I work with multiple monoliths that span anywhere from 100k to 500k lines of code, in a non-mainstream language (Elixir). Opus 4.5 crushes everything I throw at it: complex bugs, extending existing features, adding new features in a way that matches conventions, refactors, migrations... The only time it struggles is if my instructions are unclear or incomplete. For example if I ask it to fix a bug but don't specify that such-and-such should continue to work the way it does due to an undocumented business requirement, Opus might mess that up. But I consider that normal because a human developer would also do fail at it.

Or a Rails app.