[JS] regression - schema accepted under v1.22 fail from v1.23 onwards

[christophe-g]

Describe the bug

Relatively simple output schema break under v1.23 (up to v1.27) - but works with v1.22:

GenkitError: Error fetching from https://generativelanguage.googleapis.com/v1beta/models/gemini-3-flash-preview:generateContent: [400 Bad Request] Invalid JSON payload received. Unknown name "$ref" at 'generation_config.response_schema.properties[1].value.items.properties[1].value.items.properties[1].value.items.any_of[0][1].properties[0].value': Cannot find field.
Invalid JSON payload received. Unknown name "$ref" at 'generation_config.response_schema.properties[1].value.items.properties[1].value.items.properties[1].value.items.any_of[0][1].properties[1].value': Cannot find field.

To Reproduce

import { googleAI } from '@genkit-ai/google-genai';
import { getDownloadURL, getStorage } from 'firebase-admin/storage';
import { genkit, z } from 'genkit';
import { beforeEach, describe, expect, it } from 'vitest';

describe('importFormFlow Regression Test', () => {

  // 1. Simplified Schema
  const questionTypeEnums = z.enum([
    'textfield',
    'radio',
  ])

  const baseQuestionSchema = z.object({
    code: z.string().describe('The code of the question, camelCase'),
    label: z.string().describe('The question text'),
  });

  const textfieldSchema = baseQuestionSchema.extend({
    subType: questionTypeEnums.extract(['textfield']),
  }).describe('Text field question, allowing users to enter a single line of text');

  const optionSchema = z.object({
    code: z.string().describe('The code of the question, camelCase'),
    label: z.string().describe('The question text'),
  });

  const radioSchema = baseQuestionSchema.extend({
    subType: questionTypeEnums.extract(['radio']),
    items: z.array(optionSchema).describe('The options for the radio question'),
  }).describe('Radio question, allowing users to select one option from a list');

  const questionSchema = z.discriminatedUnion('subType', [
    textfieldSchema,
    radioSchema,
  ]);

  const textSchema = z.object({
    code: z.string().describe('The code of the question, camelCase'),
    label: z.string().describe('The question text'),
    // items: z.array(optionSchema).describe('List of option for the question'),
  });

  const sectionSchema = z.object({
    title: z.string().optional().describe('Section title'),
    items: z.array(z.union([questionSchema, textSchema])).describe('List of questions in the section'),
  });

  const pageSchema = z.object({
    heading: z.string().describe('Page heading'),
    sections: z.array(sectionSchema).describe('Sections in the page'),
  });

  const formSchema = z.object({
    title: z.string().describe('Form title to display'),
    pages: z.array(pageSchema).describe('Pages in the form'),
  });

  const ai = genkit({
    plugins: [
      googleAI({
        apiKey: geminiApiKey,
        // legacyResponseSchema: true // this is necessary from v1.23, otherwise another error is triggered
      })
    ]
  });

  const model = googleAI.model('gemini-3-flash-preview')

  const importFlow = ai.defineFlow(
    {
      name: 'importFlow',
      inputSchema: z.string(), // Input is just a prompt string for this test
      outputSchema: formSchema,
    },
    // @ts-expect-error - temporary workaround for genkit typing issue
    async (prompt) => {
      const { output } = await ai.generate({
        model,
        prompt: prompt,
        output: { schema: formSchema },
      });
      return output;
    }
  );

  it('should successfully parse generated form data', async () => {
    const result = await importFlow('Generate a form');
    console.info('Generated Result:', JSON.stringify(result, null, 2));
    expect(result).not.toBeNull();
    expect(result?.pages).toBeInstanceOf(Array);
    expect(result?.pages[0].sections[0].items).toBeInstanceOf(Array);
  }, 10000);
});

Expected behavior

The schema is accepted as an output schema.

Runtime (please complete the following information):

• OS: linux
• Version: 1.23

** Node version

• 23.6.1