Claude suggested editing vendor/magento, why is that a hard no?

Three reasons. (1) Composer wipes vendor/ on every composer install, your edit vanishes silently in CI. (2) Adobe's security patches replace the file on the next composer update, taking your fix with it but also reintroducing whatever bug you fixed. (3) Marketplace tech-review hard-fails any module that ships diffs against vendor. The right pattern: copy the file's logic into a plugin (around if you need to short-circuit, after for mutation), or a preference if you really must replace. Add a CI guard: git diff --name-only origin/main HEAD | grep '^vendor/' && exit 1.