The initial mock4net version relies on Responses starting point to create checks.

When I read it, I assume this is an enum type (due the plural form). Following this conception, it implies that syntax would be:

Responses.Success.WithCode(xxx);

where success happens to be a specific value/instance of response.

Alternatively, the check could take the form of a verb, using Responds instead

Responds.WithSuccess().AndCode(xxx)

Maybe I am going to far in my analysis of this, but tt is still appropriate time to refine wording, and I feel choosing the best wording is instrumental in the success of a library.