OP means “given the same input, produce the same output” determinism. This isn’t really much different from normal compilers, you might have a language spec, but at the end of the day the results are determined by the concrete compiler’s implementation.

But most LLM services on purpose introduce randomness, so you don’t get the same result for the same input you control as a user.

You can get deterministic output if just turn the temperature all the way down. The problem is that you usually get really bad results, deterministically. It turns out the randomness helps in finding solutions.