> why can't there be an LLM that would always give the exact same output for the exact same input

LLMs are inherently deterministic, but LLM providers add randomness through “temperature” and random seeds.

Without the random seed and variable randomness (temperature setting), LLMs will always produce the same output for the same input.

Of course, the context you pass to the LLM also affects the determinism in a production system.

Theoretically, with a detailed enough spec, the LLM would produce the same output, regardless of temp/seed.

Side note: A neat trick to force more “random” output for prompts (when temperature isn’t variable enough), is to add some “noise” data to the input (i.e. off-topic data that the LLM “ignores” in it’s response).