I was able to realize huge productivity gains working on a 20 years old codebase with 2+ million loc, as I mentioned in the sister post. So I disagree that big productivity gains are only on greenfield projects. Realizing productivity gains on mature code based requires more skill and upfront setup. You need to put some work in your claude.md and give Claude tools for accessing necessary data, logs, build process. It should be able to test your code autonomously as much as possible. In my experience, people who say they are not able to realize productivity gains don't put enough effort to understand these new tools and setup them properly for their project.

You should write a blog post on this! We need more discussion of how to get traction on mature codebases and less of the youtube influencers making toy greenfield apps. Of course at a high level it's all going to be "give the model the right context" (in Claude.md etc.) but the devil is in the details.