diff --git a/entrypoints.json b/entrypoints.json
index 6b507911ae..dd17bd46fb 100644
--- a/entrypoints.json
+++ b/entrypoints.json
@@ -20,6 +20,10 @@
     "typings": "./lib/database/index.d.ts",
     "dist": "./lib/database/index.js"
   },
+  "firebase-admin/extensions": {
+    "typings": "./lib/extensions/index.d.ts",
+    "dist": "./lib/extensions/index.js"
+  },
   "firebase-admin/firestore": {
     "typings": "./lib/firestore/index.d.ts",
     "dist": "./lib/firestore/index.js"
diff --git a/etc/firebase-admin.extensions.api.md b/etc/firebase-admin.extensions.api.md
new file mode 100644
index 0000000000..ed9090aaee
--- /dev/null
+++ b/etc/firebase-admin.extensions.api.md
@@ -0,0 +1,24 @@
+## API Report File for "firebase-admin.extensions"
+
+> Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
+
+```ts
+
+/// <reference types="node" />
+
+import { Agent } from 'http';
+
+// @public
+export class Extensions {
+    // Warning: (ae-forgotten-export) The symbol "App" needs to be exported by the entry point index.d.ts
+    //
+    // (undocumented)
+    readonly app: App;
+    // Warning: (ae-forgotten-export) The symbol "Runtime" needs to be exported by the entry point index.d.ts
+    runtime(): Runtime;
+}
+
+// @public
+export function getExtensions(app?: App): Extensions;
+
+```
diff --git a/package.json b/package.json
index 00cdee8e35..d5f678f5ec 100644
--- a/package.json
+++ b/package.json
@@ -74,6 +74,9 @@
       "eventarc": [
         "lib/eventarc"
       ],
+      "extensions": [
+        "lib/extensions"
+      ],
       "database": [
         "lib/database"
       ],
@@ -136,6 +139,11 @@
       "require": "./lib/eventarc/index.js",
       "import": "./lib/esm/eventarc/index.js"
     },
+    "./extensions": {
+      "types": "./lib/extensions/index.d.ts",
+      "require": "./lib/extensions/index.js",
+      "import": "./lib/esm/extensions/index.js"
+    },
     "./firestore": {
       "types": "./lib/firestore/index.d.ts",
       "require": "./lib/firestore/index.js",
diff --git a/src/extensions/extensions-api-client-internal.ts b/src/extensions/extensions-api-client-internal.ts
new file mode 100644
index 0000000000..407dd5a82e
--- /dev/null
+++ b/src/extensions/extensions-api-client-internal.ts
@@ -0,0 +1,150 @@
+/*!
+ * @license
+ * Copyright 2022 Google Inc.
+ *
+ * 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 { App } from '../app';
+import { FirebaseApp } from '../app/firebase-app';
+import { AuthorizedHttpClient, HttpClient, HttpError, HttpRequestConfig } from '../utils/api-request';
+import { FirebaseAppError, PrefixedFirebaseError } from '../utils/error';
+import * as validator from '../utils/validator';
+import * as utils from '../utils';
+
+const FIREBASE_FUNCTIONS_CONFIG_HEADERS = {
+  'X-Firebase-Client': `fire-admin-node/${utils.getSdkVersion()}`
+};
+const EXTENSIONS_API_VERSION = 'v1beta';
+// Note - use getExtensionsApiUri() instead so that changing environments is consistent.
+const EXTENSIONS_URL = 'https://firebaseextensions.googleapis.com';
+
+/**
+ * Class that facilitates sending requests to the Firebase Extensions backend API.
+ *
+ * @internal
+ */
+export class ExtensionsApiClient {
+  private readonly httpClient: HttpClient;
+  
+  constructor(private readonly app: App) {
+    if (!validator.isNonNullObject(app) || !('options' in app)) {
+      throw new FirebaseAppError(
+        'invalid-argument',
+        'First argument passed to getExtensions() must be a valid Firebase app instance.');
+    }
+    this.httpClient = new AuthorizedHttpClient(this.app as FirebaseApp);
+  }
+
+  async updateRuntimeData(
+    projectId: string,
+    instanceId: string,
+    runtimeData: RuntimeData
+  ): Promise<RuntimeDataResponse> {
+    const url = this.getRuntimeDataUri(projectId, instanceId);
+    const request: HttpRequestConfig = {
+      method: 'PATCH',
+      url,
+      headers: FIREBASE_FUNCTIONS_CONFIG_HEADERS,
+      data: runtimeData,
+    };
+    try {
+      const res = await this.httpClient.send(request);
+      return res.data
+    } catch (err: any) {
+      throw this.toFirebaseError(err);
+    }
+  }
+
+  private getExtensionsApiUri(): string {
+    return process.env['FIREBASE_EXT_URL'] ?? EXTENSIONS_URL;
+  }
+
+  private getRuntimeDataUri(projectId: string, instanceId: string): string {
+    return `${
+      this.getExtensionsApiUri()
+    }/${EXTENSIONS_API_VERSION}/projects/${projectId}/instances/${instanceId}/runtimeData`;
+  }
+
+  private toFirebaseError(err: HttpError): PrefixedFirebaseError {
+    if (err instanceof PrefixedFirebaseError) {
+      return err;
+    }
+
+    const response = err.response;
+    if (!response?.isJson()) {
+      return new FirebaseExtensionsError(
+        'unknown-error',
+        `Unexpected response with status: ${response.status} and body: ${response.text}`);
+    }
+    const error = response.data?.error;
+    const message = error?.message || `Unknown server error: ${response.text}`;
+    switch (error.code) {
+    case 403:
+      return  new FirebaseExtensionsError('forbidden', message);
+    case 404:
+      return new FirebaseExtensionsError('not-found', message);
+    case 500:
+      return new FirebaseExtensionsError('internal-error', message);
+    }
+    return new FirebaseExtensionsError('unknown-error', message);
+  }
+}
+
+interface RuntimeData {
+
+  //oneof
+  processingState?: ProcessingState;
+  fatalError?: FatalError;
+}
+
+interface RuntimeDataResponse extends RuntimeData{
+  name: string,
+  updateTime: string,
+}
+
+interface FatalError {
+  errorMessage: string;
+}
+
+interface ProcessingState {
+  detailMessage: string;
+  state: State;
+}
+
+type State = 'STATE_UNSPECIFIED' |
+  'NONE' |
+  'PROCESSING' |
+  'PROCESSING_COMPLETE' |
+  'PROCESSING_WARNING' |
+  'PROCESSING_FAILED';
+
+type ExtensionsErrorCode = 'invalid-argument' | 'not-found' | 'forbidden' | 'internal-error' | 'unknown-error';
+/**
+ * Firebase Extensions error code structure. This extends PrefixedFirebaseError.
+ *
+ * @param code - The error code.
+ * @param message - The error message.
+ * @constructor
+ */
+export class FirebaseExtensionsError extends PrefixedFirebaseError {
+  constructor(code: ExtensionsErrorCode, message: string) {
+    super('Extensions', code, message);
+
+    /* tslint:disable:max-line-length */
+    // Set the prototype explicitly. See the following link for more details:
+    // https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work
+    /* tslint:enable:max-line-length */
+    (this as any).__proto__ = FirebaseExtensionsError.prototype;
+  }
+}
diff --git a/src/extensions/extensions-api.ts b/src/extensions/extensions-api.ts
new file mode 100644
index 0000000000..b5ef4fcb56
--- /dev/null
+++ b/src/extensions/extensions-api.ts
@@ -0,0 +1,29 @@
+/*!
+ * @license
+ * Copyright 2022 Google Inc.
+ *
+ * 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.
+ */
+
+/**
+ * SettableProcessingState represents all the Processing states that can be set on an ExtensionInstance's runtimeData.
+ * 
+ * - NONE: No relevant lifecycle event work has been done. Set this to clear out old statuses.
+ * - PROCESSING_COMPLETE: Lifecycle event work completed with no errors.
+ * - PROCESSING_WARNING: Lifecycle event work succeeded partially,
+ *                        or something happened that the user should be warned about.
+ * - PROCESSING_FAILED: Lifecycle event work failed completely,
+ *                        but the instance will still work correctly going forward.
+ * - If the extension instance is in a broken state due to the errors, instead set FatalError.
+ */
+export type SettableProcessingState = 'NONE' | 'PROCESSING_COMPLETE' | 'PROCESSING_WARNING' | 'PROCESSING_FAILED';
diff --git a/src/extensions/extensions.ts b/src/extensions/extensions.ts
new file mode 100644
index 0000000000..6e98f3575c
--- /dev/null
+++ b/src/extensions/extensions.ts
@@ -0,0 +1,138 @@
+/*!
+ * @license
+ * Copyright 2022 Google Inc.
+ *
+ * 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 { App } from '../app';
+import { SettableProcessingState } from './extensions-api';
+import { ExtensionsApiClient, FirebaseExtensionsError } from './extensions-api-client-internal';
+import * as validator from '../utils/validator';
+
+/**
+ * The Firebase `Extensions` service interface.
+ */
+export class Extensions {
+  private readonly client: ExtensionsApiClient;
+  /**
+   * @param app - The app for this `Extensions` service.
+   * @constructor
+   * @internal
+   */
+  constructor(readonly app: App) {
+    this.client = new ExtensionsApiClient(app);
+  }
+
+  /**
+   * The runtime() method returns a new Runtime, which provides methods to modify an extension instance's runtime data.
+   * 
+   * @returns A new Runtime object.
+   */
+  public runtime(): Runtime {
+    return new Runtime(this.client);
+  }
+}
+
+/**
+ * Runtime provides methods to modify an extension instance's runtime data.
+ */
+class Runtime {
+  private projectId: string;
+  private extensionInstanceId: string;
+  private readonly client: ExtensionsApiClient;
+  /**
+   * @param client - The API client for this `Runtime` service.
+   * @constructor
+   * @internal
+   */
+  constructor(client: ExtensionsApiClient) {
+    this.projectId = this.getProjectId();
+    if (!validator.isNonEmptyString(process.env['EXT_INSTANCE_ID'])) {
+      throw new FirebaseExtensionsError(
+        'invalid-argument',
+        'Runtime is only available from within a running Extension instance.'
+      );
+    }
+    this.extensionInstanceId = process.env['EXT_INSTANCE_ID'];
+    if (!validator.isNonNullObject(client) || !('updateRuntimeData' in client)) {
+      throw new FirebaseExtensionsError(
+        'invalid-argument',
+        'Must provide a valid ExtensionsApiClient instance to create a new Runtime.');
+    }
+    this.client = client;
+  }
+
+  /**
+   * Sets the processing state of an extension instance.
+   *
+   * Use this method to report the results of a lifecycle event handler. If the 
+   * lifecycle event failed & the extension instance will no longer work 
+   * correctly, use `setFatalError` instead.
+   * 
+   * @param state - The state to set the instance to.
+   * @param detailMessage - A message explaining the results of the lifecycle function.
+   */
+  public async setProcessingState(state: SettableProcessingState, detailMessage: string): Promise<void> {
+    await this.client.updateRuntimeData(
+      this.projectId,
+      this.extensionInstanceId,
+      {
+        processingState: {
+          state,
+          detailMessage,
+        },
+      },
+    );
+  }
+
+  /**
+   * Reports a fatal error while running a lifecycle event handler.
+   *
+   * Call this method when a lifecycle event handler fails in a way that makes 
+   * the Instance inoperable.
+   * If the lifecycle event failed but the instance will still work as expected,
+   * call `setProcessingState` with the "PROCESSING_WARNING" or 
+   * "PROCESSING_FAILED" state instead.
+   * 
+   * @param errorMessage - A message explaining what went wrong and how to fix it.
+   */
+  public async setFatalError(errorMessage: string): Promise<void> {
+    if (!validator.isNonEmptyString(errorMessage)) {
+      throw new FirebaseExtensionsError(
+        'invalid-argument',
+        'errorMessage must not be empty'
+      );
+    }
+    await this.client.updateRuntimeData(
+      this.projectId,
+      this.extensionInstanceId,
+      {
+        fatalError: {
+          errorMessage,
+        },
+      },
+    );
+  }
+
+  private getProjectId(): string {
+    const projectId = process.env['PROJECT_ID'];
+    if (!validator.isNonEmptyString(projectId)) {
+      throw new FirebaseExtensionsError(
+        'invalid-argument',
+        'PROJECT_ID must not be undefined in Extensions runtime environment'
+      );
+    }
+    return projectId;
+  }
+}
diff --git a/src/extensions/index.ts b/src/extensions/index.ts
new file mode 100644
index 0000000000..6283e630dc
--- /dev/null
+++ b/src/extensions/index.ts
@@ -0,0 +1,63 @@
+/*!
+ * @license
+ * Copyright 2021 Google Inc.
+ *
+ * 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.
+ */
+
+/**
+ * Firebase Extensions service.
+ *
+ * @packageDocumentation
+ */
+
+import { App, getApp } from '../app';
+import { FirebaseApp } from '../app/firebase-app';
+import { Extensions } from './extensions';
+
+export { Extensions } from './extensions';
+ 
+/**
+  * Gets the {@link Extensions} service for the default app
+  * or a given app.
+  *
+  * `getExtensions()` can be called with no arguments to access the default
+  * app's `Extensions` service or as `getExtensions(app)` to access the
+  * `Extensions` service associated with a specific app.
+  *
+  * @example
+  * ```javascript
+  * // Get the `Extensions` service for the default app
+  * const defaultExtensions = getExtensions();
+  * ```
+  *
+  * @example
+  * ```javascript
+  * // Get the `Extensions` service for a given app
+  * const otherExtensions = getExtensions(otherApp);
+  * ```
+  *
+  * @param app - Optional app for which to return the `Extensions` service.
+  *   If not provided, the default `Extensions` service is returned.
+  *
+  * @returns The default `Extensions` service if no app is provided, or the `Extensions`
+  *   service associated with the provided app.
+  */
+export function getExtensions(app?: App): Extensions {
+  if (typeof app === 'undefined') {
+    app = getApp();
+  }
+   
+  const firebaseApp: FirebaseApp = app as FirebaseApp;
+  return firebaseApp.getOrInitService('extensions', (app) => new Extensions(app));
+}
diff --git a/test/unit/extensions/extensions-api-client-internal.spec.ts b/test/unit/extensions/extensions-api-client-internal.spec.ts
new file mode 100644
index 0000000000..a95baff154
--- /dev/null
+++ b/test/unit/extensions/extensions-api-client-internal.spec.ts
@@ -0,0 +1,90 @@
+/*!
+ * @license
+ * Copyright 2022 Google Inc.
+ *
+ * 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 sinon from 'sinon';
+
+import * as utils from '../utils';
+import * as mocks from '../../resources/mocks';
+import { FirebaseApp } from '../../../src/app/firebase-app';
+import { ExtensionsApiClient, FirebaseExtensionsError } from '../../../src/extensions/extensions-api-client-internal';
+import { HttpClient, HttpRequestConfig } from '../../../src/utils/api-request';
+import { SettableProcessingState } from '../../../src/extensions/extensions-api';
+
+const testProjectId = 'test-project';
+const testInstanceId = 'test-instance';
+
+describe('Extension API client', () => {
+  let app: FirebaseApp;
+  let apiClient: ExtensionsApiClient;
+
+  let httpClientStub: sinon.SinonStub;
+  const mockOptions = {
+    credential: new mocks.MockCredential(),
+    projectId: 'test-project',
+    serviceAccountId: 'service-acct@email.com'
+  };
+  
+  before(() => {
+    app = mocks.appWithOptions(mockOptions);
+    apiClient = new ExtensionsApiClient(app);
+  });
+
+  after(() => {
+    return app.delete();
+  });
+
+  beforeEach(() => {
+    httpClientStub = sinon.stub(HttpClient.prototype, 'send');
+  });
+
+  afterEach(() => {
+    httpClientStub.restore();
+  });
+
+  describe('Constructor', () => {
+    it('should reject when the app is null', () => {
+      expect(() => new ExtensionsApiClient(null as unknown as FirebaseApp))
+        .to.throw('First argument passed to getExtensions() must be a valid Firebase app instance.');
+    });
+  });
+
+  describe('updateRuntimeData', () => {
+    it('should updateRuntimeData', async () => {
+      const testRuntimeData = {
+        processingState: {
+          state: 'PROCESSING_COMPLETE' as SettableProcessingState,
+          detailMessage: 'done processing',
+        },
+      }
+      const expected =  sinon.match((req: HttpRequestConfig) => { 
+        const url = 'https://firebaseextensions.googleapis.com/' +
+        'v1beta/projects/test-project/instances/test-instance/runtimeData';
+        return req.method == 'PATCH' && req.url == url && req.data == testRuntimeData;
+      }, 'Incorrect URL or Method');
+      httpClientStub.withArgs(expected).resolves(utils.responseFrom(testRuntimeData, 200));
+      await expect(apiClient.updateRuntimeData(testProjectId, testInstanceId, testRuntimeData))
+        .to.eventually.deep.equal(testRuntimeData);
+    });
+
+    it('should convert errors in FirebaseErrors', async () => {
+      httpClientStub.rejects(utils.errorFrom('Something went wrong', 404));
+      await expect(apiClient.updateRuntimeData(testProjectId, testInstanceId, {}))
+        .to.eventually.be.rejectedWith(FirebaseExtensionsError);
+    });
+  });
+});
diff --git a/test/unit/extensions/extensions.spec.ts b/test/unit/extensions/extensions.spec.ts
new file mode 100644
index 0000000000..0b3ffb3004
--- /dev/null
+++ b/test/unit/extensions/extensions.spec.ts
@@ -0,0 +1,179 @@
+/*!
+ * @license
+ * Copyright 2022 Google Inc.
+ *
+ * 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 * as sinon from 'sinon';
+import { expect } from 'chai';
+
+import * as mocks from '../../resources/mocks';
+import * as utils from '../utils';
+import { FirebaseApp } from '../../../src/app/firebase-app';
+import { Extensions } from '../../../src/extensions/extensions';
+import { FirebaseAppError } from '../../../src/utils/error';
+import { HttpClient, HttpRequestConfig } from '../../../src/utils/api-request';
+import { SettableProcessingState } from '../../../src/extensions/extensions-api';
+import { FirebaseExtensionsError } from '../../../src/extensions/extensions-api-client-internal';
+
+describe('Extensions', () => {
+  const mockOptions = {
+    credential: new mocks.MockCredential(),
+    projectId: 'test-project',
+  };
+
+  let extensions: Extensions;
+  let mockApp: FirebaseApp;
+
+  beforeEach(() => {
+    mockApp = mocks.appWithOptions(mockOptions);
+    extensions = new Extensions(mockApp);
+  });
+
+  afterEach(() => {
+    return mockApp.delete();
+  });
+
+  describe('Constructor', () => {
+    it('should reject when the app is null', () => {
+      expect(() => new Extensions(null as unknown as FirebaseApp))
+        .to.throw(FirebaseAppError);
+    });
+  });
+
+  describe('app', () => {
+    it('returns the app from the constructor', () => {
+      // We expect referential equality here
+      expect(extensions.app).to.equal(mockApp);
+    });
+  });
+
+  describe('Runtime', () => {
+    let processEnvCopy: Record<string, string>;
+    beforeEach(() => {
+      processEnvCopy = JSON.parse(JSON.stringify(process.env)) as Record<string, string>;
+    });
+
+    afterEach(() => {
+      process.env = processEnvCopy;
+    });
+
+    describe('Constructor', () => {
+      it('should error if called without PROJECT_ID', () => {
+        process.env['EXT_INSTANCE_ID'] = 'test-instance';
+        expect(() => extensions.runtime())
+          .to.throw('PROJECT_ID must not be undefined in Extensions runtime environment');
+      });
+  
+  
+      it('should error if called without EXT_INSTANCE_ID', () => {
+        process.env['PROJECT_ID'] = 'test-project';
+        expect(() => extensions.runtime())
+          .to.throw('Runtime is only available from within a running Extension instance.');
+      });
+  
+      it('should not error if called from an extension', () => {
+        process.env['PROJECT_ID'] = 'test-project';
+        process.env['EXT_INSTANCE_ID'] = 'test-instance';
+        expect(() => extensions.runtime()).not.to.throw();
+      });
+    });
+    
+    describe('setProcessingState', () => {
+      let httpClientStub: sinon.SinonStub;
+      beforeEach(() => {
+        process.env['PROJECT_ID'] = 'test-project';
+        process.env['EXT_INSTANCE_ID'] = 'test-instance';
+        httpClientStub = sinon.stub(HttpClient.prototype, 'send');
+      });
+  
+      afterEach(() => {
+        httpClientStub.restore();
+      });
+  
+      for (const state of ['PROCESSING_FAILED', 'PROCESSING_WARNING','PROCESSING_COMPLETE', 'NONE']) {
+        it(`should set ${state} state`, async () => {
+          const expectedRuntimeData = {
+            processingState: {
+              state: state as SettableProcessingState,
+              detailMessage: 'done processing',
+            },
+          }
+          const expected =  sinon.match((req: HttpRequestConfig) => { 
+            const url = 'https://firebaseextensions.googleapis.com/' +
+              'v1beta/projects/test-project/instances/test-instance/runtimeData';
+            return req.method == 'PATCH' &&
+              req.url == url &&
+              JSON.stringify(req.data) == JSON.stringify(expectedRuntimeData);
+          }, 'Incorrect URL or Method');
+          httpClientStub.withArgs(expected).resolves(utils.responseFrom(expectedRuntimeData, 200));
+
+
+          await extensions.runtime().setProcessingState(state as SettableProcessingState, 'done processing');
+          expect(httpClientStub).to.have.been.calledOnce;
+        });
+      }
+  
+      it('should covert errors in FirebaseErrors', async () => {
+        httpClientStub.rejects(utils.errorFrom('Something went wrong', 404));
+        await expect(extensions.runtime().setProcessingState('PROCESSING_COMPLETE', 'a message'))
+          .to.eventually.be.rejectedWith(FirebaseExtensionsError);
+      });
+    });
+
+    describe('setFatalError', () => {
+      let httpClientStub: sinon.SinonStub;
+      beforeEach(() => {
+        process.env['PROJECT_ID'] = 'test-project';
+        process.env['EXT_INSTANCE_ID'] = 'test-instance';
+        httpClientStub = sinon.stub(HttpClient.prototype, 'send');
+      });
+  
+      afterEach(() => {
+        httpClientStub.restore();
+      });
+  
+      it('should set fatal error', async () => {
+        const expectedRuntimeData = {
+          fatalError: {
+            errorMessage: 'A bad error!',
+          },
+        };
+        const expected =  sinon.match((req: HttpRequestConfig) => { 
+          const url = 'https://firebaseextensions.googleapis.com/' +
+            'v1beta/projects/test-project/instances/test-instance/runtimeData';
+          return req.method == 'PATCH' &&
+            req.url == url &&
+            JSON.stringify(req.data) == JSON.stringify(expectedRuntimeData);
+        }, 'Incorrect URL or Method');
+        httpClientStub.withArgs(expected).resolves(utils.responseFrom(expectedRuntimeData, 200));
+
+
+        await extensions.runtime().setFatalError('A bad error!');
+        expect(httpClientStub).to.have.been.calledOnce;
+      });
+        
+      it('should error if errorMessage is empty', async () => {
+        await expect(extensions.runtime().setFatalError(''))
+          .to.eventually.be.rejectedWith(FirebaseExtensionsError, 'errorMessage must not be empty');
+      });
+  
+      it('should convert errors in FirebaseErrors', async () => {
+        httpClientStub.rejects(utils.errorFrom('Something went wrong', 404));
+        await expect(extensions.runtime().setFatalError('a message'))
+          .to.eventually.be.rejectedWith(FirebaseExtensionsError);
+      });
+    })
+  });
+});
diff --git a/test/unit/index.spec.ts b/test/unit/index.spec.ts
index aa7b262111..ffa46e7ef4 100644
--- a/test/unit/index.spec.ts
+++ b/test/unit/index.spec.ts
@@ -107,7 +107,12 @@ import './app-check/token-verifier.spec.ts';
 // Eventarc
 import './eventarc/eventarc.spec';
 import './eventarc/eventarc-utils.spec';
+
 // Functions
 import './functions/index.spec';
 import './functions/functions.spec';
 import './functions/functions-api-client-internal.spec';
+
+// Extensions
+import './extensions/extensions.spec';
+import './extensions/extensions-api-client-internal.spec';