[apapalns]

> codebase with hundreds of thousands of lines of code and go from 0% to 80%+ coverage in the next few weeks

I had a coworker do this with windsurf + manual driving awhile back and it was an absolute mess. Awful tests that were unmaintainable and next to useless (too much mocking, testing that the code “works the way it was written”, etc.). Writing a useful test suite is one of the most important parts of a codebase and requires careful deliberate thought. Without deep understanding of business logic (which takes time and is often lost after the initial devs move on) you’re not gonna get great tests.

To be fair to AI, we hired a “consultant” that also got us this same level of testing so it’s not like there is a high bar out there. It’s just not the kind of problem you can solve in 2 weeks.

[simonw]

I find coding agents can produce very high quality tests if and only if you give them detailed guidance and good starting examples.

Ask a coding agent to build tests for a project that has none and you're likely to get all sorts of messy mocks and tests that exercise internals when really you want them to exercise the top level public API of the project.

Give them just a few starting examples that demonstrate how to create a good testable environment without mocking and test the higher level APIs and they are much less likely to make a catastrophic mess.

You're still going to have to keep an eye on what they're doing and carefully review their work though!

[cortesoft]

> I find coding agents can produce very high quality tests if and only if you give them detailed guidance and good starting examples.

I find this to be true for all AI coding, period. When I have the problem fully solved in my head, and I write the instructions to explicitly and fully describe my solution, the code that is generated works remarkably well. If I am not sure how it should work and give more vague instructions, things don't work so well.

[Vinnl]

I feel like that leaves me with the hard part of writing tests, and only saves me the bit I can usually power through quickly because it's easy to get into a flow state for it.

[omgbear]

Left to his own devices, I found Claude liked to copy the code under test into the test files to 'remove dependencies' :/

Or would return early from playwright tests when the desired targets couldn't be found instead of failing.

But I agree that with some guidance and a better CLAUDE.md, can work well!

[throwup238]

I've think they're also much better at creating useful end to end UI tests than unit or integration tests, but unfortunately those are hard to create self contained environments for without bringing a lot of baggage and docker containers, which not all agent VMs might support yet. Getting headless QT running was a pain too, but now ChatGPT Codex can see screenshots and show them in chat (Claude Code can't show them in the chat for some reason) and it's been generating much better end to end tests than I've seen for unit/integration.

[btown]

Has anyone had success with specific prompts to avoid the agent over-indexing on implementation details? For instance, something like: "Before each test case, add a comment justifying the business case for every assumption made here, without regards to implementation details. If this cannot be made succinct, or if there is ambiguity in the business case, the test case should not be generated."

[andai]

Does it depend on the model? I would have expected the bigger ones to be better with common sense and not fixating on irrelevant details. But I have only used them with quite small codebases so far. (Which have basically no internals to exercise!)

[krschacht]

I find most human agents can only produce high quality tests if you give them detailed guidance and good starting examples. :)

[anandchowdhary]

Indeed the case - luckily my codebase had some tests already and a pretty decent CLAUDE.md file so I got results I’m happy with.

[typpilol]

I was able to do this with vitest and a ton of lint rules.

[LASR]

There is no free lunch. The amount of prompt writing to give the LLM enough context about your codebase etc is comparable to writing the tests yourself.

Code assistance tools might speed up your workflow by maybe 50% or even 100%, but it's not the geometric scaling that is commonly touted as the benefits of autonomous agentic AI.

And this is not a model capability issue that goes away with newer generations. But it's a human input problem.

[anandchowdhary]

I don't know if this is true.

For example, you can spend a few hours writing a really good set of initial tests that cover 10% of your codebase, and another few hours with an AGENTS.md that gives the LLM enough context about the rest of the codebase. But after that, there's a free* lunch because the agent can write all the other tests for you using that initial set and the context.

This also works with "here's how I created the Slack API integration, please create the Teams integration now" because it has enough to learn from, so that's free* too. This kind of pattern recognition means that prompting is O(1) but the model can do O(n) from that (I know, terrible analogy).

*Also literally becomes free as the cost of tokens approaches zero

[nl]

It depends on the problem domain.

I recently had a bunch of Claude credits so got it to write a language implementation for me. It probably took 4 hours of my time, but judging by other implementations online I'd say the average implementation time is hundreds of hours.

The fact that the model knew the language and there are existing tests I could use is a radical difference.

[id00]

I agree. It is very easy to fall in the trap: "I let AI write all the tests" and then find yourself in a situation where you have an unmaintainable mess with the only way to fix broken test within a reasonable time is to blindly accept AI to do that. Which exposes you to the similar level of risk as running any unchecked AI code - you just can't trust that it works correctly

[piker]

"My code isn't working. I know, I'll have an AI write my unit tests." Now you have two problems.

[colechristensen]

With recent experience I'm thinking the correct solution is a separate agent with prompting to exclusively be a test critic given a growing list of bad testing patterns to avoid, agent 2 gives feedback to agent 1. Separating agents into having unique jobs.

An agent does a good job fixing it's own bad ideas when it can run tests, but the biggest blocker I've been having is the agent writing bad tests and getting stuck or claiming success by lobotomizing a test. I got pretty far with myself being the test critic and that being mostly the only input the agent got after the initial prompt. I'm just betting it could be done with a second agent.

[andai]

I had a funny experience with Claude (Web) the other day.

Uploaded a Prolog interpreter in Python and asked for a JS version. It surprised my by not just giving me a code block, but actually running a bunch of commands in its little VM, setting up a npm project, it even wrote a test suite and ran it to make sure all the tests pass!

I was very impressed, then I opened the tests script and saw like 15 lines of code, which ran some random functions, did nothing to test their correctness, and just printed "Test passed!" regardless of the result.

[PunchyHamster]

Cleanroom design of "this is a function's interface, it does this and that, write tests for that function to pass" generally can get you pretty decent results.

But "throw vague prompt at AI direction" does about as well as doing same thing with an intern.

[gloosx]

Ah yes, classic "increase test coverage for the sake of increasing test coverage".

Aligns with vibe-coding values well: number go up – exec happy.

[cpursley]

Which language? I've found Claude very good at Elixir test coverage (surprisingly) but a dumpster fire with any sort JS/TS testing.