Improve error for structured output backend selection

[hmellor]

cc @russellb @DarkLight1337

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[russellb]

Thanks for improving the message!

Who is the log message for though? The person setting the request-level backend is going to see the exception, either in a Python program using vllm as a library, or in the HTTP response from the API.

[hmellor]

Who is the log message for though? The person setting the request-level backend is going to see the exception, either in a Python program using vllm as a library, or in the HTTP response from the API.

My thinking was:

• A library user who sets the backend in the request will see the warning telling them to not do that
• A library user who sets the backend to something different to the engine backend will see the warning and the error
• A HTTP user who sets the backend to something different to the engine backend will see the error

[russellb]

Who is the log message for though? The person setting the request-level backend is going to see the exception, either in a Python program using vllm as a library, or in the HTTP response from the API.

My thinking was:

• A library user who sets the backend in the request will see the warning telling them to not do that
• A library user who sets the backend to something different to the engine backend will see the warning and the error
• A HTTP user who sets the backend to something different to the engine backend will see the error

Won't all of these cases see the exception (either as the exception, or the HTTP error response) ?

[hmellor]

Won't all of these cases see the exception (either as the exception, or the HTTP error response) ?

I thought that if a user set the request structured output backend to the same as the engine structured output backend, they wouldn't see the error?

edit: from offline discussion we decided that the warning only helps offline users and makes server logs super noisy when the problem doesn't concern the server admin. I've removed the warning.

[russellb]

thanks!