The large US technology company builds systems where LLM calls are in the critical path. These systems are hard to test in the traditional sense — the outputs are probabilistic, the failure modes are subtle, and the “correct” answer for many queries doesn’t have a binary definition. After two years of building and evaluating LLM-integrated systems, here’s what actually works. ...
The first LLM feature I shipped was embarrassingly under-tested. I prompted the model, looked at a few outputs, thought “that looks right,” and deployed it. Users found failure modes within hours that I hadn’t imagined, much less tested for. This isn’t unusual. LLM applications have a testing problem that’s distinct from traditional software testing: the output space is too large to enumerate, the failure modes are semantic rather than syntactic, and “correctness” is often subjective. The standard response — “it’s hard, so test less” — produces unreliable products. Here’s what a functional evaluation framework looks like. ...