diff --git a/src/client.ts b/src/client.ts
index d77a85ba4..38f4a501d 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -330,7 +330,9 @@ export class OpenAI {
   fetchOptions: MergedRequestInit | undefined;
 
   private fetch: Fetch;
-  #encoder: Opts.RequestEncoder;
+  // Use a normal private field instead of JS #private to avoid
+  // brand-check crashes when methods are invoked across copies.
+  private encoder: Opts.RequestEncoder;
   protected idempotencyHeader?: string;
   protected _options: ClientOptions;
 
@@ -392,7 +394,7 @@ export class OpenAI {
     this.fetchOptions = options.fetchOptions;
     this.maxRetries = options.maxRetries ?? 2;
     this.fetch = options.fetch ?? Shims.getDefaultFetch();
-    this.#encoder = Opts.FallbackEncoder;
+    this.encoder = Opts.FallbackEncoder;
 
     this._options = options;
 
@@ -427,7 +429,7 @@ export class OpenAI {
   /**
    * Check whether the base URL is set to its default.
    */
-  #baseURLOverridden(): boolean {
+  private baseURLOverridden(): boolean {
     return this.baseURL !== 'https://api.openai.com/v1';
   }
 
@@ -494,7 +496,7 @@ export class OpenAI {
     query: Record<string, unknown> | null | undefined,
     defaultBaseURL?: string | undefined,
   ): string {
-    const baseURL = (!this.#baseURLOverridden() && defaultBaseURL) || this.baseURL;
+    const baseURL = (!this.baseURLOverridden() && defaultBaseURL) || this.baseURL;
     const url =
       isAbsoluteURL(path) ?
         new URL(path)
@@ -960,7 +962,7 @@ export class OpenAI {
     ) {
       return { bodyHeaders: undefined, body: Shims.ReadableStreamFrom(body as AsyncIterable<Uint8Array>) };
     } else {
-      return this.#encoder({ body, headers });
+      return this.encoder({ body, headers });
     }
   }
 
diff --git a/src/core/api-promise.ts b/src/core/api-promise.ts
index 9e6c756c8..769e539f4 100644
--- a/src/core/api-promise.ts
+++ b/src/core/api-promise.ts
@@ -1,6 +1,7 @@
 // File generated from our OpenAPI spec by Stainless. See CONTRIBUTING.md for details.
 
 import { type OpenAI } from '../client';
+import { OpenAIError } from './error';
 
 import { type PromiseOrValue } from '../internal/types';
 import {
@@ -14,9 +15,12 @@ import {
  * A subclass of `Promise` providing additional helper methods
  * for interacting with the SDK.
  */
+// Associate instance with client via a module WeakMap to avoid
+// JS private-field brand checks across bundles/copies.
+const apiPromiseClient = /* @__PURE__ */ new WeakMap<object, OpenAI>();
+
 export class APIPromise<T> extends Promise<WithRequestID<T>> {
   private parsedPromise: Promise<WithRequestID<T>> | undefined;
-  #client: OpenAI;
 
   constructor(
     client: OpenAI,
@@ -32,11 +36,13 @@ export class APIPromise<T> extends Promise<WithRequestID<T>> {
       // to parse the response
       resolve(null as any);
     });
-    this.#client = client;
+    apiPromiseClient.set(this, client);
   }
 
   _thenUnwrap<U>(transform: (data: T, props: APIResponseProps) => U): APIPromise<U> {
-    return new APIPromise(this.#client, this.responsePromise, async (client, props) =>
+    const client = apiPromiseClient.get(this);
+    if (!client) throw new OpenAIError('Illegal invocation of APIPromise method');
+    return new APIPromise(client, this.responsePromise, async (client, props) =>
       addRequestID(transform(await this.parseResponse(client, props), props), props.response),
     );
   }
@@ -75,8 +81,10 @@ export class APIPromise<T> extends Promise<WithRequestID<T>> {
 
   private parse(): Promise<WithRequestID<T>> {
     if (!this.parsedPromise) {
+      const client = apiPromiseClient.get(this);
+      if (!client) throw new OpenAIError('Illegal invocation of APIPromise method');
       this.parsedPromise = this.responsePromise.then((data) =>
-        this.parseResponse(this.#client, data),
+        this.parseResponse(client, data),
       ) as any as Promise<WithRequestID<T>>;
     }
     return this.parsedPromise;
diff --git a/src/core/pagination.ts b/src/core/pagination.ts
index 9bef26447..eeedabdd8 100644
--- a/src/core/pagination.ts
+++ b/src/core/pagination.ts
@@ -10,15 +10,18 @@ import { maybeObj } from '../internal/utils/values';
 
 export type PageRequestOptions = Pick<FinalRequestOptions, 'query' | 'headers' | 'body' | 'path' | 'method'>;
 
+// Associate pages with their client via a module WeakMap to avoid
+// JS private-field brand checks across bundles/copies.
+const pageClient = /* @__PURE__ */ new WeakMap<object, OpenAI>();
+
 export abstract class AbstractPage<Item> implements AsyncIterable<Item> {
-  #client: OpenAI;
   protected options: FinalRequestOptions;
 
   protected response: Response;
   protected body: unknown;
 
   constructor(client: OpenAI, response: Response, body: unknown, options: FinalRequestOptions) {
-    this.#client = client;
+    pageClient.set(this, client);
     this.options = options;
     this.response = response;
     this.body = body;
@@ -42,7 +45,9 @@ export abstract class AbstractPage<Item> implements AsyncIterable<Item> {
       );
     }
 
-    return await this.#client.requestAPIList(this.constructor as any, nextOptions);
+    const client = pageClient.get(this);
+    if (!client) throw new OpenAIError('Illegal invocation of Page method');
+    return await client.requestAPIList(this.constructor as any, nextOptions);
   }
 
   async *iterPages(): AsyncGenerator<this> {
diff --git a/src/core/streaming.ts b/src/core/streaming.ts
index efd854046..01cd3dd38 100644
--- a/src/core/streaming.ts
+++ b/src/core/streaming.ts
@@ -18,9 +18,12 @@ export type ServerSentEvent = {
   raw: string[];
 };
 
+// Associate Stream instances with their client via a module WeakMap to avoid
+// JS private-field brand checks across bundles/copies.
+const streamClient = /* @__PURE__ */ new WeakMap<object, OpenAI | undefined>();
+
 export class Stream<Item> implements AsyncIterable<Item> {
   controller: AbortController;
-  #client: OpenAI | undefined;
 
   constructor(
     private iterator: () => AsyncIterator<Item>,
@@ -28,7 +31,7 @@ export class Stream<Item> implements AsyncIterable<Item> {
     client?: OpenAI,
   ) {
     this.controller = controller;
-    this.#client = client;
+    streamClient.set(this, client);
   }
 
   static fromSSEResponse<Item>(
@@ -75,8 +78,8 @@ export class Stream<Item> implements AsyncIterable<Item> {
             try {
               data = JSON.parse(sse.data);
             } catch (e) {
-              console.error(`Could not parse message into JSON:`, sse.data);
-              console.error(`From chunk:`, sse.raw);
+              logger.error(`Could not parse message into JSON:`, sse.data);
+              logger.error(`From chunk:`, sse.raw);
               throw e;
             }
             // TODO: Is this where the error should be thrown?
@@ -177,9 +180,10 @@ export class Stream<Item> implements AsyncIterable<Item> {
       };
     };
 
+    const client = streamClient.get(this);
     return [
-      new Stream(() => teeIterator(left), this.controller, this.#client),
-      new Stream(() => teeIterator(right), this.controller, this.#client),
+      new Stream(() => teeIterator(left), this.controller, client),
+      new Stream(() => teeIterator(right), this.controller, client),
     ];
   }
 
diff --git a/tests/core/brand-guards.test.ts b/tests/core/brand-guards.test.ts
new file mode 100644
index 000000000..8a10012d8
--- /dev/null
+++ b/tests/core/brand-guards.test.ts
@@ -0,0 +1,65 @@
+import { APIPromise } from 'openai/core/api-promise';
+import { AbstractPage, Page } from 'openai/core/pagination';
+import { Stream } from 'openai/core/streaming';
+
+const dummyResponseProps: any = { response: new Response(), options: {} };
+const dummyParse = (_client: any, _props: any) => ({ data: null, response: new Response() });
+
+describe('core brand-guard stability', () => {
+  test('APIPromise.then with mismatched this does not throw private-field TypeError', async () => {
+    const fake: any = Object.create(APIPromise.prototype);
+    fake.responsePromise = Promise.resolve(dummyResponseProps);
+    fake.parseResponse = dummyParse;
+
+    const call = () => (APIPromise.prototype.then as any).call(fake, () => {});
+    expect(call).toThrow(Error);
+    expect(call).not.toThrow(/private member/i);
+  });
+
+  test('APIPromise.catch with mismatched this does not throw private-field TypeError', async () => {
+    const fake: any = Object.create(APIPromise.prototype);
+    fake.responsePromise = Promise.resolve(dummyResponseProps);
+    fake.parseResponse = dummyParse;
+
+    const call = () => (APIPromise.prototype.catch as any).call(fake, () => {});
+    expect(call).toThrow(Error);
+    expect(call).not.toThrow(/private member/i);
+  });
+
+  test('APIPromise.finally with mismatched this does not throw private-field TypeError', async () => {
+    const fake: any = Object.create(APIPromise.prototype);
+    fake.responsePromise = Promise.resolve(dummyResponseProps);
+    fake.parseResponse = dummyParse;
+
+    const call = () => (APIPromise.prototype.finally as any).call(fake, () => {});
+    expect(call).toThrow(Error);
+    expect(call).not.toThrow(/private member/i);
+  });
+
+  test('AbstractPage.getNextPage with mismatched this does not throw private-field TypeError', async () => {
+    class TestPage<Item> extends Page<Item> {
+      override nextPageRequestOptions() {
+        return { path: '/v1/anything', method: 'get' } as any;
+      }
+    }
+
+    const fake: any = Object.create(TestPage.prototype);
+    fake.options = { path: '/v1/anything', method: 'get' };
+    fake.getPaginatedItems = () => [1];
+    fake.response = new Response();
+    fake.body = {};
+
+    const call = () => (AbstractPage.prototype.getNextPage as any).call(fake);
+    await expect(call()).rejects.toBeInstanceOf(Error);
+    await expect(call()).rejects.not.toThrow(/private member/i);
+  });
+
+  test('Stream.tee with mismatched this does not throw private-field TypeError', () => {
+    const fake: any = Object.create(Stream.prototype);
+    fake.controller = new AbortController();
+    fake.iterator = async function* () {};
+
+    const call = () => (Stream.prototype.tee as any).call(fake);
+    expect(call).not.toThrow(/private member/i);
+  });
+});