Steam sets STEAM_COMPAT_DATA_PATH to compatdata/<appid> when launching a non-Steam shortcut. The prefix was named 'kyber' so Steam was launching into an empty prefix, causing immediate exit. Change KYBER_COMPAT_ID to match KYBER_APPID (9900000001) and migrate any existing 'kyber' prefix. Also handle cmd.exe being a symlink: resolve and copy the real file as cmd-real.exe, then remove the symlink before writing the shim so we only affect the Kyber prefix and not Proton globally. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01D8ckUJQtj1pH8jtAddBDZs