diff --git a/lib/web/cache/cache.js b/lib/web/cache/cache.js
index 597de52bb4a..50c0eeaf0bc 100644
--- a/lib/web/cache/cache.js
+++ b/lib/web/cache/cache.js
@@ -4,7 +4,7 @@ const { kConstruct } = require('../../core/symbols')
 const { urlEquals, getFieldValues } = require('./util')
 const { kEnumerableProperty, isDisturbed } = require('../../core/util')
 const { webidl } = require('../fetch/webidl')
-const { Response, cloneResponse, fromInnerResponse, getResponseState } = require('../fetch/response')
+const { cloneResponse, fromInnerResponse, getResponseState } = require('../fetch/response')
 const { Request, fromInnerRequest, getRequestState } = require('../fetch/request')
 const { fetching } = require('../fetch/index')
 const { urlIsHttpHttpsScheme, createDeferredPromise, readAllBytes } = require('../fetch/util')
@@ -268,7 +268,7 @@ class Cache {
     let innerRequest = null
 
     // 2.
-    if (request instanceof Request) {
+    if (webidl.is.Request(request)) {
       innerRequest = getRequestState(request)
     } else { // 3.
       innerRequest = getRequestState(new Request(request))
@@ -400,7 +400,7 @@ class Cache {
      */
     let r = null
 
-    if (request instanceof Request) {
+    if (webidl.is.Request(request)) {
       r = getRequestState(request)
 
       if (r.method !== 'GET' && !options.ignoreMethod) {
@@ -466,7 +466,7 @@ class Cache {
     // 2.
     if (request !== undefined) {
       // 2.1
-      if (request instanceof Request) {
+      if (webidl.is.Request(request)) {
         // 2.1.1
         r = getRequestState(request)
 
@@ -748,7 +748,7 @@ class Cache {
 
     // 2.
     if (request !== undefined) {
-      if (request instanceof Request) {
+      if (webidl.is.Request(request)) {
         // 2.1.1
         r = getRequestState(request)
 
@@ -847,7 +847,10 @@ webidl.converters.MultiCacheQueryOptions = webidl.dictionaryConverter([
   }
 ])
 
-webidl.converters.Response = webidl.interfaceConverter(Response)
+webidl.converters.Response = webidl.interfaceConverter(
+  webidl.is.Response,
+  'Response'
+)
 
 webidl.converters['sequence<RequestInfo>'] = webidl.sequenceConverter(
   webidl.converters.RequestInfo
diff --git a/lib/web/cookies/index.js b/lib/web/cookies/index.js
index 23986aa148f..372112307bc 100644
--- a/lib/web/cookies/index.js
+++ b/lib/web/cookies/index.js
@@ -5,9 +5,7 @@ const { stringify } = require('./util')
 const { webidl } = require('../fetch/webidl')
 const { Headers } = require('../fetch/headers')
 
-const headersToBrandCheck = globalThis.Headers
-  ? [Headers, globalThis.Headers]
-  : [Headers]
+const brandChecks = webidl.brandCheckMultiple([Headers, globalThis.Headers].filter(Boolean))
 
 /**
  * @typedef {Object} Cookie
@@ -30,7 +28,7 @@ const headersToBrandCheck = globalThis.Headers
 function getCookies (headers) {
   webidl.argumentLengthCheck(arguments, 1, 'getCookies')
 
-  webidl.brandCheckMultiple(headers, headersToBrandCheck)
+  brandChecks(headers)
 
   const cookie = headers.get('cookie')
 
@@ -57,7 +55,7 @@ function getCookies (headers) {
  * @returns {void}
  */
 function deleteCookie (headers, name, attributes) {
-  webidl.brandCheckMultiple(headers, headersToBrandCheck)
+  brandChecks(headers)
 
   const prefix = 'deleteCookie'
   webidl.argumentLengthCheck(arguments, 2, prefix)
@@ -82,7 +80,7 @@ function deleteCookie (headers, name, attributes) {
 function getSetCookies (headers) {
   webidl.argumentLengthCheck(arguments, 1, 'getSetCookies')
 
-  webidl.brandCheckMultiple(headers, headersToBrandCheck)
+  brandChecks(headers)
 
   const cookies = headers.getSetCookie()
 
@@ -101,7 +99,7 @@ function getSetCookies (headers) {
 function setCookie (headers, cookie) {
   webidl.argumentLengthCheck(arguments, 2, 'setCookie')
 
-  webidl.brandCheckMultiple(headers, headersToBrandCheck)
+  brandChecks(headers)
 
   cookie = webidl.converters.Cookie(cookie)
 
diff --git a/lib/web/fetch/body.js b/lib/web/fetch/body.js
index 83426cd586a..99355d32e78 100644
--- a/lib/web/fetch/body.js
+++ b/lib/web/fetch/body.js
@@ -39,9 +39,9 @@ function extractBody (object, keepalive = false) {
   let stream = null
 
   // 2. If object is a ReadableStream object, then set stream to object.
-  if (object instanceof ReadableStream) {
+  if (webidl.is.ReadableStream(object)) {
     stream = object
-  } else if (object instanceof Blob) {
+  } else if (webidl.is.Blob(object)) {
     // 3. Otherwise, if object is a Blob object, set stream to the
     //    result of running object’s get stream.
     stream = object.stream()
@@ -64,7 +64,7 @@ function extractBody (object, keepalive = false) {
   }
 
   // 5. Assert: stream is a ReadableStream object.
-  assert(stream instanceof ReadableStream)
+  assert(webidl.is.ReadableStream(stream))
 
   // 6. Let action be null.
   let action = null
@@ -86,7 +86,7 @@ function extractBody (object, keepalive = false) {
 
     // Set type to `text/plain;charset=UTF-8`.
     type = 'text/plain;charset=UTF-8'
-  } else if (object instanceof URLSearchParams) {
+  } else if (webidl.is.URLSearchParams(object)) {
     // URLSearchParams
 
     // spec says to run application/x-www-form-urlencoded on body.list
@@ -109,7 +109,7 @@ function extractBody (object, keepalive = false) {
 
     // Set source to a copy of the bytes held by object.
     source = new Uint8Array(object.buffer.slice(object.byteOffset, object.byteOffset + object.byteLength))
-  } else if (object instanceof FormData) {
+  } else if (webidl.is.FormData(object)) {
     const boundary = `----formdata-undici-0${`${Math.floor(Math.random() * 1e11)}`.padStart(11, '0')}`
     const prefix = `--${boundary}\r\nContent-Disposition: form-data`
 
@@ -178,7 +178,7 @@ function extractBody (object, keepalive = false) {
     // followed by the multipart/form-data boundary string generated
     // by the multipart/form-data encoding algorithm.
     type = `multipart/form-data; boundary=${boundary}`
-  } else if (object instanceof Blob) {
+  } else if (webidl.is.Blob(object)) {
     // Blob
 
     // Set source to object.
@@ -206,7 +206,7 @@ function extractBody (object, keepalive = false) {
     }
 
     stream =
-      object instanceof ReadableStream ? object : ReadableStreamFrom(object)
+      webidl.is.ReadableStream(object) ? object : ReadableStreamFrom(object)
   }
 
   // 11. If source is a byte sequence, then set action to a
@@ -265,7 +265,7 @@ function safelyExtractBody (object, keepalive = false) {
   // a byte sequence or BodyInit object object, run these steps:
 
   // 1. If object is a ReadableStream object, then:
-  if (object instanceof ReadableStream) {
+  if (webidl.is.ReadableStream(object)) {
     // Assert: object is neither disturbed nor locked.
     // istanbul ignore next
     assert(!util.isDisturbed(object), 'The body has already been consumed.')
diff --git a/lib/web/fetch/formdata-parser.js b/lib/web/fetch/formdata-parser.js
index 1890b27422f..4bbc2c909fe 100644
--- a/lib/web/fetch/formdata-parser.js
+++ b/lib/web/fetch/formdata-parser.js
@@ -4,6 +4,7 @@ const { isUSVString, bufferToLowerCasedHeaderName } = require('../../core/util')
 const { utf8DecodeBytes } = require('./util')
 const { HTTP_TOKEN_CODEPOINTS, isomorphicDecode } = require('./data-url')
 const { makeEntry } = require('./formdata')
+const { webidl } = require('./webidl')
 const assert = require('node:assert')
 const { File: NodeFile } = require('node:buffer')
 
@@ -204,7 +205,7 @@ function multipartFormDataParser (input, mimeType) {
 
     // 5.12. Assert: name is a scalar value string and value is either a scalar value string or a File object.
     assert(isUSVString(name))
-    assert((typeof value === 'string' && isUSVString(value)) || value instanceof File)
+    assert((typeof value === 'string' && isUSVString(value)) || webidl.is.File(value))
 
     // 5.13. Create an entry with name and value, and append it to entry list.
     entryList.push(makeEntry(name, value, filename))
diff --git a/lib/web/fetch/formdata.js b/lib/web/fetch/formdata.js
index 4909e6dbc98..ef45cefb369 100644
--- a/lib/web/fetch/formdata.js
+++ b/lib/web/fetch/formdata.js
@@ -31,7 +31,7 @@ class FormData {
 
     name = webidl.converters.USVString(name)
 
-    if (arguments.length === 3 || value instanceof Blob) {
+    if (arguments.length === 3 || webidl.is.Blob(value)) {
       value = webidl.converters.Blob(value, prefix, 'value')
 
       if (filename !== undefined) {
@@ -122,7 +122,7 @@ class FormData {
 
     name = webidl.converters.USVString(name)
 
-    if (arguments.length === 3 || value instanceof Blob) {
+    if (arguments.length === 3 || webidl.is.Blob(value)) {
       value = webidl.converters.Blob(value, prefix, 'value')
 
       if (filename !== undefined) {
@@ -235,7 +235,7 @@ function makeEntry (name, value, filename) {
 
     // 1. If value is not a File object, then set value to a new File object,
     //    representing the same bytes, whose name attribute value is "blob"
-    if (!(value instanceof File)) {
+    if (!webidl.is.File(value)) {
       value = new File([value], 'blob', { type: value.type })
     }
 
@@ -256,4 +256,6 @@ function makeEntry (name, value, filename) {
   return { name, value }
 }
 
+webidl.is.FormData = webidl.util.MakeTypeAssertion(FormData.prototype)
+
 module.exports = { FormData, makeEntry, setFormDataState }
diff --git a/lib/web/fetch/index.js b/lib/web/fetch/index.js
index b2c95d94a09..00d34071f7f 100644
--- a/lib/web/fetch/index.js
+++ b/lib/web/fetch/index.js
@@ -803,7 +803,7 @@ function schemeFetch (fetchParams) {
 
       // 2. If request’s method is not `GET`, blobURLEntry is null, or blobURLEntry’s
       //    object is not a Blob object, then return a network error.
-      if (request.method !== 'GET' || !(blob instanceof Blob)) {
+      if (request.method !== 'GET' || !webidl.is.Blob(blob)) {
         return Promise.resolve(makeNetworkError('invalid method'))
       }
 
@@ -1439,7 +1439,7 @@ async function httpNetworkOrCacheFetch (
   //    11. If httpRequest’s referrer is a URL, then append
   //    `Referer`/httpRequest’s referrer, serialized and isomorphic encoded,
   //     to httpRequest’s header list.
-  if (httpRequest.referrer instanceof URL) {
+  if (webidl.is.URL(httpRequest.referrer)) {
     httpRequest.headersList.append('referer', isomorphicEncode(httpRequest.referrer.href), true)
   }
 
diff --git a/lib/web/fetch/request.js b/lib/web/fetch/request.js
index b6a0d6dfdfb..fec12b58308 100644
--- a/lib/web/fetch/request.js
+++ b/lib/web/fetch/request.js
@@ -144,7 +144,7 @@ class Request {
       // 6. Otherwise:
 
       // 7. Assert: input is a Request object.
-      assert(input instanceof Request)
+      assert(webidl.is.Request(input))
 
       // 8. Set request to input’s request.
       request = input.#state
@@ -490,7 +490,7 @@ class Request {
 
     // 33. Let inputBody be input’s request’s body if input is a Request
     // object; otherwise null.
-    const inputBody = input instanceof Request ? input.#state.body : null
+    const inputBody = webidl.is.Request(input) ? input.#state.body : null
 
     // 34. If either init["body"] exists and is non-null or inputBody is
     // non-null, and request’s method is `GET` or `HEAD`, then throw a
@@ -986,23 +986,21 @@ Object.defineProperties(Request.prototype, {
   }
 })
 
+webidl.is.Request = webidl.util.MakeTypeAssertion(Request.prototype)
+
 // https://fetch.spec.whatwg.org/#requestinfo
 webidl.converters.RequestInfo = function (V, prefix, argument) {
   if (typeof V === 'string') {
     return webidl.converters.USVString(V)
   }
 
-  if (V instanceof Request) {
+  if (webidl.is.Request(V)) {
     return V
   }
 
   return webidl.converters.USVString(V)
 }
 
-webidl.converters.AbortSignal = webidl.interfaceConverter(
-  AbortSignal
-)
-
 // https://fetch.spec.whatwg.org/#requestinit
 webidl.converters.RequestInit = webidl.dictionaryConverter([
   {
diff --git a/lib/web/fetch/response.js b/lib/web/fetch/response.js
index 904752f6c98..60cee8bb26e 100644
--- a/lib/web/fetch/response.js
+++ b/lib/web/fetch/response.js
@@ -19,7 +19,6 @@ const {
   nullBodyStatus
 } = require('./constants')
 const { webidl } = require('./webidl')
-const { FormData } = require('./formdata')
 const { URLSerializer } = require('./data-url')
 const { kConstruct } = require('../../core/symbols')
 const assert = require('node:assert')
@@ -569,7 +568,7 @@ webidl.converters.XMLHttpRequestBodyInit = function (V, prefix, name) {
     return webidl.converters.USVString(V, prefix, name)
   }
 
-  if (V instanceof Blob) {
+  if (webidl.is.Blob(V)) {
     return V
   }
 
@@ -577,11 +576,11 @@ webidl.converters.XMLHttpRequestBodyInit = function (V, prefix, name) {
     return V
   }
 
-  if (V instanceof FormData) {
+  if (webidl.is.FormData(V)) {
     return V
   }
 
-  if (V instanceof URLSearchParams) {
+  if (webidl.is.URLSearchParams(V)) {
     return V
   }
 
@@ -590,7 +589,7 @@ webidl.converters.XMLHttpRequestBodyInit = function (V, prefix, name) {
 
 // https://fetch.spec.whatwg.org/#bodyinit
 webidl.converters.BodyInit = function (V, prefix, argument) {
-  if (V instanceof ReadableStream) {
+  if (webidl.is.ReadableStream(V)) {
     return V
   }
 
@@ -620,6 +619,8 @@ webidl.converters.ResponseInit = webidl.dictionaryConverter([
   }
 ])
 
+webidl.is.Response = webidl.util.MakeTypeAssertion(Response.prototype)
+
 module.exports = {
   isNetworkError,
   makeNetworkError,
diff --git a/lib/web/fetch/util.js b/lib/web/fetch/util.js
index daa2483c943..cb9f3dd5ba3 100644
--- a/lib/web/fetch/util.js
+++ b/lib/web/fetch/util.js
@@ -399,7 +399,7 @@ function determineRequestsReferrer (request) {
 
     // note: we need to clone it as it's mutated
     referrerSource = new URL(globalOrigin)
-  } else if (request.referrer instanceof URL) {
+  } else if (webidl.is.URL(request.referrer)) {
     // Let referrerSource be request’s referrer.
     referrerSource = request.referrer
   }
@@ -476,7 +476,7 @@ function determineRequestsReferrer (request) {
  */
 function stripURLForReferrer (url, originOnly) {
   // 1. Assert: url is a URL.
-  assert(url instanceof URL)
+  assert(webidl.is.URL(url))
 
   url = new URL(url)
 
@@ -508,7 +508,7 @@ function stripURLForReferrer (url, originOnly) {
 }
 
 function isURLPotentiallyTrustworthy (url) {
-  if (!(url instanceof URL)) {
+  if (!webidl.is.URL(url)) {
     return false
   }
 
diff --git a/lib/web/fetch/webidl.js b/lib/web/fetch/webidl.js
index 5397344e57b..34653e6439e 100644
--- a/lib/web/fetch/webidl.js
+++ b/lib/web/fetch/webidl.js
@@ -13,10 +13,12 @@ const NULL = 7
 const OBJECT = 8 // function and object
 
 /** @type {import('../../../types/webidl').Webidl} */
-const webidl = {}
-webidl.converters = {}
-webidl.util = {}
-webidl.errors = {}
+const webidl = {
+  converters: {},
+  util: {},
+  errors: {},
+  is: {}
+}
 
 webidl.errors.exception = function (message) {
   return new TypeError(`${message.header}: ${message.message}`)
@@ -43,18 +45,22 @@ webidl.errors.invalidArgument = function (context) {
 
 // https://webidl.spec.whatwg.org/#implements
 webidl.brandCheck = function (V, I) {
-  if (!(V instanceof I)) {
+  if (!I.prototype.isPrototypeOf(V)) { // eslint-disable-line no-prototype-builtins
     const err = new TypeError('Illegal invocation')
     err.code = 'ERR_INVALID_THIS' // node compat.
     throw err
   }
 }
 
-webidl.brandCheckMultiple = function (V, List) {
-  if (List.every((clazz) => !(V instanceof clazz))) {
-    const err = new TypeError('Illegal invocation')
-    err.code = 'ERR_INVALID_THIS' // node compat.
-    throw err
+webidl.brandCheckMultiple = function (List) {
+  const prototypes = List.map((c) => webidl.util.MakeTypeAssertion(c.prototype))
+
+  return (V) => {
+    if (prototypes.every(typeCheck => !typeCheck(V))) {
+      const err = new TypeError('Illegal invocation')
+      err.code = 'ERR_INVALID_THIS' // node compat.
+      throw err
+    }
   }
 }
 
@@ -75,6 +81,11 @@ webidl.illegalConstructor = function () {
   })
 }
 
+const isPrototypeOf = Object.prototype.isPrototypeOf
+webidl.util.MakeTypeAssertion = function (Prototype) {
+  return (O) => isPrototypeOf.call(Prototype, O)
+}
+
 // https://tc39.es/ecma262/#sec-ecmascript-data-types-and-values
 webidl.util.Type = function (V) {
   switch (typeof V) {
@@ -372,12 +383,12 @@ webidl.recordConverter = function (keyConverter, valueConverter) {
   }
 }
 
-webidl.interfaceConverter = function (i) {
+webidl.interfaceConverter = function (TypeCheck, name) {
   return (V, prefix, argument) => {
-    if (!(V instanceof i)) {
+    if (!TypeCheck(V)) {
       throw webidl.errors.exception({
         header: prefix,
-        message: `Expected ${argument} ("${webidl.util.Stringify(V)}") to be an instance of ${i.name}.`
+        message: `Expected ${argument} ("${webidl.util.Stringify(V)}") to be an instance of ${name}.`
       })
     }
 
@@ -451,6 +462,14 @@ webidl.nullableConverter = function (converter) {
   }
 }
 
+webidl.is.ReadableStream = webidl.util.MakeTypeAssertion(ReadableStream.prototype)
+webidl.is.Blob = webidl.util.MakeTypeAssertion(Blob.prototype)
+webidl.is.URLSearchParams = webidl.util.MakeTypeAssertion(URLSearchParams.prototype)
+webidl.is.File = webidl.util.MakeTypeAssertion((globalThis.File ?? require('node:buffer').File).prototype)
+webidl.is.URL = webidl.util.MakeTypeAssertion(URL.prototype)
+webidl.is.AbortSignal = webidl.util.MakeTypeAssertion(AbortSignal.prototype)
+webidl.is.MessagePort = webidl.util.MakeTypeAssertion(MessagePort.prototype)
+
 // https://webidl.spec.whatwg.org/#es-DOMString
 webidl.converters.DOMString = function (V, prefix, argument, opts) {
   // 1. If V is null and the conversion is to an IDL type
@@ -697,7 +716,12 @@ webidl.converters['record<ByteString, ByteString>'] = webidl.recordConverter(
   webidl.converters.ByteString
 )
 
-webidl.converters.Blob = webidl.interfaceConverter(Blob)
+webidl.converters.Blob = webidl.interfaceConverter(webidl.is.Blob, 'Blob')
+
+webidl.converters.AbortSignal = webidl.interfaceConverter(
+  webidl.is.AbortSignal,
+  'AbortSignal'
+)
 
 module.exports = {
   webidl
diff --git a/lib/web/websocket/events.js b/lib/web/websocket/events.js
index 760b7297359..d61f6e9634a 100644
--- a/lib/web/websocket/events.js
+++ b/lib/web/websocket/events.js
@@ -3,7 +3,6 @@
 const { webidl } = require('../fetch/webidl')
 const { kEnumerableProperty } = require('../../core/util')
 const { kConstruct } = require('../../core/symbols')
-const { MessagePort } = require('node:worker_threads')
 
 /**
  * @see https://html.spec.whatwg.org/multipage/comms.html#messageevent
@@ -215,7 +214,10 @@ Object.defineProperties(ErrorEvent.prototype, {
   error: kEnumerableProperty
 })
 
-webidl.converters.MessagePort = webidl.interfaceConverter(MessagePort)
+webidl.converters.MessagePort = webidl.interfaceConverter(
+  webidl.is.MessagePort,
+  'MessagePort'
+)
 
 webidl.converters['sequence<MessagePort>'] = webidl.sequenceConverter(
   webidl.converters.MessagePort
diff --git a/lib/web/websocket/stream/websocketerror.js b/lib/web/websocket/stream/websocketerror.js
index ebe751efd71..e47551a96b9 100644
--- a/lib/web/websocket/stream/websocketerror.js
+++ b/lib/web/websocket/stream/websocketerror.js
@@ -78,4 +78,6 @@ Object.defineProperties(WebSocketError.prototype, {
   }
 })
 
+webidl.is.WebSocketError = webidl.util.MakeTypeAssertion(WebSocketError.prototype)
+
 module.exports = { WebSocketError, createUnvalidatedWebSocketError }
diff --git a/lib/web/websocket/stream/websocketstream.js b/lib/web/websocket/stream/websocketstream.js
index 937d1ba4766..71d63a5767a 100644
--- a/lib/web/websocket/stream/websocketstream.js
+++ b/lib/web/websocket/stream/websocketstream.js
@@ -425,7 +425,7 @@ class WebSocketStream {
     let reasonString = ''
 
     // 3. If reason implements WebSocketError ,
-    if (reason instanceof WebSocketError) {
+    if (webidl.is.WebSocketError(reason)) {
       // 3.1. Set code to reason ’s closeCode .
       code = reason.closeCode
 
@@ -470,11 +470,6 @@ webidl.converters.WebSocketStreamOptions = webidl.dictionaryConverter([
   }
 ])
 
-// TODO: take from request,hs
-webidl.converters.AbortSignal ??= webidl.interfaceConverter(
-  AbortSignal
-)
-
 webidl.converters.WebSocketCloseInfo = webidl.dictionaryConverter([
   {
     key: 'closeCode',
diff --git a/lib/web/websocket/websocket.js b/lib/web/websocket/websocket.js
index a0cd86c92c5..483b8c9f189 100644
--- a/lib/web/websocket/websocket.js
+++ b/lib/web/websocket/websocket.js
@@ -270,7 +270,7 @@ class WebSocket extends EventTarget {
       this.#sendQueue.add(data, () => {
         this.#bufferedAmount -= data.byteLength
       }, sendHints.typedArray)
-    } else if (data instanceof Blob) {
+    } else if (webidl.is.Blob(data)) {
       // If the WebSocket connection is established, and the WebSocket
       // closing handshake has not yet started, then the user agent must
       // send a WebSocket Message comprised of data using a binary frame
@@ -666,7 +666,7 @@ webidl.converters['DOMString or sequence<DOMString> or WebSocketInit'] = functio
 
 webidl.converters.WebSocketSendData = function (V) {
   if (webidl.util.Type(V) === webidl.util.Types.OBJECT) {
-    if (V instanceof Blob) {
+    if (webidl.is.Blob(V)) {
       return V
     }
 
diff --git a/test/webidl/helpers.js b/test/webidl/helpers.js
index fcd375f68b4..fd7a7dfb97b 100644
--- a/test/webidl/helpers.js
+++ b/test/webidl/helpers.js
@@ -4,11 +4,13 @@ const { describe, test } = require('node:test')
 const assert = require('node:assert')
 const { webidl } = require('../../lib/web/fetch/webidl')
 
-test('webidl.interfaceConverter', () => {
+test('webidl.interfaceConverter', (t) => {
   class A {}
   class B {}
 
-  const converter = webidl.interfaceConverter(A)
+  const converter = webidl.interfaceConverter(
+    webidl.util.MakeTypeAssertion(A.prototype)
+  )
 
   assert.throws(() => {
     converter(new B(), 'converter', 'converter')
@@ -17,6 +19,19 @@ test('webidl.interfaceConverter', () => {
   assert.doesNotThrow(() => {
     converter(new A(), 'converter', 'converter')
   })
+
+  t.test('interfaceConverters ignore Symbol.hasInstance', () => {
+    class V {}
+
+    Object.defineProperty(Blob.prototype, Symbol.hasInstance, {
+      value: () => true
+    })
+
+    const blobConverter = webidl.interfaceConverter(webidl.is.Blob, 'Blob')
+
+    assert.throws(() => blobConverter(new V()))
+    assert.equal(webidl.is.Blob(new V()), false)
+  })
 })
 
 describe('webidl.dictionaryConverter', () => {
diff --git a/types/webidl.d.ts b/types/webidl.d.ts
index c42a2617fd2..ac73af521d7 100644
--- a/types/webidl.d.ts
+++ b/types/webidl.d.ts
@@ -1,4 +1,5 @@
 // These types are not exported, and are only used internally
+import * as undici from './index'
 
 /**
  * Take in an unknown value and return one that is of type T
@@ -82,6 +83,8 @@ interface WebidlUtil {
    * Stringifies {@param V}
    */
   Stringify (V: any): string
+
+  MakeTypeAssertion <T extends { prototype: T }>(Prototype: T['prototype']): (arg: any) => arg is T
 }
 
 interface WebidlConverters {
@@ -173,10 +176,27 @@ interface WebidlConverters {
   [Key: string]: (...args: any[]) => unknown
 }
 
+type IsAssertion<T> = (arg: any) => arg is T
+
+interface WebidlIs {
+  Request: IsAssertion<undici.Request>
+  Response: IsAssertion<undici.Response>
+  ReadableStream: IsAssertion<ReadableStream>
+  Blob: IsAssertion<Blob>
+  URLSearchParams: IsAssertion<URLSearchParams>
+  File: IsAssertion<File>
+  FormData: IsAssertion<undici.FormData>
+  URL: IsAssertion<URL>
+  WebSocketError: IsAssertion<undici.WebSocketError>
+  AbortSignal: IsAssertion<AbortSignal>
+  MessagePort: IsAssertion<MessagePort>
+}
+
 export interface Webidl {
   errors: WebidlErrors
   util: WebidlUtil
   converters: WebidlConverters
+  is: WebidlIs
 
   /**
    * @description Performs a brand-check on {@param V} to ensure it is a
@@ -184,7 +204,7 @@ export interface Webidl {
    */
   brandCheck <Interface extends new () => unknown>(V: unknown, cls: Interface): asserts V is Interface
 
-  brandCheckMultiple <Interfaces extends (new () => unknown)[]> (V: unknown, list: Interfaces): asserts V is Interfaces[number]
+  brandCheckMultiple <Interfaces extends (new () => unknown)[]> (list: Interfaces): (V: any) => asserts V is Interfaces[number]
 
   /**
    * @see https://webidl.spec.whatwg.org/#es-sequence
@@ -207,11 +227,11 @@ export interface Webidl {
    * Similar to {@link Webidl.brandCheck} but allows skipping the check if third party
    * interfaces are allowed.
    */
-  interfaceConverter <Interface>(cls: Interface): (
+  interfaceConverter <Interface>(typeCheck: IsAssertion<Interface>, name: string): (
     V: unknown,
     prefix: string,
     argument: string
-  ) => asserts V is typeof cls
+  ) => asserts V is Interface
 
   // TODO(@KhafraDev): a type could likely be implemented that can infer the return type
   // from the converters given?