@schlessera The tests fail in danielbachhuber/host-check-command#4 because of this issue. The source of the problem is the same as wp-cli/scaffold-command#10

There are three paths router.php can be located at, relative to WP_CLI root:

WP-CLI as a standalone project

WP_CLI_ROOT -> ~/
router.php -> ~/vendor/wp-cli/server-command/router.php

WP-CLI as a dependency of the server-command project

WP_CLI_ROOT -> ~/vendor/wp-cli/wp-cli/
router.php -> ~/router.php

WP-CLI and server-command as dependencies of another project

WP_CLI_ROOT -> ~/vendor/wp-cli/wp-cli/
router.php -> ~/vendor/wp-cli/server-command/router.php

I still suggest the same approach as I recommended for wp-cli/scaffold-command#10 - base the relative path for router.php on the location of the command, not the location of the WP-CLI framework. This way, you don't have three different scenarios anymore, you only have:

WP_CLI_ROOT -> <base of wp-cli/wp-cli location>
router.php  -> <base of wp-cli/server-command location> . 'router.php'

The fact that they can change their relative positioning between them becomes irrelevant.