Hi @benjismith ,
Sorry for the late reply.
Currently the only way you can do that is by using
 "inputs":"Once upon a time, there was a horrible witch who",
          "options":{"wait_for_model":true}
          "parameters": {"max_length": 10}

but that IS an issue because you need to know how many tokens your prompt is to be precise.
We’re going to add a better parameter for this and document it.