Why am I unable to use the Agent with an API Key in Cursor?

I am trying to use the Agent feature with an API key in Cursor, but I am encountering the following error message:

“Agent and Edit rely on custom models that cannot be billed to an API key. Please use a Pro or Business subscription and/or disable API keys. Ask should still work. (Request ID: e0f675cb-ad65-4c5a-a7e1-3f0d97d4548f)”

Unfortunately, I cannot afford a Pro or Business subscription. I would like to know why this error occurs and if there’s a way to use a free Google API key instead, as I believe Google offers a free tier. Could someone please explain the limitations of using an API key with the Agent feature and guide me on how to configure or disable API keys to work with a free Google API key? Any help or suggestions would be greatly appreciated!

Thank you.