Function param 

pub fn param(
    target: &dyn ReplayTarget,
    request: &CompletionRequest,
    key: &str,
) -> Option<Value>

The value top-level key gets from the mapped options, the provider options and additional_params, merged as request_params merges them, with no base. For a reader that runs before encoding and must agree with the body. Read only, so it cannot rank anything, and it reports nothing: a refused option adds nothing here. The one sanctioned reader of these layers on the completion path outside the merge. A wire’s map_options must not call it, since it calls map_options.