>the LLM constantly guesses arguments or dictionary formats wrong [...] it's better to avoid that whole class of runtime errors altogether.

Use Mypy in strict mode and run it in the post-turn hook of your LLM harness so the LLM has no choice but to obey it. And don't use overly general dictionary types when the keys are known at development time; use TypedDicts for annotations if you must use dicts at runtime.