diff --git a/packages-exp/auth-exp/karma.conf.js b/packages-exp/auth-exp/karma.conf.js
index fe4ace4252f..6e8b47df149 100644
--- a/packages-exp/auth-exp/karma.conf.js
+++ b/packages-exp/auth-exp/karma.conf.js
@@ -17,7 +17,7 @@
 
 const karmaBase = require('../../config/karma.base');
 
-const files = ['src/**/*.test.ts'];
+const files = ['src/**/*.test.ts', 'test/**/*.test.ts'];
 
 module.exports = function(config) {
   const karmaConfig = Object.assign({}, karmaBase, {
diff --git a/packages-exp/auth-exp/package.json b/packages-exp/auth-exp/package.json
index ea17f40c598..73fb1a354ed 100644
--- a/packages-exp/auth-exp/package.json
+++ b/packages-exp/auth-exp/package.json
@@ -19,6 +19,7 @@
     "dev": "rollup -c -w",
     "test": "yarn type-check && run-p lint test:browser test:node",
     "test:browser": "karma start --single-run",
+    "test:browser:debug": "karma start",
     "test:node": "TS_NODE_COMPILER_OPTIONS='{\"module\":\"commonjs\"}' nyc --reporter lcovonly -- mocha src/**/*.test.* --opts ../../config/mocha.node.opts",
     "type-check": "tsc -p . --noEmit",
     "prepare": "yarn build"
diff --git a/packages-exp/auth-exp/test/mock_fetch.test.ts b/packages-exp/auth-exp/test/mock_fetch.test.ts
new file mode 100644
index 00000000000..db88a57bed3
--- /dev/null
+++ b/packages-exp/auth-exp/test/mock_fetch.test.ts
@@ -0,0 +1,111 @@
+/**
+ * @license
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { expect } from 'chai';
+import * as mockFetch from './mock_fetch';
+
+async function fetchJson(path: string, req?: object): Promise<object> {
+  const body = req
+    ? {
+        body: JSON.stringify(req)
+      }
+    : {};
+  const request: RequestInit = {
+    method: 'POST',
+    headers: {
+      'Content-Type': 'application/json'
+    },
+    referrerPolicy: 'no-referrer',
+    ...body
+  };
+
+  const response = await fetch(path, request);
+  return response.json();
+}
+
+describe('mock fetch utility', () => {
+  beforeEach(mockFetch.setUp);
+  afterEach(mockFetch.tearDown);
+
+  describe('matched requests', () => {
+    it('returns the correct object for multiple routes', async () => {
+      mockFetch.mock('/a', { a: 1 });
+      mockFetch.mock('/b', { b: 2 });
+
+      expect(await fetchJson('/a')).to.eql({ a: 1 });
+      expect(await fetchJson('/b')).to.eql({ b: 2 });
+    });
+
+    it('passes through the status of the mock', async () => {
+      mockFetch.mock('/not-ok', {}, 500);
+      expect((await fetch('/not-ok')).status).to.equal(500);
+    });
+
+    it('records calls to the mock', async () => {
+      const someRequest = {
+        sentence:
+          'Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo'
+      };
+
+      const mock = mockFetch.mock('/word', {});
+      await fetchJson('/word', someRequest);
+      await fetchJson('/word', { a: 'b' });
+      await fetch('/word');
+
+      expect(mock.calls.length).to.equal(3);
+      expect(mock.calls[0].request).to.eql(someRequest);
+      expect(mock.calls[1].request).to.eql({ a: 'b' });
+      expect(mock.calls[2].request).to.equal(undefined);
+    });
+  });
+
+  describe('route rejection', () => {
+    it('if the route is not in the map', () => {
+      mockFetch.mock('/test', {});
+      expect(() => fetch('/not-test')).to.throw(
+        'Unknown route being requested: /not-test'
+      );
+    });
+
+    it('if call is not a string', () => {
+      mockFetch.mock('/blah', {});
+      expect(() => fetch(new Request({} as any))).to.throw(
+        'URL passed to fetch was not a string'
+      );
+    });
+  });
+});
+
+describe('mock fetch utility (no setUp/tearDown)', () => {
+  it('errors if mock attempted without setup', () => {
+    expect(() => mockFetch.mock('/test', {})).to.throw(
+      'Mock fetch is not set up'
+    );
+  });
+
+  it('routes do not carry to next run', async () => {
+    mockFetch.setUp();
+    mockFetch.mock('/test', { first: 'first' });
+    expect(await fetchJson('/test')).to.eql({ first: 'first' });
+    mockFetch.tearDown();
+    mockFetch.setUp();
+    expect(() => fetch('/test')).to.throw(
+      'Unknown route being requested: /test'
+    );
+    mockFetch.tearDown();
+  });
+});
diff --git a/packages-exp/auth-exp/test/mock_fetch.ts b/packages-exp/auth-exp/test/mock_fetch.ts
new file mode 100644
index 00000000000..1645e8a2453
--- /dev/null
+++ b/packages-exp/auth-exp/test/mock_fetch.ts
@@ -0,0 +1,82 @@
+/**
+ * @license
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { stub, SinonStub } from 'sinon';
+
+export interface Call {
+  request?: object;
+}
+
+export interface Route {
+  response: object;
+  status: number;
+  calls: Call[];
+}
+
+let fetchStub: SinonStub | null = null;
+let routes = new Map<string, Route>();
+
+// Using a constant rather than a function to enforce the type matches fetch()
+const fakeFetch: typeof fetch = (input: RequestInfo, request?: RequestInit) => {
+  if (typeof input !== 'string') {
+    throw new Error('URL passed to fetch was not a string');
+  }
+
+  if (!routes.has(input)) {
+    throw new Error(`Unknown route being requested: ${input}`);
+  }
+
+  // Bang-assertion is fine since we check for routes.has() above
+  const { response, status, calls } = routes.get(input)!;
+
+  calls.push({
+    request: request?.body ? JSON.parse(request.body as string) : undefined
+  });
+
+  const blob = new Blob([JSON.stringify(response)]);
+  return Promise.resolve(
+    new Response(blob, {
+      status
+    })
+  );
+};
+
+export function setUp(): void {
+  fetchStub = stub(self, 'fetch');
+  fetchStub.callsFake(fakeFetch);
+}
+
+export function mock(url: string, response: object, status = 200): Route {
+  if (!fetchStub) {
+    throw new Error('Mock fetch is not set up. Call setUp() first');
+  }
+
+  const route: Route = {
+    response,
+    status,
+    calls: []
+  };
+
+  routes.set(url, route);
+  return route;
+}
+
+export function tearDown(): void {
+  fetchStub?.restore();
+  fetchStub = null;
+  routes = new Map();
+}