alt Hacker NewsTesting works because tests are (essentially) a second, crappy implementation of your software. Tests only pass if both implementations of your software behave the same way. Usually that will only happen if the test and the code are both correct. Imagine if your code (without tests) has a 5% defect rate. And the tests have a 5% defect rate (with 100% test coverage). Then ideally, you will have a 5%^2 defect rate after fixing all the bugs. Which is 0.25%.
The price you pay for tests is that they need to be written and maintained. Writing and maintaining code is much more expensive than people think.
Or at least it used to be. Writing code with claude code is essentially free. But the defect rate has gone up. This makes TDD a better value proposition than ever.
TDD is also great because claude can fix bugs autonomously when it has a clear failing test case. A few weeks ago I used claude code and experts to write a big 300+ conformance test suite for JMAP. (JMAP is a protocol for email). For fun, I asked claude to implement a simple JMAP-only mail server in rust. Then I ran the test suite against claude's output. Something like 100 of the tests failed. Then I asked claude to fix all the bugs found by the test suite. It took about 45 minutes, but now the conformance test suite fully passes. I didn't need to prompt claude at all during that time. This style of TDD is a very human-time efficient way to work with an LLM.
Replies
This is great. The tests in this case are the spec. When you give the agent something concrete to fail against, it knows what done looks like.
The problem is if you skip that step and ask Claude to write the tests after.
I think there is a difference whether you do TDD or write tests after the fact to avoid regression. TDD can only work decently if you already know your specs very well, but not so much when you still need to figure them out, and need to build something actual to be able to figure it out.
I even addressed this in my comment as did Dijkstra