LLMs have default personalities - shaped by RLHF and other post-training methods. There is a lot of variance to it, but variance from one LLM to another is much higher than that within the same LLM.
If you want an LLM to retain the same default personality, you pretty much have to use an open weights model. That's the only way to be sure it wouldn't be deprecated or updated without your knowledge.
If you want an LLM to retain the same default personality, you pretty much have to use an open weights model. That's the only way to be sure it wouldn't be deprecated or updated without your knowledge.