Devtools Must Be Open Source: Agents Have Made Personalizing Software Cheap

David Crawshaw, an engineer at exe.dev, published a post titled “Devtools must be open source” on 2 August 2026, arguing that AI agents have made personalizing software so cheap that access to source code is becoming essential.

From config files to personal software

Crawshaw recalls that five years ago most engineers he spoke to had no programs they had written for themselves: all day they used programs written by others in order to write programs for others. Writing and, above all, maintaining software for yourself had a questionable return — there was only so much code one could write in a day, and coming back to a project after a year to maintain it was painful; some years he threw his custom software away and used the most standard environments available. That economics is why complex software shipped with configuration files and plugin and extension systems: designing for many users justified the cost.

Two prompts that change the calculus

Today, he writes, personalizing software is astonishingly easy. Two kinds of prompts do most of the work: first, download the source for a piece of software, build it for local use and modify it, telling the agent’s memory that future changes mean editing the sources and replacing the current version, and recording the motivation for the change in version control. Second, and more importantly, set up a nightly cron job that fetches upstream changes, rebases the local modifications on top of them, checks that the software still works, and replaces the current version. Agents, he notes, can manage synchronization with upstream releases — so the return on customization improves on two fronts at once: it is much easier to start personalizing, and much easier to keep going. Because those two prompts can live inside an agent as a skill, no programming is required; exe.dev built this into their own agent, Shelley, so typing “make Shelley’s UI high-contrast” is enough to personalize it.

The source code is the extension system

As a worked example, Crawshaw describes building his diff-minimizing tool, meat.dev, into Shelley with a single prompt — including pre-processing commits in the background and a toggle in the Diffs view. Trying to do the same through the VS Code extensions API, he writes, would be “convoluted misery”, because the points of extension are the wrong shape. What such personalization requires, he argues, is access to the source code — his own blog is bespoke software for exactly that reason.

Where Codex and Claude Code diverge

The skill-based approach works trivially with other open-source agents such as Pi; Crawshaw wonders why Pi needs a built-in extension system at all, since “the source code is the extension system”. The same could be done to Codex, which is open source, though it would require many more tokens. The wall is Claude Code: it is closed-source, so users cannot personalize it, only use the customization hooks that ship with it. His advice is blunt — if your desired way of working does not fit those hooks, switch to an agent that lets you personalize it.