MAGICAL×SPIRAL is not a blockbuster game. In fact, it's a game that is only known by a very small group of people who always like to delve deep into the doujin games rabbit hole and try out whatever they find interesting. This causes a major hurdle in the way of troubleshooting potential problems, as a small fanbase/playerbase means less chances to find actual help for problems that might arise. Case in point, this game and a rare Steam API error that might pop up after trying to run it.

There's a chance that the game might not start upon launching it either from the Steam client or from the executable, and instead show you this error message:



This error means that the game is unable to communicate with Steam, to verify game ownership and/or activate other related features. On the web, various troubleshooting sources lead you to allow the game through your firewall, run Steam as the administrator, or opt into the Steam beta program.
You don't need to do any of those steps. The actual solution is much more simple.

To fix the game, you need to have another Steam game installed on your system. Navigate to the other game's installation directory by right clicking on it's name in your games list, or by pressing the cog button on its client page, then selecting "Manage" and "Browse local files" afterwards.



The installation directory for that game will open. Find "steam_api.dll", copy it and paste it in MAGICALxSPIRAL's installation directory, then confirm the changes when Windows asks you to replace an existing file. Make sure you find and copy the exact file I have mentioned in this guide, as the game will still refuse to run with any other DLL file.

After you did all of the steps above, try running the game again to see the error message is gone: