feat(client): support passing a custom fetch function (#46)

src/core.ts

@@ -21,7 +21,7 @@ export {
 
 const MAX_RETRIES = 2;
 
-type Fetch = (url: RequestInfo, init?: RequestInit) => Promise<Response>;
+export type Fetch = (url: RequestInfo, init?: RequestInit) => Promise<Response>;
 
 export abstract class APIClient {
   baseURL: string;
@@ -37,18 +37,20 @@ export abstract class APIClient {
     maxRetries,
     timeout = 60 * 1000, // 60s
     httpAgent,
+    fetch: overridenFetch,
   }: {
     baseURL: string;
     maxRetries?: number | undefined;
     timeout: number | undefined;
     httpAgent: Agent | undefined;
+    fetch: Fetch | undefined;
   }) {
     this.baseURL = baseURL;
     this.maxRetries = validatePositiveInteger('maxRetries', maxRetries ?? MAX_RETRIES);
     this.timeout = validatePositiveInteger('timeout', timeout);
     this.httpAgent = httpAgent;
 
-    this.fetch = fetch;
+    this.fetch = overridenFetch ?? fetch;
   }
 
   protected authHeaders(): Headers {

src/index.ts

@@ -35,6 +35,14 @@ type Config = {
    */
   httpAgent?: Agent;
 
+  /**
+   * Specify a custom `fetch` function implementation.
+   *
+   * If not provided, we use `node-fetch` on Node.js and otherwise expect that `fetch` is
+   * defined globally.
+   */
+  fetch?: Core.Fetch | undefined;
+
   /**
    * The maximum number of times that the client will retry a request in case of a
    * temporary failure, like a network error or a 5XX error from the server.
@@ -81,6 +89,7 @@ export class Anthropic extends Core.APIClient {
       timeout: options.timeout,
       httpAgent: options.httpAgent,
       maxRetries: options.maxRetries,
+      fetch: options.fetch,
     });
     this.apiKey = options.apiKey || null;
     this._options = options;

tests/index.test.ts

@@ -2,6 +2,7 @@
 
 import { Headers } from '@anthropic-ai/sdk/core';
 import Anthropic from '@anthropic-ai/sdk';
+import { Response } from '@anthropic-ai/sdk/_shims/fetch';
 
 describe('instantiate client', () => {
   const env = process.env;
@@ -77,6 +78,23 @@ describe('instantiate client', () => {
     });
   });
 
+  test('custom fetch', async () => {
+    const client = new Anthropic({
+      baseURL: 'http://localhost:5000/',
+      apiKey: 'my api key',
+      fetch: (url) => {
+        return Promise.resolve(
+          new Response(JSON.stringify({ url, custom: true }), {
+            headers: { 'Content-Type': 'application/json' },
+          }),
+        );
+      },
+    });
+
+    const response = await client.get('/foo');
+    expect(response).toEqual({ url: 'http://localhost:5000/foo', custom: true });
+  });
+
   describe('baseUrl', () => {
     test('trailing slash', () => {
       const client = new Anthropic({ baseURL: 'http://localhost:5000/custom/path/', apiKey: 'my api key' });