Powerful AI Is Forcing Us To Write Good Code

For decades, we’ve all known what “good code” looks like. Thorough tests. Clear documentation. Small, well-scoped modules. Static typing. Dev environments you can spin up without a minor religious ritual.

These things were always optional, and time pressure usually meant optional got cut.

Agents need these optional things though. They aren’t great at making a mess and cleaning it up later. Agents will happily be the Roomba that rolls over dog poop and drags it all over your house.

The only guardrails are the ones you set and enforce. If the agentic context is lacking and the guardrails aren’t sufficient, you’ll find yourself in a world of pain1. But if the guardrails are solid, the LLM can bounce around tirelessly until the only path out is the correct one.

Our six-person team has made a lot of specific and, sometimes, controversial investments to accommodate our agentic coders. Let’s talk about some of the less obvious ones.

The most controversial guideline we have is our most valuable: We require 100% code coverage2.

Everyone is skeptical when they hear this until they live with it for a day. It feels like a secret weapon at times.

Coverage, as we use it, isn’t strictly about bug prevention; it’s about guaranteeing the agent has double-checked the behavior of every line of code it wrote.

The usual misinterpretation is that people think we believe 100% coverage means “no bugs”. Or that we’re chasing a metric, and metrics get gamed. Neither of those are the case here.

Why 100%? At 95% coverage, you’re still making decisions about what’s “important enough” to test. At 99.99%, you don’t know if that uncovered line in ./src/foo.ts was there before you started work on the new feature. At 100%, there’s a phase change and all of that ambiguity goes away3. If a line isn’t covered, it’s because of something you actively just did.The coverage report becomes a simple todo list of tests you still need to write. It’s also one less degree of freedom we have to give to the agent to reason about.

Source: HackerNews