[JS] zod.literal breaks schema parsing

[christophe-g]

Describe the bug
Schema containing zod.literals seem to break schema, with error message difficult to interpret

To Reproduce

For instance:

// run this through `pnpx tsx test.ts` 

import { gemini25FlashPreview0417, googleAI } from '@genkit-ai/googleai';
import { genkit, z } from 'genkit';

const optionSchema = z.object({
  locale: z.object({
    label: z.string().describe('The text/label of the option'),
    supportingText: z.string().optional().describe('The supporting text of the option'),
  }),
  description: z.string().optional().describe('The description of the option'),
}).describe('The option for a question');

// Base schema with common fields including the locale structure
const baseQuestionSchema = z.object({
  locale: z.object({
    label: z.string().describe('The text/label of the question being asked'),
    supportingText: z.string().optional().describe('The supporting text of the question'),
  }),
  items: z.array(optionSchema).optional().describe('The options for the checkbox question'),
  description: z.string().optional().describe('The description of the question'),
  type: z.string().describe('The type of the question'),
  // type: z.enum(['text', 'textarea', 'number']).describe('The type of the question'),
});
export const textItemSchema = z.object({
  type: z.literal('text'),
  locale: z.object({
    content: z.string().describe('text content of the presentation item'),
  }),
  description: z.string().optional().describe('The description of the option'),
}).describe('a text item, containing text content in markdown format');

export const sectionSchema = z.object({
  type: z.literal('section'),
  locale: z.object({
    heading: z.string().optional().describe('The heading of the section'),
  }),
  description: z.string().optional().describe('The description of the option'),
  items: z.array(baseQuestionSchema).describe('The questions or presentation items in the section'), // Allow both questions and presentation items
  // items: z.array(z.union([questionSchema, textItemSchema])).describe('The questions or presentation items in the section'), // Allow both questions and presentation items
}).describe('The section containing questions or presentation items');

export const pageSchema = z.object({
  type: z.literal('page').describe('Type for the page'),
  locale: z.object({
    heading: z.string().describe('The heading/title of the page'),
  }),
  description: z.string().optional().describe('The description of the option'),
  items: z.array(sectionSchema).describe('The sections in the page'),
}).describe('The page containing sections');

export const formSchema = z.object({
  type: z.literal('form').describe('Type for the form'),
  locale: z.object({
    heading: z.string().describe('title of the form'),
  }),
  description: z.string().optional().describe('The description of the option'),
  items: z.array(pageSchema).describe('The pages in the form'),
}).describe('The form containing pages');

const systemPromptForForm = `
  You are a editor of a Form. A Form is a collection of pages, sections, questions and text. 
  Each page has heading. Each page has section items. Each page can have a description. 
  Each section can have a heading and a description. Section group questions together under a common theme. 
  Section also contain text items written in markdown. A text item is a presentation item, it only contains content.
  Question types can be one of the following: text, textarea, number. 

  
`
const ai = genkit({
  plugins: [googleAI({ apiKey: process.env.GEMINI_API_KEY! })],
  model: gemini25FlashPreview0417, // set default model
});


async function main() {
  // make a generation request
  const { text } = await ai.generate({
    model: gemini25FlashPreview0417,
    system: systemPromptForForm,
    prompt: `Create a form that will be used to collect data from users about satisfaction with their current workplace.`,
    output: { schema: formSchema },
  });
  console.log(text);
}

main();

produces:

Restarting 'src/ai/generateForm.ts'
Shutting down all Genkit servers...
node:internal/process/promises:394
    triggerUncaughtException(err, true /* fromPromise */);
    ^

GoogleGenerativeAIFetchError: [GoogleGenerativeAI Error]: Error fetching from https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash-preview-04-17:generateContent: [400 Bad Request] Invalid JSON payload received. Unknown name "const" at 'generation_config.response_schema.properties[0].value': Cannot find field.
Invalid JSON payload received. Unknown name "const" at 'generation_config.response_schema.properties[3].value.items.properties[0].value': Cannot find field.
Invalid JSON payload received. Unknown name "const" at 'generation_config.response_schema.properties[3].value.items.properties[3].value.items.properties[0].value': Cannot find field. [{"@type":"type.googleapis.com/google.rpc.BadRequest","fieldViolations":[{"field":"generation_config.response_schema.properties[0].value","description":"Invalid JSON payload received. Unknown name \"const\" at 'generation_config.response_schema.properties[0].value': Cannot find field."},{"field":"generation_config.response_schema.properties[3].value.items.properties[0].value","description":"Invalid JSON payload received. Unknown name \"const\" at 'generation_config.response_schema.properties[3].value.items.properties[0].value': Cannot find field."},{"field":"generation_config.response_schema.properties[3].value.items.properties[3].value.items.properties[0].value","description":"Invalid JSON payload received. Unknown name \"const\" at 'generation_config.response_schema.properties[3].value.items.properties[3].value.items.properties[0].value': Cannot find field."}]}]
    at handleResponseNotOk (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@google+generative-ai@0.24.1/node_modules/@google/generative-ai/dist/index.js:434:11)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async makeRequest (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@google+generative-ai@0.24.1/node_modules/@google/generative-ai/dist/index.js:403:9)
    at async generateContent (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@google+generative-ai@0.24.1/node_modules/@google/generative-ai/dist/index.js:867:22)
    at async ChatSession.sendMessage (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@google+generative-ai@0.24.1/node_modules/@google/generative-ai/dist/index.js:1210:9)
    at async callGemini (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@genkit-ai+googleai@1.7.0_genkit@1.8.0/node_modules/@genkit-ai/googleai/src/gemini.ts:1045:26)
    at async <anonymous> (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@genkit-ai+googleai@1.7.0_genkit@1.8.0/node_modules/@genkit-ai/googleai/src/gemini.ts:1102:11)
    at async <anonymous> (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@genkit-ai+core@1.8.0/node_modules/@genkit-ai/core/src/action.ts:439:14)
    at async <anonymous> (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@genkit-ai+core@1.8.0/node_modules/@genkit-ai/core/src/action.ts:335:26)
    at async <anonymous> (/home/christophe/Programming/preignition/app/app-survey/functions/node_modules/.pnpm/@genkit-ai+core@1.8.0/node_modules/@genkit-ai/core/src/tracing/instrumentation.ts:73:16) {
  status: 400,
  statusText: 'Bad Request',
  errorDetails: [
    {
      '@type': 'type.googleapis.com/google.rpc.BadRequest',
      fieldViolations: [
        {
          field: 'generation_config.response_schema.properties[0].value',
          description: `Invalid JSON payload received. Unknown name "const" at 'generation_config.response_schema.properties[0].value': Cannot find field.`
        },
        {
          field: 'generation_config.response_schema.properties[3].value.items.properties[0].value',
          description: `Invalid JSON payload received. Unknown name "const" at 'generation_config.response_schema.properties[3].value.items.properties[0].value': Cannot find field.`
        },
        {
          field: 'generation_config.response_schema.properties[3].value.items.properties[3].value.items.properties[0].value',
          description: `Invalid JSON payload received. Unknown name "const" at 'generation_config.response_schema.properties[3].value.items.properties[3].value.items.properties[0].value': Cannot find field.`
        }
      ]
    }
  ],
  traceId: '36cf7d26589b35ed1965c2ee010dd081'
}

Node.js v23.6.1
Failed running 'src/ai/generateForm.ts'

Expected behavior

• zod basic features are not breaking schema parting
• errors message convey useful information

Runtime (please complete the following information):

• OS: Linux mint latest
• Version [e.g. 22]

** Node version

• 23.6.1

Additional context
For others hitting this, using enums as Literals seem to work better:

const typeEnums = z.enum([
  'text',
  'textarea',
  'number',
  'radio',
  'checkbox',
  'dropdown',
  'rating',
  'textfield',
  'tel',
  'date',
  'email',
  'url',
  'option',
  'section',
  'page',
  'form',
])

...
export const pageSchema = z.object({
  type: typeEnums.extract(['page']).describe('Type for the section'),
  locale: z.object({
    heading: z.string().describe('The heading/title of the page'),
  }),
  description: z.string().optional().describe('The description of the option'),
  items: z.array(sectionSchema).describe('The sections in the page'),
}).describe('The page containing sections');