Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires phpunit/phpunit ^12.0.0 -> satisfiable by phpunit/phpunit[12.0.0, 12.0.1, 12.0.2]. - Root composer.json requires vimeo/psalm ^5.26.1 -> satisfiable by vimeo/psalm[5.26.1]. - phpunit/phpunit[12.0.0, ..., 12.0.2] require sebastian/diff ^7.0.0 -> satisfiable by sebastian/diff[7.0.0]. - Conclusion: don't install sebastian/diff 7.0.0 (conflict analysis result) Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.