Most any LLM has a "temperature" setting, a set of randomness added to the otherwise fixed weights to intentionally cause exactly this nondeterministic behavior

Most any LLM has a "temperature" setting, a set of randomness added to the otherwise fixed weights to intentionally cause exactly this nondeterministic behavior. Good for creative tasks, bad for repeatability. If you're running one of the open models, set the temperature down to 0 and it suddenly becomes perfectly consistent.