Evals are the actual product.
A demo proves an agent can work once. An eval suite proves it keeps working. Only one of those is safe to put in front of customers.
The most dangerous moment in an agent project is the first great demo. It feels like the finish line. It's actually the start: you've shown the thing can work once, under conditions you controlled, with an audience rooting for it. Production is none of those.
What a demo hides
A demo is a single sample. It can't tell you how often the agent is right, where it fails, or whether a prompt tweak made things better or quietly worse. The instant you change a model, a prompt, or a tool, the demo is meaningless and you're back to vibes.
An eval suite turns all of that into a number you can move. It's the difference between "I think this is better" and "this is three points better on the cases we care about."
Build the suite first
- Collect real inputs from day one; every run is a future test case.
- Write down what "correct" means before you optimize anything.
- Score every change against the suite, not against the last good demo.
- Treat a regression in evals like a failing test: it blocks the ship.
The reframe
The agent is the part that's fun to build. The eval suite is the part that lets you ship it, change it, and sleep. When a project stalls, it's almost never the agent that's missing; it's the way to know whether the agent is good. Build that first.