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.
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.
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.
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.
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.
