Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires infection/infection ^0.29.0 -> satisfiable by infection/infection[0.29.0, ..., 0.29.10]. - Root composer.json requires vimeo/psalm ^5.17.0 -> satisfiable by vimeo/psalm[5.17.0, ..., 5.26.1]. - infection/infection 0.29.10 requires nikic/php-parser ^5.3 -> satisfiable by nikic/php-parser[v5.3.0, v5.3.1, v5.4.0]. - infection/infection[0.29.0, ..., 0.29.8] require nikic/php-parser ^5.0 -> satisfiable by nikic/php-parser[v5.0.0, ..., v5.4.0]. - infection/infection 0.29.9 requires thecodingmachine/safe dev-master as 2.5.0 -> found thecodingmachine/safe[dev-master] but it does not match your minimum-stability. - Conclusion: don't install nikic/php-parser v5.4.0 (conflict analysis result) Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.