Thank you for the direct actionable feedback, we will improve that messaging.

Regarding debugging your specific problem, when an API is attempted to be added, the local process attempts a 1-token request to the cheapest model with the GPT platform (in your case, gpt-4o-mini on OpenAI) to verify that the key works. Though, if the account has no balance, this request may fail even though it costs a fraction of a fraction of a penny (though anything that fails that request will cause the API key to be considered invalid).