If you have a type like:

public record Product(int Name, decimal Price);

... and make a call like:

var response = await chatClient.CompleteAsync<Product>(
    "Extract info about the following: we sell eggs for fifty dollars");
if (response.TryGetResult(out var product))
{
    Console.WriteLine($"{product.Name} costs {product.Price:c}");
}

... you would expect it to work quite reliably. But it does not - often the result JSON can't be parsed.

The cause

On debugging, I found this is because the JSON schema we generate contains this:

"price": {
    "type": ["string", "number" ]
}

The LLM sees the invitation to supply a string, and often returns JSON like:

{ "Name": "Eggs", "Price": "$50" }

or sometimes

{ "Name": "eggs", "Price": "fifty dollars" }

... neither of which can be JSON-parsed as a Product.

The solution

By default, for decimal/double/float, the schema should not invite the LLM to respond with a string, because suggesting it can return an arbitrary string will allow it to put in literally anything. It has no reason even to favour values that are numeric-like.

@eiriktsarpalis Should we be passing some other option to the JSON schema generator to advise it not to use string for numeric types?