I can't code, only script; Can experienced devs make me understand why even Claude sometimes starts to fail?

Sorry if the title sounds stupid, I'm trying to word my issue as coherently as I can

So basically when the codebase starts to become very, very big, even Sonnet 3.7 (I don't use 'Thinking' mode at all, only 'normal') stops working. I give it all the logs, I give it all the files, we're talking ten of class files etc, my github project files, changelogs.md etc etc, and still, it fails.

Is there simply still a huge limit to the capacity of AI when handling complex projects consisting of 1000s of lines of code? Even if I log every single step and use git?