The kernel of the CMS: content, users, abilities, the admin and everything plugins build on. A site doesn't install this directly: start from the letterncms/lettern project, or the zip.

composer create-project letterncms/lettern my-site

Working on it

You need PHP 8.4 and Composer:

git clone https://github.com/letterncms/cms
cd cms
composer install
composer test

Tests that turn on a plugin skip themselves here. The admin takes Node 22 and pnpm:

pnpm install
pnpm build
pnpm typecheck
pnpm test
pnpm test:browser   # the admin in headless Chrome against fixtures (needs the plugins next to it, built; CHROME=path)

To try a change in a real site, build the admin, then make a site next to your clone and point it there:

composer create-project letterncms/lettern ../sandbox
cd ../sandbox
composer config repositories.cms path ../cms
composer update letterncms/cms
php cms install
php cms serve