Missing parameters to AIFunctionFactory functions result in default value being passed

[stephentoub]

If I have a non-optional parameter in an AIFunction returned by AIFunctionFactory, and the values passed to InvokeAsync do not include a value for that parameter, the default value ends up getting used, rather than an exception being thrown. Is that the desired behavior?

Repro:

using Microsoft.Extensions.AI;

AIFunction f = AIFunctionFactory.Create((int a, int b) => a + b);
Console.WriteLine(await f.InvokeAsync([new KeyValuePair<string, object?>("a", 42)]));

That prints 42 rather than throwing an exception, even though there's no value for the non-optional b parameter.

cc: @eiriktsarpalis, @SteveSandersonMS

[SteveSandersonMS]

In my opinion this isn't the desired behavior. Handling "missing" as default(T) means we're baking in .NET's semantics about default(T) into something that's inherently cross-language. There's no reason for these .NET-specific semantics to show up in the API contract here.

My guess is it doesn't bite us very much right now because, for non-optional params, models are generally going to provide a value. Especially when producing JSON in strict conformance to the JSON schema.

However rather than being loose with this and having to support that looseness forever, I agree it would be better to throw now.

[eiriktsarpalis]

Is that the desired behavior?

From my perspective probably not. I think I had raised that issue a while back but I think we decided to pass in defaults so that invalid LLM inputs are handled more gracefully.

[eiriktsarpalis]

However rather than being loose with this and having to support that looseness forever, I agree it would be better to throw now.

100%. STJ made a similar mistake with constructor deserialization and we now need to support that mode forever.