I built a next-generation forum with Claude Fable 5.1, and now Claude writes its plugins

Sharing a Claude project that turned out better than I expected: Flatbb, a flat, lightweight forum built with Claude Fable 5.1.

Why a new forum at all? The two big open-source options both come with baggage. Discourse needs Ruby, Docker, Redis, Postgres and a beefy server before it shows a single topic. Flarum is PHP but still needs Composer and a command line, and extensions tend to break on every major upgrade. Neither runs on the cheap shared hosting most small communities actually have.

What Fable 5.1 produced instead: plain PHP 8 with no framework, no Composer and no build step. Unzip a folder, open the URL, and the installer sets up SQLite or MySQL. The whole core is small plain functions, with rules that keep it that way (no file over 30 KB, no queries in loops, central CSRF, a static check that fails the build on any unescaped output). Fable was noticeably good at holding those constraints across a large codebase, and at portable SQL that runs identically on SQLite and MySQL 5.7.

The part that matters for this community: plugins are written by Claude. A plugin is one folder with one manifest, and the entire plugin API (hooks, regions, helpers, database rules) is documented in a single text file. The workflow is: paste https://www.flatbb.com/dev/plugins.md into Claude Code, describe the plugin, upload the folder it gives you in the admin panel. Every bundled plugin was built this way, and there is a marketplace where anyone can publish theirs. On the marketplace side, submissions get an automatic quality review that is also Claude reading the source against the rules.

Free and MIT licensed: Site: https://www.flatbb.com Source: https://github.com/nwnuyhs/flatbb Plugin guide for AI assistants: https://www.flatbb.com/dev/plugins

Happy to answer questions about the prompts, the constraints that worked, and what Fable still got wrong.