkg');",
+  );
+  myVfs.mount('/mh7');
+
+  const result = require('/mh7/app/entry5.js');
+  assert.strictEqual(result.fromIndex, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Extension resolution (require without file extension)
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/lib', { recursive: true });
+  // File without .js extension in specifier
+  myVfs.writeFileSync('/lib/utils.js', 'module.exports = { ext: "js" };');
+  myVfs.writeFileSync(
+    '/lib/main.js',
+    "module.exports = require('/mh8/lib/utils');",
+  );
+  myVfs.mount('/mh8');
+
+  const result = require('/mh8/lib/main.js');
+  assert.strictEqual(result.ext, 'js');
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Extension resolution with .json
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/data.json', JSON.stringify({ ext: 'json' }));
+  myVfs.writeFileSync(
+    '/reader.js',
+    "module.exports = require('/mh9/data');",
+  );
+  myVfs.mount('/mh9');
+
+  const result = require('/mh9/reader.js');
+  assert.strictEqual(result.ext, 'json');
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Scoped package resolution (@scope/pkg)
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/@myorg/mylib', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/@myorg/mylib/package.json', JSON.stringify({
+    name: '@myorg/mylib',
+    type: 'module',
+    exports: './index.mjs',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/@myorg/mylib/index.mjs',
+    'export const scoped = true;',
+  );
+  myVfs.writeFileSync(
+    '/app/scoped-entry.mjs',
+    "export { scoped } from '@myorg/mylib';",
+  );
+  myVfs.mount('/mh11');
+
+  const result = await import('/mh11/app/scoped-entry.mjs');
+  assert.strictEqual(result.scoped, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Scoped package with subpath
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/@myorg/utils/lib', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/@myorg/utils/package.json', JSON.stringify({
+    name: '@myorg/utils',
+    exports: {
+      '.': './lib/index.mjs',
+      './helpers': './lib/helpers.mjs',
+    },
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/@myorg/utils/lib/index.mjs',
+    'export const main = true;',
+  );
+  myVfs.writeFileSync(
+    '/app/node_modules/@myorg/utils/lib/helpers.mjs',
+    'export const helpers = true;',
+  );
+  myVfs.writeFileSync(
+    '/app/scoped-sub-entry.mjs',
+    `
+    import { helpers } from '@myorg/utils/helpers';
+    export { helpers };
+    `,
+  );
+  myVfs.mount('/mh12');
+
+  const result = await import('/mh12/app/scoped-sub-entry.mjs');
+  assert.strictEqual(result.helpers, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: .js file with type: module in package.json → ESM format
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/package.json', JSON.stringify({ type: 'module' }));
+  myVfs.writeFileSync('/mod.js', 'export const fromModule = true;');
+  myVfs.mount('/mh13');
+
+  const result = await import('/mh13/mod.js');
+  assert.strictEqual(result.fromModule, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: .cjs always treated as CommonJS regardless of package type
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/package.json', JSON.stringify({ type: 'module' }));
+  myVfs.writeFileSync('/helper.cjs', 'module.exports = { cjsAlways: true };');
+  myVfs.writeFileSync(
+    '/use-cjs.js',
+    `
+    import { createRequire } from 'module';
+    const require = createRequire(import.meta.url);
+    const result = require('/mh14/helper.cjs');
+    export const cjsAlways = result.cjsAlways;
+    `,
+  );
+  myVfs.mount('/mh14');
+
+  const result = await import('/mh14/use-cjs.js');
+  assert.strictEqual(result.cjsAlways, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: file: URL specifier
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/fileurl.mjs', 'export const fromFileUrl = true;');
+  myVfs.mount('/mh15');
+
+  const result = await import('file:///mh15/fileurl.mjs');
+  assert.strictEqual(result.fromFileUrl, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Package with main field requiring extension resolution
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/dir-pkg', { recursive: true });
+  myVfs.writeFileSync('/dir-pkg/package.json', JSON.stringify({
+    name: 'dir-pkg',
+    main: './entry',
+  }));
+  myVfs.writeFileSync(
+    '/dir-pkg/entry.js',
+    'module.exports = { dirPkg: true };',
+  );
+  myVfs.mount('/mh16');
+
+  // Main field has no extension - tryExtensions should resolve entry → entry.js
+  const result = require('/mh16/dir-pkg');
+  assert.strictEqual(result.dirPkg, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Bare specifier with package subpath (no exports, direct file)
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/direct-pkg/lib', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/direct-pkg/package.json', JSON.stringify({
+    name: 'direct-pkg',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/direct-pkg/lib/util.js',
+    'module.exports = { directSub: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry-sub.js',
+    "module.exports = require('direct-pkg/lib/util.js');",
+  );
+  myVfs.mount('/mh17');
+
+  const result = require('/mh17/app/entry-sub.js');
+  assert.strictEqual(result.directSub, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Bare specifier subpath with extension resolution
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/ext-pkg/lib', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/ext-pkg/package.json', JSON.stringify({
+    name: 'ext-pkg',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/ext-pkg/lib/util.js',
+    'module.exports = { extSub: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry-ext.js',
+    // No .js extension - should be resolved by tryExtensions
+    "module.exports = require('ext-pkg/lib/util');",
+  );
+  myVfs.mount('/mh18');
+
+  const result = require('/mh18/app/entry-ext.js');
+  assert.strictEqual(result.extSub, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Bare specifier main field with extension resolution
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/main-ext-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/main-ext-pkg/package.json', JSON.stringify({
+    name: 'main-ext-pkg',
+    main: './entry',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/main-ext-pkg/entry.js',
+    'module.exports = { mainExt: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry-main-ext.js',
+    "module.exports = require('main-ext-pkg');",
+  );
+  myVfs.mount('/mh19');
+
+  const result = require('/mh19/app/entry-main-ext.js');
+  assert.strictEqual(result.mainExt, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: exports with array value (fallback array)
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/arr-exp-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/arr-exp-pkg/package.json', JSON.stringify({
+    name: 'arr-exp-pkg',
+    exports: {
+      '.': ['./index.js', './fallback.js'],
+    },
+    main: './index.js',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/arr-exp-pkg/index.js',
+    'module.exports = { arrExport: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry-arr.js',
+    "module.exports = require('arr-exp-pkg');",
+  );
+  myVfs.mount('/mh22');
+
+  // Array target in exports: canonical resolver tries each entry in order
+  const result = require('/mh22/app/entry-arr.js');
+  assert.strictEqual(result.arrExport, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Exports with "default" condition
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/default-pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/default-pkg/package.json', JSON.stringify({
+    name: 'default-pkg',
+    exports: {
+      '.': {
+        browser: './browser.js',
+        default: './default.mjs',
+      },
+    },
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/default-pkg/default.mjs',
+    'export const fromDefault = true;',
+  );
+  myVfs.writeFileSync(
+    '/app/entry-default.mjs',
+    "export { fromDefault } from 'default-pkg';",
+  );
+  myVfs.mount('/mh23');
+
+  // 'browser' condition not active in Node, 'default' should match
+  const result = await import('/mh23/app/entry-default.mjs');
+  assert.strictEqual(result.fromDefault, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Package.json type "commonjs" explicitly set for .js
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/package.json', JSON.stringify({ type: 'commonjs' }));
+  myVfs.writeFileSync('/explicit-cjs.js', 'module.exports = { explicitCjs: true };');
+  myVfs.mount('/mh24');
+
+  const result = require('/mh24/explicit-cjs.js');
+  assert.strictEqual(result.explicitCjs, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: .js file with no package.json → defaults to commonjs
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/no-pkg.js', 'module.exports = { noPkg: true };');
+  myVfs.mount('/mh25');
+
+  const result = require('/mh25/no-pkg.js');
+  assert.strictEqual(result.noPkg, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Package.json type walk stops at node_modules boundary
+// =================================================================
+{
+  const myVfs = vfs.create();
+  // Root has type: module
+  myVfs.writeFileSync('/package.json', JSON.stringify({ type: 'module' }));
+  // But file is inside node_modules with no local package.json
+  myVfs.mkdirSync('/node_modules/inner', { recursive: true });
+  myVfs.writeFileSync(
+    '/node_modules/inner/index.js',
+    'module.exports = { inner: true };',
+  );
+  myVfs.mount('/mh26');
+
+  // The walk should stop at node_modules, not inherit type:module from root
+  const result = require('/mh26/node_modules/inner/index.js');
+  assert.strictEqual(result.inner, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Invalid package.json in directory resolution
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/bad-json-dir', { recursive: true });
+  myVfs.writeFileSync('/bad-json-dir/package.json', '{ invalid json }');
+  myVfs.writeFileSync(
+    '/bad-json-dir/index.js',
+    'module.exports = { fallbackIndex: true };',
+  );
+  myVfs.mount('/mh28');
+
+  // Should fall through to index.js after failing to parse package.json
+  const result = require('/mh28/bad-json-dir');
+  assert.strictEqual(result.fallbackIndex, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Invalid package.json in type walk-up
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/package.json', '{ broken json }');
+  myVfs.writeFileSync('/no-type.js', 'module.exports = { noType: true };');
+  myVfs.mount('/mh29');
+
+  // Should treat as 'none' (commonjs) since package.json is invalid
+  const result = require('/mh29/no-type.js');
+  assert.strictEqual(result.noType, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: Scoped package without slash (just @scope/name)
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/@solo/pkg', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/@solo/pkg/package.json', JSON.stringify({
+    name: '@solo/pkg',
+    main: './index.js',
+  }));
+  myVfs.writeFileSync(
+    '/app/node_modules/@solo/pkg/index.js',
+    'module.exports = { solo: true };',
+  );
+  myVfs.writeFileSync(
+    '/app/entry-solo.js',
+    "module.exports = require('@solo/pkg');",
+  );
+  myVfs.mount('/mh30');
+
+  const result = require('/mh30/app/entry-solo.js');
+  assert.strictEqual(result.solo, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: node: builtin passthrough
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/use-builtin.mjs', `
+    import path from 'node:path';
+    export const sep = path.sep;
+  `);
+  myVfs.mount('/mh31');
+
+  const result = await import('/mh31/use-builtin.mjs');
+  assert.strictEqual(typeof result.sep, 'string');
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: JSON import with type assertion
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/data.json', JSON.stringify({ preformat: true }));
+  myVfs.mount('/mh32');
+
+  const result = await import('/mh32/data.json', { with: { type: 'json' } });
+  assert.strictEqual(result.default.preformat, true);
+
+  myVfs.unmount();
+}
+
+// =================================================================
+// Test: File with unknown extension → defaults to commonjs
+// =================================================================
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/data.txt', 'module.exports = { txt: true };');
+  myVfs.mount('/mh33');
+
+  // .txt extension → falls back to 'commonjs' via VFS_FORMAT_MAP default
+  const result = require('/mh33/data.txt');
+  assert.strictEqual(result.txt, true);
+
+  myVfs.unmount();
+}
diff --git a/test/parallel/test-vfs-package-json-cache.js b/test/parallel/test-vfs-package-json-cache.js
new file mode 100644
index 00000000000000..0c8c61e1facdc8
--- /dev/null
+++ b/test/parallel/test-vfs-package-json-cache.js
@@ -0,0 +1,23 @@
+// Flags: --experimental-vfs
+'use strict';
+
+// Package.json caches must be cleared on VFS unmount.
+
+require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const vfs = require('node:vfs');
+
+const myVfs = vfs.create();
+myVfs.mkdirSync('/pkg');
+myVfs.writeFileSync('/pkg/package.json', '{"name":"test","type":"module"}');
+myVfs.writeFileSync('/pkg/index.js', 'module.exports = 42');
+myVfs.mount('/mnt_pjcache');
+
+// Access the file so caches are populated
+assert.ok(fs.existsSync('/mnt_pjcache/pkg/package.json'));
+
+// After unmount, cache should be cleared (no stale entries)
+myVfs.unmount();
+
+assert.strictEqual(fs.existsSync('/mnt_pjcache/pkg/package.json'), false);
diff --git a/test/parallel/test-vfs-package-json.js b/test/parallel/test-vfs-package-json.js
new file mode 100644
index 00000000000000..c9ffe063e99d12
--- /dev/null
+++ b/test/parallel/test-vfs-package-json.js
@@ -0,0 +1,184 @@
+// Flags: --experimental-vfs --expose-internals
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const path = require('path');
+const vfs = require('node:vfs');
+
+// Test 1: read() reads package.json from VFS
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/pkg', { recursive: true });
+  myVfs.writeFileSync('/pkg/package.json', JSON.stringify({
+    name: 'test-pkg',
+    type: 'module',
+    main: './index.js',
+    exports: { '.': './index.js' },
+  }));
+  myVfs.mount('/mnt/read-test');
+
+  const packageJsonReader = require('internal/modules/package_json_reader');
+  const result = packageJsonReader.read(
+    path.resolve('/mnt/read-test/pkg/package.json'), {},
+  );
+
+  assert.strictEqual(result.exists, true);
+  assert.strictEqual(result.name, 'test-pkg');
+  assert.strictEqual(result.type, 'module');
+  assert.strictEqual(result.main, './index.js');
+  assert.deepStrictEqual(result.exports, { '.': './index.js' });
+  assert.strictEqual(
+    result.pjsonPath,
+    path.resolve('/mnt/read-test/pkg/package.json'),
+  );
+
+  // Non-existent package.json returns exists: false
+  const missing = packageJsonReader.read(
+    path.resolve('/mnt/read-test/nope/package.json'), {},
+  );
+  assert.strictEqual(missing.exists, false);
+
+  myVfs.unmount();
+}
+
+// Test 2: getNearestParentPackageJSON() walks up VFS directories
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/src/lib/deep', { recursive: true });
+  myVfs.writeFileSync('/app/package.json', JSON.stringify({
+    name: 'my-app',
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/app/src/lib/deep/module.js', '');
+  myVfs.mount('/mnt/parent-test');
+
+  const packageJsonReader = require('internal/modules/package_json_reader');
+  const result = packageJsonReader.getNearestParentPackageJSON(
+    path.resolve('/mnt/parent-test/app/src/lib/deep/module.js'),
+  );
+
+  assert.ok(result);
+  assert.strictEqual(result.exists, true);
+  assert.strictEqual(result.data.name, 'my-app');
+  assert.strictEqual(result.data.type, 'module');
+  assert.strictEqual(
+    result.path,
+    path.resolve('/mnt/parent-test/app/package.json'),
+  );
+
+  myVfs.unmount();
+}
+
+// Test 3: getPackageScopeConfig() returns correct scope from VFS
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/project/src', { recursive: true });
+  myVfs.writeFileSync('/project/package.json', JSON.stringify({
+    name: 'my-project',
+    type: 'commonjs',
+    exports: { '.': './main.js' },
+  }));
+  myVfs.writeFileSync('/project/src/index.js', '');
+  myVfs.mount('/mnt/scope-test');
+
+  const packageJsonReader = require('internal/modules/package_json_reader');
+  const { pathToFileURL } = require('url');
+  const scopeUrl = pathToFileURL(
+    path.resolve('/mnt/scope-test/project/src/index.js'),
+  ).href;
+  const result = packageJsonReader.getPackageScopeConfig(scopeUrl);
+
+  assert.strictEqual(result.exists, true);
+  assert.strictEqual(result.type, 'commonjs');
+  assert.strictEqual(result.name, 'my-project');
+  assert.strictEqual(
+    result.pjsonPath,
+    path.resolve('/mnt/scope-test/project/package.json'),
+  );
+
+  // Path with no package.json returns exists: false
+  const myVfs2 = vfs.create();
+  myVfs2.mkdirSync('/empty/src', { recursive: true });
+  myVfs2.writeFileSync('/empty/src/file.js', '');
+  myVfs2.mount('/mnt/scope-empty');
+
+  const emptyUrl = pathToFileURL(
+    path.resolve('/mnt/scope-empty/empty/src/file.js'),
+  ).href;
+  const emptyResult = packageJsonReader.getPackageScopeConfig(emptyUrl);
+  assert.strictEqual(emptyResult.exists, false);
+
+  myVfs2.unmount();
+  myVfs.unmount();
+}
+
+// Test 4: getPackageType() returns correct type from VFS
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/esm-app', { recursive: true });
+  myVfs.writeFileSync('/esm-app/package.json', JSON.stringify({
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/esm-app/index.js', '');
+  myVfs.mount('/mnt/type-test');
+
+  const packageJsonReader = require('internal/modules/package_json_reader');
+  const { pathToFileURL } = require('url');
+  const typeUrl = pathToFileURL(
+    path.resolve('/mnt/type-test/esm-app/index.js'),
+  ).href;
+  const type = packageJsonReader.getPackageType(typeUrl);
+  assert.strictEqual(type, 'module');
+
+  // No package.json => 'none'
+  const myVfs2 = vfs.create();
+  myVfs2.mkdirSync('/bare', { recursive: true });
+  myVfs2.writeFileSync('/bare/file.js', '');
+  myVfs2.mount('/mnt/type-empty');
+
+  const noneUrl = pathToFileURL(
+    path.resolve('/mnt/type-empty/bare/file.js'),
+  ).href;
+  const noneType = packageJsonReader.getPackageType(noneUrl);
+  assert.strictEqual(noneType, 'none');
+
+  myVfs2.unmount();
+  myVfs.unmount();
+}
+
+// Test 5: End-to-end CJS require with package.json type detection
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/cjs-app', { recursive: true });
+  myVfs.writeFileSync('/cjs-app/package.json', JSON.stringify({
+    name: 'cjs-app',
+    type: 'commonjs',
+  }));
+  myVfs.writeFileSync('/cjs-app/main.js',
+                      'module.exports = { format: "cjs", ok: true };');
+  myVfs.mount('/mnt/e2e-cjs');
+
+  const result = require('/mnt/e2e-cjs/cjs-app/main.js');
+  assert.strictEqual(result.format, 'cjs');
+  assert.strictEqual(result.ok, true);
+
+  myVfs.unmount();
+}
+
+// Test 6: End-to-end ESM import with VFS package type
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/esm', { recursive: true });
+  myVfs.writeFileSync('/esm/package.json', JSON.stringify({
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/esm/mod.mjs', 'export const x = 42;');
+  myVfs.mount('/mnt/e2e-esm');
+
+  // Use .mjs to ensure ESM treatment regardless of package type
+  const mod = require('/mnt/e2e-esm/esm/mod.mjs');
+  assert.strictEqual(mod.x, 42);
+
+  myVfs.unmount();
+}
diff --git a/test/parallel/test-vfs-require.js b/test/parallel/test-vfs-require.js
new file mode 100644
index 00000000000000..8c6c49c73ada5d
--- /dev/null
+++ b/test/parallel/test-vfs-require.js
@@ -0,0 +1,405 @@
+// Flags: --experimental-vfs
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+const vfs = require('node:vfs');
+
+// Test requiring a simple virtual module
+// VFS internal path: /hello.js
+// Mount point: /virtual
+// External path: /virtual/hello.js
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/hello.js', 'module.exports = "hello from vfs";');
+  myVfs.mount('/virtual');
+
+  const result = require('/virtual/hello.js');
+  assert.strictEqual(result, 'hello from vfs');
+
+  myVfs.unmount();
+}
+
+// Test requiring a virtual module that exports an object
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/config.js', `
+    module.exports = {
+      name: 'test-config',
+      version: '1.0.0',
+      getValue: function() { return 42; }
+    };
+  `);
+  myVfs.mount('/virtual2');
+
+  const config = require('/virtual2/config.js');
+  assert.strictEqual(config.name, 'test-config');
+  assert.strictEqual(config.version, '1.0.0');
+  assert.strictEqual(config.getValue(), 42);
+
+  myVfs.unmount();
+}
+
+// Test requiring a virtual module that requires another virtual module
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/utils.js', `
+    module.exports = {
+      add: function(a, b) { return a + b; }
+    };
+  `);
+  myVfs.writeFileSync('/main.js', `
+    const utils = require('/virtual3/utils.js');
+    module.exports = {
+      sum: utils.add(10, 20)
+    };
+  `);
+  myVfs.mount('/virtual3');
+
+  const main = require('/virtual3/main.js');
+  assert.strictEqual(main.sum, 30);
+
+  myVfs.unmount();
+}
+
+// Test requiring a JSON file from VFS
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/data.json', JSON.stringify({
+    items: [1, 2, 3],
+    enabled: true,
+  }));
+  myVfs.mount('/virtual4');
+
+  const data = require('/virtual4/data.json');
+  assert.deepStrictEqual(data.items, [1, 2, 3]);
+  assert.strictEqual(data.enabled, true);
+
+  myVfs.unmount();
+}
+
+// Test virtual package.json resolution
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/my-package', { recursive: true });
+  myVfs.writeFileSync('/my-package/package.json', JSON.stringify({
+    name: 'my-package',
+    main: 'index.js',
+  }));
+  myVfs.writeFileSync('/my-package/index.js', `
+    module.exports = { loaded: true };
+  `);
+  myVfs.mount('/virtual5');
+
+  const pkg = require('/virtual5/my-package');
+  assert.strictEqual(pkg.loaded, true);
+
+  myVfs.unmount();
+}
+
+// Test that real modules still work when VFS is mounted
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/test.js', 'module.exports = 1;');
+  myVfs.mount('/virtual6');
+
+  // require('assert') should still work (builtin)
+  assert.strictEqual(typeof assert.strictEqual, 'function');
+
+  // Real file requires should still work
+  const commonMod = require('../common');
+  assert.ok(commonMod);
+
+  myVfs.unmount();
+}
+
+// Test require with relative paths inside VFS module
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/lib', { recursive: true });
+  myVfs.writeFileSync('/lib/helper.js', `
+    module.exports = { help: function() { return 'helped'; } };
+  `);
+  myVfs.writeFileSync('/lib/index.js', `
+    const helper = require('./helper.js');
+    module.exports = helper.help();
+  `);
+  myVfs.mount('/virtual8');
+
+  const result = require('/virtual8/lib/index.js');
+  assert.strictEqual(result, 'helped');
+
+  myVfs.unmount();
+}
+
+// Test fs.readFileSync interception when VFS is active
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/file.txt', 'virtual content');
+  myVfs.mount('/virtual9');
+
+  const content = fs.readFileSync('/virtual9/file.txt', 'utf8');
+  assert.strictEqual(content, 'virtual content');
+
+  myVfs.unmount();
+}
+
+// Test requiring an ESM .mjs module from VFS
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/esm.mjs', 'export const msg = "hello from esm";');
+  myVfs.mount('/virtual11');
+
+  const mod = require('/virtual11/esm.mjs');
+  assert.strictEqual(mod.msg, 'hello from esm');
+
+  myVfs.unmount();
+}
+
+// Test requiring an ESM .mjs module with default export from VFS
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/esm-default.mjs', 'export default function() { return 42; }');
+  myVfs.mount('/virtual12');
+
+  const mod = require('/virtual12/esm-default.mjs');
+  assert.strictEqual(mod.default(), 42);
+
+  myVfs.unmount();
+}
+
+// Test require(esm): .js file detected as ESM via VFS package.json type:module
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app', { recursive: true });
+  myVfs.writeFileSync('/app/package.json', JSON.stringify({
+    name: 'esm-app',
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/app/lib.js',
+                      'export const value = 42;' +
+                      ' export function hello() { return "hi"; }');
+  myVfs.mount('/virtual13');
+
+  const mod = require('/virtual13/app/lib.js');
+  assert.strictEqual(mod.value, 42);
+  assert.strictEqual(mod.hello(), 'hi');
+
+  myVfs.unmount();
+}
+
+// Test require(esm): nested .js walks up to parent package.json in VFS
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/project/src/utils', { recursive: true });
+  myVfs.writeFileSync('/project/package.json', JSON.stringify({
+    name: 'nested-esm',
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/project/src/utils/math.js',
+                      'export const add = (a, b) => a + b;' +
+                      ' export default 99;');
+  myVfs.mount('/virtual14');
+
+  const mod = require('/virtual14/project/src/utils/math.js');
+  assert.strictEqual(mod.add(3, 4), 7);
+  assert.strictEqual(mod.default, 99);
+
+  myVfs.unmount();
+}
+
+// Test require(esm): .js without type field stays CJS
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/cjs-app', { recursive: true });
+  myVfs.writeFileSync('/cjs-app/package.json', JSON.stringify({
+    name: 'cjs-app',
+  }));
+  myVfs.writeFileSync('/cjs-app/index.js',
+                      'module.exports = { cjs: true };');
+  myVfs.mount('/virtual15');
+
+  const mod = require('/virtual15/cjs-app/index.js');
+  assert.strictEqual(mod.cjs, true);
+
+  myVfs.unmount();
+}
+
+// Test require(esm): ESM .js that imports another ESM .js in VFS
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/multi/src', { recursive: true });
+  myVfs.writeFileSync('/multi/package.json', JSON.stringify({
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/multi/src/dep.js', 'export const X = 100;');
+  myVfs.writeFileSync('/multi/src/main.js',
+                      'import { X } from "./dep.js";' +
+                      ' export const result = X + 1;');
+  myVfs.mount('/virtual16');
+
+  const mod = require('/virtual16/multi/src/main.js');
+  assert.strictEqual(mod.result, 101);
+
+  myVfs.unmount();
+}
+
+// Test require(esm): .mjs without any package.json loads as ESM
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/no-pkg.mjs',
+                      'export const x = 1; export default "hello";');
+  myVfs.mount('/virtual17');
+
+  const mod = require('/virtual17/no-pkg.mjs');
+  assert.strictEqual(mod.x, 1);
+  assert.strictEqual(mod.default, 'hello');
+
+  myVfs.unmount();
+}
+
+// Test require(esm): .mjs with package.json that has no type field
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app', { recursive: true });
+  myVfs.writeFileSync('/app/package.json',
+                      JSON.stringify({ name: 'no-type' }));
+  myVfs.writeFileSync('/app/lib.mjs', 'export const val = 42;');
+  myVfs.mount('/virtual18');
+
+  const mod = require('/virtual18/app/lib.mjs');
+  assert.strictEqual(mod.val, 42);
+
+  myVfs.unmount();
+}
+
+// Test require(esm): .mjs in type:commonjs package still loads as ESM
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/cjs-pkg', { recursive: true });
+  myVfs.writeFileSync('/cjs-pkg/package.json', JSON.stringify({
+    name: 'cjs-pkg',
+    type: 'commonjs',
+  }));
+  myVfs.writeFileSync('/cjs-pkg/esm.mjs', 'export const z = 99;');
+  myVfs.mount('/virtual19');
+
+  const mod = require('/virtual19/cjs-pkg/esm.mjs');
+  assert.strictEqual(mod.z, 99);
+
+  myVfs.unmount();
+}
+
+// Test CJS: package with "main" field resolves through VFS
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/pkg/lib', { recursive: true });
+  myVfs.writeFileSync('/pkg/package.json', JSON.stringify({
+    name: 'legacy-main-pkg',
+    main: './lib/entry',
+  }));
+  myVfs.writeFileSync('/pkg/lib/entry.js', 'module.exports = "legacy-main";');
+  myVfs.mount('/virtual20');
+
+  const result = require('/virtual20/pkg');
+  assert.strictEqual(result, 'legacy-main');
+
+  myVfs.unmount();
+}
+
+// Test CJS: package with no "main" field resolves index.js
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/pkg2', { recursive: true });
+  myVfs.writeFileSync('/pkg2/package.json', JSON.stringify({
+    name: 'no-main-pkg',
+  }));
+  myVfs.writeFileSync('/pkg2/index.js', 'module.exports = "index-fallback";');
+  myVfs.mount('/virtual21');
+
+  const result = require('/virtual21/pkg2');
+  assert.strictEqual(result, 'index-fallback');
+
+  myVfs.unmount();
+}
+
+// Test ESM legacyMainResolve: import() a VFS package with "main" (no "exports")
+// This triggers the ESM legacyMainResolve path in resolve.js via bare specifier
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app/node_modules/esm-legacy-main/lib', { recursive: true });
+  myVfs.writeFileSync('/app/node_modules/esm-legacy-main/package.json', JSON.stringify({
+    name: 'esm-legacy-main',
+    type: 'module',
+    main: './lib/entry.js',
+  }));
+  myVfs.writeFileSync('/app/node_modules/esm-legacy-main/lib/entry.js',
+                      'export const value = "esm-legacy-main";');
+  myVfs.writeFileSync('/app/main.mjs',
+                      'export { value } from "esm-legacy-main";');
+  myVfs.mount('/virtual20b');
+
+  import('/virtual20b/app/main.mjs').then(common.mustCall((mod) => {
+    assert.strictEqual(mod.value, 'esm-legacy-main');
+    myVfs.unmount();
+  }));
+}
+
+// Test ESM legacyMainResolve: import() a VFS package with no "main" (index.js fallback)
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/app2/node_modules/esm-nomain', { recursive: true });
+  myVfs.writeFileSync('/app2/node_modules/esm-nomain/package.json', JSON.stringify({
+    name: 'esm-nomain',
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/app2/node_modules/esm-nomain/index.js',
+                      'export const value = "esm-index-fallback";');
+  myVfs.writeFileSync('/app2/main.mjs',
+                      'export { value } from "esm-nomain";');
+  myVfs.mount('/virtual21b');
+
+  import('/virtual21b/app2/main.mjs').then(common.mustCall((mod) => {
+    assert.strictEqual(mod.value, 'esm-index-fallback');
+    myVfs.unmount();
+  }));
+}
+
+// Test getFormatOfExtensionlessFile: extensionless JS file in type:module package
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/esm-pkg', { recursive: true });
+  myVfs.writeFileSync('/esm-pkg/package.json', JSON.stringify({
+    name: 'esm-pkg',
+    type: 'module',
+  }));
+  myVfs.writeFileSync('/esm-pkg/entry', 'export const x = 123;');
+  myVfs.mount('/virtual22');
+
+  // Use import() to trigger ESM loader path for extensionless file detection
+  import('/virtual22/esm-pkg/entry').then(common.mustCall((mod) => {
+    assert.strictEqual(mod.x, 123);
+    myVfs.unmount();
+  }));
+}
+
+// Test that unmounting stops interception
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/unmount-test.js', 'module.exports = "before unmount";');
+  myVfs.mount('/virtual10');
+
+  const result = require('/virtual10/unmount-test.js');
+  assert.strictEqual(result, 'before unmount');
+
+  myVfs.unmount();
+
+  // After unmounting, the file should not be found
+  assert.throws(() => {
+    // Clear require cache first — the cache key is the platform-resolved path
+    delete require.cache[path.resolve('/virtual10/unmount-test.js')];
+    require('/virtual10/unmount-test.js');
+  }, { code: 'MODULE_NOT_FOUND' });
+}