Default strict to true for OpenAI

[verdie-g]

When using an OpenAI provider, the strict option for the JSON schema is not set so it will only be best effort only which I feel like is not the expected behavior when using IChatClient.GetResponseAsync<T>.

It the OpenAI doc, it states

We recommend always enabling strict mode.

Would it make sense to always enable strict mode?

[stephentoub]

We previously tried and had to revert it. The OpenAI rules are too strict and end up breaking lots of consumption, especially when used with arbitrary MCP tools.

Note that it's not the default for the OpenAI clients when used directly, either: you have to opt-in. You can opt-in via IChatClient by putting "strictJsonSchema"=true into AdditionalProperties for either the ChatOptions you're using or the specific AIFunction in question.

[verdie-g]

Understood. Could you share an example of how it can break consumers? Is it related to this line

Some features of JSON schema are not supported. (See supported schemas.)

[stephentoub]

Is it related to this line

Yes. Only a subset of JSON schema is supported; things outside of that fail the request.

[ouvreboite]

@verdie-g I think there are also those requirements that are specific to strict mode:

Under the hood, strict mode works by leveraging our structured outputs feature and therefore introduces a couple requirements:

• additionalProperties must be set to false for each object in the parameters.
• All fields in properties must be marked as required.

You can denote optional fields by adding null as a type option