diff --git a/packages/core/src/utils/openai/index.ts b/packages/core/src/utils/openai/index.ts
index 2b5fdbef9c11..9bab70c2ae7c 100644
--- a/packages/core/src/utils/openai/index.ts
+++ b/packages/core/src/utils/openai/index.ts
@@ -264,6 +264,12 @@ function createDeepProxy(target: object, currentPath = '', options?: OpenAiOptio
         return instrumentMethod(value as (...args: unknown[]) => Promise<unknown>, methodPath, obj, options);
       }
 
+      if (typeof value === 'function') {
+        // Bind non-instrumented functions to preserve the original `this` context,
+        // which is required for accessing private class fields (e.g. #baseURL) in OpenAI SDK v5.
+        return value.bind(obj);
+      }
+
       if (value && typeof value === 'object') {
         return createDeepProxy(value as object, methodPath, options);
       }
diff --git a/packages/core/test/utils/openai-integration-functions.test.ts b/packages/core/test/utils/openai-integration-functions.test.ts
new file mode 100644
index 000000000000..d032fb1e69ed
--- /dev/null
+++ b/packages/core/test/utils/openai-integration-functions.test.ts
@@ -0,0 +1,210 @@
+import { beforeEach, describe, expect, it } from 'vitest';
+import type { OpenAiClient } from '../../src';
+import { instrumentOpenAiClient } from '../../src/utils/openai';
+
+interface FullOpenAIClient {
+  chat: {
+    completions: {
+      create: (params: ChatCompletionParams) => Promise<ChatCompletionResponse>;
+      parse: (params: ParseCompletionParams) => Promise<ParseCompletionResponse>;
+    };
+  };
+}
+interface ChatCompletionParams {
+  model: string;
+  messages: Array<{ role: string; content: string }>;
+}
+
+interface ChatCompletionResponse {
+  id: string;
+  model: string;
+  choices: Array<{ message: { content: string } }>;
+}
+
+interface ParseCompletionParams {
+  model: string;
+  messages: Array<{ role: string; content: string }>;
+  response_format: {
+    type: string;
+    json_schema: {
+      name: string;
+      schema: {
+        type: string;
+        properties: Record<string, { type: string }>;
+      };
+    };
+  };
+}
+
+interface ParseCompletionResponse {
+  id: string;
+  model: string;
+  choices: Array<{
+    message: {
+      content: string;
+      parsed: { name: string; age: number };
+    };
+  }>;
+  parsed: { name: string; age: number };
+}
+
+/**
+ * Mock OpenAI client that simulates the private field behavior
+ * that causes the "Cannot read private member" error
+ */
+class MockOpenAIClient implements FullOpenAIClient {
+  // Simulate private fields using WeakMap (similar to how TypeScript private fields work)
+  static #privateData = new WeakMap();
+
+  // Simulate instrumented methods
+  chat = {
+    completions: {
+      create: async (params: ChatCompletionParams): Promise<ChatCompletionResponse> => {
+        this.#buildURL('/chat/completions');
+        return { id: 'test', model: params.model, choices: [{ message: { content: 'Hello!' } }] };
+      },
+
+      // This is NOT instrumented
+      parse: async (params: ParseCompletionParams): Promise<ParseCompletionResponse> => {
+        this.#buildURL('/chat/completions');
+        return {
+          id: 'test',
+          model: params.model,
+          choices: [
+            {
+              message: {
+                content: 'Hello!',
+                parsed: { name: 'John', age: 30 },
+              },
+            },
+          ],
+          parsed: { name: 'John', age: 30 },
+        };
+      },
+    },
+  };
+
+  constructor() {
+    MockOpenAIClient.#privateData.set(this, {
+      apiKey: 'test-key',
+      baseURL: 'https://api.openai.com',
+    });
+  }
+
+  // Simulate the buildURL method that accesses private fields
+  #buildURL(path: string): string {
+    const data = MockOpenAIClient.#privateData.get(this);
+    if (!data) {
+      throw new TypeError('Cannot read private member from an object whose class did not declare it');
+    }
+    return `${data.baseURL}${path}`;
+  }
+}
+
+describe('OpenAI Integration Private Field Fix', () => {
+  let mockClient: MockOpenAIClient;
+  let instrumentedClient: FullOpenAIClient & OpenAiClient;
+
+  beforeEach(() => {
+    mockClient = new MockOpenAIClient();
+    instrumentedClient = instrumentOpenAiClient(mockClient as unknown as OpenAiClient) as FullOpenAIClient &
+      OpenAiClient;
+  });
+
+  it('should work with instrumented methods (chat.completions.create)', async () => {
+    // This should work because it's instrumented and we handle it properly
+    const result = await instrumentedClient.chat.completions.create({
+      model: 'gpt-4',
+      messages: [{ role: 'user', content: 'test' }],
+    });
+
+    expect(result.model).toBe('gpt-4');
+  });
+
+  it('should work with non-instrumented methods without breaking private fields', async () => {
+    // The parse method should work now with our fix - previously it would throw:
+    // "TypeError: Cannot read private member from an object whose class did not declare it"
+
+    await expect(
+      instrumentedClient.chat.completions.parse({
+        model: 'gpt-4',
+        messages: [{ role: 'user', content: 'Extract name and age from: John is 30 years old' }],
+        response_format: {
+          type: 'json_schema',
+          json_schema: {
+            name: 'person',
+            schema: {
+              type: 'object',
+              properties: {
+                name: { type: 'string' },
+                age: { type: 'number' },
+              },
+            },
+          },
+        },
+      }),
+    ).resolves.toBeDefined();
+  });
+
+  it('should preserve the original context for all method calls', async () => {
+    // Verify that 'this' context is preserved for instrumented methods
+    const createResult = await instrumentedClient.chat.completions.create({
+      model: 'gpt-4',
+      messages: [{ role: 'user', content: 'test' }],
+    });
+
+    expect(createResult.model).toBe('gpt-4');
+
+    // Verify that 'this' context is preserved for non-instrumented methods
+    const parseResult = await instrumentedClient.chat.completions.parse({
+      model: 'gpt-4',
+      messages: [{ role: 'user', content: 'Extract name and age from: John is 30 years old' }],
+      response_format: {
+        type: 'json_schema',
+        json_schema: {
+          name: 'person',
+          schema: {
+            type: 'object',
+            properties: {
+              name: { type: 'string' },
+              age: { type: 'number' },
+            },
+          },
+        },
+      },
+    });
+
+    expect(parseResult.parsed).toEqual({ name: 'John', age: 30 });
+  });
+
+  it('should handle nested object access correctly', async () => {
+    expect(typeof instrumentedClient.chat.completions.create).toBe('function');
+    expect(typeof instrumentedClient.chat.completions.parse).toBe('function');
+  });
+
+  it('should work with non-instrumented methods', async () => {
+    const result = await instrumentedClient.chat.completions.parse({
+      model: 'gpt-4',
+      messages: [{ role: 'user', content: 'Extract name and age from: John is 30 years old' }],
+      response_format: {
+        type: 'json_schema',
+        json_schema: {
+          name: 'person',
+          schema: {
+            type: 'object',
+            properties: {
+              name: { type: 'string' },
+              age: { type: 'number' },
+            },
+          },
+        },
+      },
+    });
+
+    expect(result.model).toBe('gpt-4');
+    expect(result.parsed).toEqual({ name: 'John', age: 30 });
+
+    // Verify we can access the parse method without issues
+    expect(typeof instrumentedClient.chat.completions.parse).toBe('function');
+  });
+});