A customer asked us a normal question. They got a reply that looked like a form letter.

It was polite. It was fast. It was completely useless. And nobody got an alert, because nothing failed in a way that looked like failing.

That is the kind of bug I want to talk about.

What we thought was happening

We have an internal helper that answers support mail. The idea is simple. Take the incoming question, add our own notes about the product, ask a model, send the answer.

The notes are not huge. About a page of text. We appended them to every prompt because it was easy, and because the model seemed happier with context.

For a while it worked. Then replies started degrading in a way nobody could name. Less specific. More filler. Shorter.

We blamed the model at first. Then we blamed the prompt. Both were wrong.

What was actually happening

Our consulting endpoint validates the incoming message length. There is a cap: 3000 characters.…