How to improve code quality of Claude Code and codex (on 2026-05)

I'm using both claude code (opus-4.7) and codex (gpt-5.5). The agents are perfectly capable of delivering most features hands free these days, but the code quality is still miserable without another few rounds of prompt. And it's worse for personal vibe project without well-written history code files.

Simple example would be a poor extension of existing util methods, agent always duplicate and create new method instead of think about gracefully extending existing methods.

How are you guys solving this issue? A carefully maintained claude.md imo is only a mitigation, not a scalable solution.