Ideally you have an MCP server (Model Context Protocol) that talks to your game. It can use existing API if it's exposed - but it's very rare if it's not a game you're developing/is modded. It could also be reversed engineered with packets (if it's an online game), web sockets, memory editing, dll injection, or OCR and input manipulation if everything else fails.

If you don't have an MCP server the AI agent might try to figure out how to talk to the game using the above ideas. But at this point you might as well ask it to help you write one.