The README's warning says the local server is no longer actively maintained and the hosted server is at https://api.elevenlabs.io/v1/mcp with OAuth, so no API key is required for that path. The older quick start still documents a local uvx or pip install that uses ELEVENLABS_API_KEY.
Example prompts in the README cover creating a speaking agent, generating voice variations, converting a recording, making a soundscape, and transcribing speech with speaker labels. Credits are required. The README says there is a free tier with 10,000 credits a month, which is why the cost label is free. Generation can use that allowance quickly.