diff --git a/lib/server.js b/lib/server.js
index ff03553eb..8b7823e43 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -82,6 +82,8 @@ class Server extends KarmaEventEmitter {
       filesPromise: ['factory', createFilesPromise],
       socketServer: ['factory', createSocketIoServer],
       executor: ['factory', Executor.factory],
+      // TODO: Deprecated. Remove in the next major
+      customFileHandlers: ['value', []],
       reporter: ['factory', reporter.createReporters],
       capturedBrowsers: ['factory', BrowserCollection.factory],
       args: ['value', {}],
diff --git a/lib/web-server.js b/lib/web-server.js
index 379818435..394066b7b 100644
--- a/lib/web-server.js
+++ b/lib/web-server.js
@@ -16,6 +16,25 @@ const proxyMiddleware = require('./middleware/proxy')
 
 const log = require('./logger').create('web-server')
 
+function createCustomHandler (customFileHandlers, config) {
+  let warningDone = false
+
+  return function (request, response, next) {
+    const handler = customFileHandlers.find((handler) => handler.urlRegex.test(request.url))
+
+    if (customFileHandlers.length > 0 && !warningDone) {
+      warningDone = true
+      log.warn('The `customFileHandlers` is deprecated and will be removed in Karma 7. Please upgrade plugins relying on this provider.')
+    }
+
+    return handler
+      ? handler.handler(request, response, 'fake/static', 'fake/adapter', config.basePath, 'fake/root')
+      : next()
+  }
+}
+
+createCustomHandler.$inject = ['customFileHandlers', 'config']
+
 function createFilesPromise (emitter, fileList) {
   // Set an empty list of files to avoid race issues with
   // file_list_modified not having been emitted yet
@@ -58,6 +77,8 @@ function createWebServer (injector, config) {
   handler.use(injector.invoke(sourceFilesMiddleware.create))
   // TODO(vojta): extract the proxy into a plugin
   handler.use(proxyMiddlewareInstance)
+  // TODO: Deprecated. Remove in the next major
+  handler.use(injector.invoke(createCustomHandler))
 
   if (config.middleware) {
     config.middleware.forEach((middleware) => handler.use(injector.get('middleware:' + middleware)))
diff --git a/test/unit/web-server.spec.js b/test/unit/web-server.spec.js
index 8517f172e..5d9f4f18e 100644
--- a/test/unit/web-server.spec.js
+++ b/test/unit/web-server.spec.js
@@ -31,7 +31,7 @@ describe('web-server', () => {
   // NOTE(vojta): only loading once, to speed things up
   // this relies on the fact that none of these tests mutate fs
   const m = mocks.loadFile(path.join(__dirname, '/../../lib/web-server.js'), _mocks, _globals)
-  server = emitter = null
+  let customFileHandlers = server = emitter = null
   let beforeMiddlewareActive = false
   let middlewareActive = false
   const servedFiles = (files) => {
@@ -40,6 +40,7 @@ describe('web-server', () => {
 
   describe('request', () => {
     beforeEach(() => {
+      customFileHandlers = []
       emitter = new EventEmitter()
       const config = {
         basePath: '/base/path',
@@ -56,6 +57,7 @@ describe('web-server', () => {
 
       const injector = new di.Injector([{
         config: ['value', config],
+        customFileHandlers: ['value', customFileHandlers],
         emitter: ['value', emitter],
         fileList: ['value', { files: { served: [], included: [] } }],
         filesPromise: ['factory', m.createFilesPromise],
@@ -180,6 +182,23 @@ describe('web-server', () => {
         })
     })
 
+    it('should load custom handlers', () => {
+      servedFiles(new Set())
+
+      // TODO(vojta): change this, only keeping because karma-dart is relying on it
+      customFileHandlers.push({
+        urlRegex: /\/some\/weird/,
+        handler (request, response, staticFolder, adapterFolder, baseFolder, urlRoot) {
+          response.writeHead(222)
+          response.end('CONTENT')
+        }
+      })
+
+      return request(server)
+        .get('/some/weird/url')
+        .expect(222, 'CONTENT')
+    })
+
     it('should serve 404 for non-existing files', () => {
       servedFiles(new Set())
 
@@ -196,6 +215,7 @@ describe('web-server', () => {
         cert: fs.readFileSync(path.join(__dirname, '/certificates/server.crt'))
       }
 
+      customFileHandlers = []
       emitter = new EventEmitter()
 
       const injector = new di.Injector([{
@@ -206,6 +226,7 @@ describe('web-server', () => {
           httpsServerOptions: credentials,
           client: { useIframe: true, useSingleWindow: false }
         }],
+        customFileHandlers: ['value', customFileHandlers],
         emitter: ['value', emitter],
         fileList: ['value', { files: { served: [], included: [] } }],
         filesPromise: ['factory', m.createFilesPromise],
@@ -244,10 +265,12 @@ describe('web-server', () => {
         cert: fs.readFileSync(path.join(__dirname, '/certificates/server.crt'))
       }
 
+      customFileHandlers = []
       emitter = new EventEmitter()
 
       const injector = new di.Injector([{
         config: ['value', { basePath: '/base/path', urlRoot: '/', httpModule: http2, protocol: 'https:', httpsServerOptions: credentials }],
+        customFileHandlers: ['value', customFileHandlers],
         emitter: ['value', emitter],
         fileList: ['value', { files: { served: [], included: [] } }],
         filesPromise: ['factory', m.createFilesPromise],