diff --git a/src/core.ts b/src/core.ts
index ea8d8dca..344bf6ac 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -556,9 +556,19 @@ export abstract class APIClient {
 
     const timeout = setTimeout(() => controller.abort(), ms);
 
+    const fetchOptions = {
+      signal: controller.signal as any,
+      ...options,
+    };
+    if (fetchOptions.method) {
+      // Custom methods like 'patch' need to be uppercased
+      // See https://github.com/nodejs/undici/issues/2294
+      fetchOptions.method = fetchOptions.method.toUpperCase();
+    }
+
     return (
       // use undefined this binding; fetch errors if bound to something else in browser/cloudflare
-      this.fetch.call(undefined, url, { signal: controller.signal as any, ...options }).finally(() => {
+      this.fetch.call(undefined, url, fetchOptions).finally(() => {
         clearTimeout(timeout);
       })
     );
diff --git a/tests/index.test.ts b/tests/index.test.ts
index b6398085..9402a819 100644
--- a/tests/index.test.ts
+++ b/tests/index.test.ts
@@ -122,6 +122,23 @@ describe('instantiate client', () => {
     expect(spy).toHaveBeenCalledTimes(1);
   });
 
+  test('normalized method', async () => {
+    let capturedRequest: RequestInit | undefined;
+    const testFetch = async (url: RequestInfo, init: RequestInit = {}): Promise<Response> => {
+      capturedRequest = init;
+      return new Response(JSON.stringify({}), { headers: { 'Content-Type': 'application/json' } });
+    };
+
+    const client = new Anthropic({
+      baseURL: 'http://localhost:5000/',
+      apiKey: 'my-anthropic-api-key',
+      fetch: testFetch,
+    });
+
+    await client.patch('/foo');
+    expect(capturedRequest?.method).toEqual('PATCH');
+  });
+
   describe('baseUrl', () => {
     test('trailing slash', () => {
       const client = new Anthropic({