Why model flexibility matters in voice: Speechify Simba 3.2 and Fish Audio S2.1-Pro top Vapi's Humanness Index

Speechify's Simba 3.2 debuted atop Vapi's Humanness Index at 99, one point under the human baseline of 100, and Fish Audio's S2.1-Pro entered at 94 with 141 ms time to first audio. The prior leader, xAI's Grok TTS, held the top spot for only three weeks at 95, and the leading score has since moved from 95 to 99. Vapi argues that model-agnostic voice platforms let teams swap TTS models as rankings shift.