Vapi Humanness Index: Grok TTS scores 95 and MiniMax Speech 2.5 scores 93 against a real human at 100
Vapi's Humanness Index, a blind crowdsourced leaderboard where listeners pick which of two cloned voices sounds more human, places xAI's Grok TTS at 95 and MiniMax Speech 2.5 at 93 against a real human recording scored at 100 after more than 11,000 votes. Every battle uses the same cloned voice and script across models, and the real human recording is included in the comparison set, so a 95 means listeners chose Grok TTS at nearly the same rate as the human on the same quote. Vapi argues naturalness is no longer the bottleneck for voice agents, since callers experience the voice rather than the reasoning behind it.