Software you can't personalize is obsolete: why devtools must be open source

The author argues that with AI agents, personalizing software is now so easy that closed-source tools are at a disadvantage. They describe how agents can modify source code and automatically sync with upstream, eliminating the need for config files and plugin systems. They share a personal example of building a custom diff-review tool into their agent Shelley, and contrast open-source agents like Codex with closed-source Claude Code, urging developers to choose tools they can truly customize.