I've also found it to keep such a constrained context window (on large codebases), that it writes a secondary block of code that already had a solution in a different area of the same file.

Nothing I do seems to fix that in its initial code writing steps. Only after it finishes, when I've asked it to go back and rewrite the changes, this time making only 2 or 3 lines of code, does it magically (or finally) find the other implementation and reuse it.

It's freakin incredible at tracing through code and figuring it out. I <3 Opus. However, it's still quite far from any kind of set-and-forget-it.