Zod schema to define structure and not just to validate

[defless]

In the documentation I can found this about the generateObject method:

However, if you want to generate structured data like JSON, you can provide a schema that describes the structure of your desired object to the generateObject function.

What I understand, is that the LLM uses the provided zod schema to generate structured object according to it and also validate it.

In my case, I use streamUi alongside tools to allow user to generate and modify structured data in a "conversation" flow.
But it seems like the zod schema provided to the tool's parameters works perfectly for validation but is not influencing my data structure.

For example:

const title = z.object({
  type: z.literal('title'),
  headingType: z.enum(['h1', 'h2', 'h3', 'h4', 'h5', 'h6']),
  content: z.string(),
});

If I prompt something slightly different like 'generate a heading', the type will likely be type: 'heading' and my validation will fail.

Is it a normal behaviour ? or maybe I'm doing it wrong.

Thank you for your answer !

[lgrammel]

Please try using .describe("can be one of: 'h1', 'h2', 'h3', 'h4', 'h5', 'h6'") on the headingType.

[defless]

Yes that's what I'm doing to make it work, for example:

.describe(`
    A title, or heading is a block that contains a headingType
    and a content.
     It can be one of: 'h1', 'h2', 'h3', 'h4', 'h5', 'h6'
    Its type is "title".
  `),

But isn't the zod schema supposed to do this "formatting job" ?

[lgrammel]

I would need to look at the details of how this is translated into a json schema by zod-to-json-schema. I know that for enums it can be tricky. We don't modify further on purpose to give you control through describe.