I removed the LLM call and replaced it with 200 lines of template code

The feature was a letter generator. Somebody fills in a few fields and gets a finished letter of recommendation, resignation letter or notice letter, in plain text, ready to paste into an email. The obvious build is a prompt and a model call. I wrote the deterministic version instead: a pure function, about two hundred lines, no network, no key, no tokens. I want to lay out the reasoning, because "just call a model" is the default now and the default is not always right. The three reasons, in order of weight 1. The output is short and the shape is fixed. A recommendation letter is a date block, a greeting, three or four paragraphs, a sign off and a name. There is no structural variation to discover. Generation is valuable when the space of good outputs is large and you cannot enumerate it. Here the space is small enough to write down, and once you have written it down the model is doing an expensive approximation of a switch statement. 2. It is a legal-adjacent document. Not legal advice, but it goes into an employment record. A resignation letter that invents a notice period, or a reference that invents a fact about a person, is a real problem for the person who sent it. Templates cannot hallucinate. Everything specific in the output either came from a form field or is a sentence I wrote and can be held to. 3. Zero marginal cost changes what the product can be. This is the one that actually decided it. A model call costs money per use, and anything that costs money per use needs an account, a rate limit and eventually a card. A pure function costs nothing, so the tool can stay open with no signup, forever, without a business case. That is a product decision expressed as an architecture decision, and it only works if the code path is free. What the code looks like The whole engine is one exported function over one input type. export type LetterKind = ' resignation ' | ' notice ' | ' recommendation ' ; export type LetterTone = ' formal ' | ' warm ' | ' brief ' ; expo