Not really, you can add a prompt to commercial LLM products to alter the persona a bit but the actual system prompt the harness is using is opaque to you. Your Claudes, Kimis, Mistrals and what have you are a polished product on top of the model, they’re not the model itself. A lot of the actual day-to-day software engineering of these things is writing a harness that’s robust enough to deal with a cantankerous non-deterministic pile of linear algebra that doesn’t know the difference between ‘looks good’ and ‘is good’.