David Crawshaw (Tailscale co-founder, now building exe.dev) argues the open-source-everything era of devtools has arrived not as ideology but as a consequence of AI agents. Five years ago, custom software rarely made sense — the cost of maintaining and learning a codebase dwarfed its benefit, so we amortized customization through config files, plugin systems, and extension APIs. Agents change both sides of the ROI: the setup prompt (“download the source, build it for local use, record why in version control”) makes personalizing trivial, and the maintenance prompt — a nightly cron that fetches upstream and rebases local changes on top, checking the software still works — makes staying in sync automatic. The thesis: for personal or small-team software, the source code is the extension system. He demonstrates it by wiring his own diff-minimizing tool (meat.dev) into his agent Shelley with a single prompt, and contrasts it with Claude Code, which is closed-source — “you don’t get to personalize it.” The practical pattern for agent users: record the why of local changes in version control, rebase onto upstream nightly, and codify the workflow as a skill.

Read the full essay at exe.dev