> The LLMs can write it fine. It wrote it almost acceptably on first sight

you are probably talking of some coding harness which looked up the existing code base and then made it and not something like first prompt to llm "write xyz testcase in my systems company dsl in tcl from early 2000".

> LLMs can write it fine

sure, coding by examples is fine (it goes back to "system prompt describing the language"). but the claim we are arguing is reliability. did the llm generate 100% test case or code after reading your existing codebase, likely not as you mention it "almost acceptably on first sight".

i would probably be in denial if was suggesting llms not good at finding and fitting patterns.