Zod schema conversion broken for unions in Zod 4.1.13+

[jfancher]

Confirm this is a Node library issue and not an underlying OpenAI API issue

• This is an issue with the Node library

Describe the bug

Zod recently started emitting oneOf instead of anyOf for discriminated unions:
colinhacks/zod#5453

This is not supported for OpenAI strict mode, so using the Zod conversion helpers will cause API failures upon upgrading to the latest Zod.

To Reproduce

• Define a discriminated union with Zod v4.1.13
• Use one of the openai/helpers/zod functions
• Use the result with the appropriate OpenAI endpoint
• It fails because the JSON schema includes oneOf, which is unsupported by OpenAI

Code snippets

import { z } from "zod";
import { zodResponseFormat } from "openai/helpers/zod";
import { OpenAI } from "openai";

const ResponseSchema = z.object({
  data: z.discriminatedUnion("type", [
    z.object({ type: z.literal("a") }),
    z.object({ type: z.literal("b") }),
  ]),
});

const client = new OpenAI({
  apiKey: process.env.OPENAI_API_KEY
});

const completion = await client.chat.completions.parse({
  model: "gpt-4o-mini",
  messages: [{ role: "user", content: "Choose a or b"}],
  response_format: zodResponseFormat(ResponseSchema, "choice"),
});

// Result:
// {
//   status: 400,
//   headers: Headers {},
//   requestID: 'req_a7f2a1dc6e19439289ff6992bec4a24d',
//   error: {
//     message: "Invalid schema for response_format 'choice': In context=('properties', 'data'), 'oneOf' is not permitted.",
//     type: 'invalid_request_error',
//     param: 'response_format',
//     code: null
//   },
//   code: null,
//   param: 'response_format',
//   type: 'invalid_request_error'
// }

OS

N/A

Node version

N/A

Library version

openai v6.9.1