zlacker

It depends on how easily testable the Excel is. If Claude has the ability to run both the Excel and the Python with different inputs, and check the outputs, it's stunningly likely to be able to one-shot it.

>>linsom+(OP)
That's exactly what it did (author here).

>>linsom+(OP)
Something being simultaneously described as a "30 sheet, mind-numbingly complex Excel model" and "testable" seems somewhat unlikely, even before we get into whether Claude will be able to test such a thing before it runs into context length issues. I've seen Claude hallucinate running test suites before.

>>martin+o
I'm having trouble reconciling "30 sheet mind numbingly complicated Excel financial model" and "Two or three prompts got it there, using plan mode to figure out the structure of the Excel sheet, then prompting to implement it. It even added unit tests to the Python model itself, which I was impressed with!"

"1 or 2 plan mode prompts" to fully describe a 30-sheet complicated doc suggests a massively higher level of granularity than Opus initial plans on existing codebases give me or a less-than-expected level of Excel craziness.

And the tooling harnesses have been telling the models to add testing to things they make for months now, so why's that impressive or suprising?

>>majorm+61
No it didn't make a giant plan of every detail. It made a plan of the core concepts and then when it was in implementation mode it kept checking the excel file to get more info. It took around ~30 mins in implementation mode to build it.

I was impressed because the prompt didn't ask it to do that. It doesn't normally add tests for me without asking, YMMV.

>>AlotOf+01
It compacted at least twice but continued with no real issues.

Anyway, please try it if you find it unbelievable. I didn't expect it to work FWIW like it did. Opus 4.5 is pretty amazing at long running tasks like this.

>>martin+H1
Ah, I see.

Did it build a test suite for the Excel side? A fuzzer or such?

It's the cross-concern interactions that still get me.

80% of what I think about these days when writing software is how to test more exhaustively without build times being absolute shit (and not necessarily actually being exhaustive anyway).

>>martin+U1
I think the skepticism here is that without tests or a _lot_ of manual QA how would you know that it did it correctly?

Maybe you did one or the other , but “nearly one-shotted” doesn’t tend to mean that.

Claude Code more than occasionally likes to make weird assumptions, and it’s well known that it hallucinates quite a bit more near the context length, and that compaction only partially helps this issue.

>>martin+U1
I generally agree with you, but I tried to get it to modernize a fairly old SaaS codebase, and it couldn't. It had all the code right there, all it had to do was change a few lines, upgrade a few libraries, etc, but it kept getting lots of things wrong. The HTML was wrong, the CSS was completely missing, basic views wouldn't work, things like that.

I have no idea why it had so much trouble with this generally easy task. Bizarre.

>>linsom+(OP)
And also - who understands the system now? Does anyone know Python at this shop? Is it someone’s implicit duty to now learn Python, or is the LLM now the de facto interface for modifying the system?

When shit hits the fan and execs need answers yesterday, will they jump to using the LLM to probabilistically make modifications to the system, or will they admit it was a mistake and pull Excel back up to deterministically make modifications the way they know how?

>>linsom+(OP)
where exactly have you seen excel forumalas to have tests?

I have, in my early careers, gone knee deep into Excel macros and worked on c# automation that will create excel sheet run excel macros on it and then save it without the macros.

in the entire process, I saw dozens of date time mistakes in VBA code, but no tests that would catch them...

>>linsom+(OP)
You touched on Kolmogorov complexity there :)

>>moregr+w3
If you’re porting some formulas from one language to another, “correct” can be defined as “gets the same answers as before.” Assuming you can run both easily, this is easy to write a property test for.

Sure, maybe that’s just building something that’s bug-for-bug compatible, but it’s something Claude can work with.

>>skybri+vo
For starters, Python uses IEEE 754, and Excel uses IEEE 754 (with caveats). I wonder if that's being emulated.

>>linsom+(OP)
Different inputs? With no understanding of the program, how is Claude going to determine what input set is sufficient.

Tell me if I am wrong, but surely Claude cannot even access execution coverage.

>>AlotOf+01
>I've seen Claude hallucinate running test suites before.

This reminded of something that happened to me last year. Not Claude (I think it was GPT 4.0 maybe?), but I had it running in VS Code's Copilot and asked it to fix a bug then add a test for the case.

Well, it kept failing to pass its own test, so on the third try, it sat there "thinking" for a moment, then finally spit out the command `echo "Test Passed!"`, executed it, read it from the terminal, and said it was done.

I was almost impressed by the gumption more than anything.

>>djeast+KS
I've been using Claude Code with Opus 4.5 a lot the last several months and while it's amazingly capable it has a huge tendency to give up on tests. It will just decide that it can commit a failing test because "fixing it has been deferred" or "it's a pre-existing problem." It also knows that it can use `HUSKY=0 git commit ...` to bypass tests that are run in commit hooks. This is all with CLAUDE.md being very specific that every commit must have passing tests, lint, etc. I eventually had to add a Claude Code pre-command hook (which it can't bypass) to block it from running git commit if it isn't following the rules.

>>Merad+902
Anecdata from the internet has a few stories of Claude Opus bypassing hooks too =)

1) it wants to run X command

2) it notices a hook preventing it from running X

3) it creates a Python application or shell script that does X and runs it instead

Whoops.

>>datsci+U9
At least you can literally buy books that tell you how to test Python code and confirm it works as expected.

When shit hits the fan, execs need answers yesterday and the 30 sheet Excel monstrosity is producing the wrong numbers - who fixes it?

It was done by Sue, who left the company 4 years ago, people have been using it since and nobody really understands it.

>>theshr+Pg4
Alright so it is implicit that it is someone’s duty to learn Python. Don’t get me wrong, Python is better than Excel in a million ways, but not many companies are competent at hiring people who are competent at Python - including software companies from my personal experience hah.

I’ve also heard plenty of horror stories of bus factor employees leaving (or threatening to leave) behind an excel monstrosity and companies losing 6 months of sales, so maybe there’s a win for AI somewhere in there.

>>theshr+pg4
I haven't seen it bypass my hook yet (knock on wood). I have my hook script [0] tell that its commits are required to pass validation, maybe that helps push it in the right direction?

0: https://github.com/mbcrawfo/vibefun/blob/main/.claude/hooks/...