Speech models split into two lines. Whisper led ASR: its MIT-licensed 1.55B model drove transcription cost toward zero in September 2022; after v2, v3, and turbo cut the decoder from 32 layers to four, OpenAI moved to closed gpt-4o-transcribe. In TTS, ElevenLabs grew to an $11B valuation and $500M ARR, while Kokoro (82M, Apache-2.0) and Chatterbox preserved self-hosting. Speech-to-speech Realtime APIs are now rewriting live conversation.
Cartesia's core is Sonic real-time TTS, Ink STT, and streaming inference. Although it offers the Line voice-agent platform in 2026, buyers must still separate the model layer from telephony orchestration and design consent, retention, and fallback for cloned voices.
Week 8 sends text through TTS and back through STT, requiring error classification, formatting-loss analysis, and accent stress tests, while Lab 4 prepares Git collaboration for the team agent project.
Deepgram combines streaming STT, LLM orchestration, turn detection, barge-in, and streaming TTS over one WebSocket while preserving paths for standalone speech models and bring-your-own LLM or TTS.
ElevenLabs has expanded from a TTS vendor into the ElevenAgents platform: Scribe Realtime listens, Flash speaks, and the platform connects the LLM, turn-taking, tools, and telephony. The key choice is whether you need a voice model or the whole agent control plane.
Vapi connects phone and web audio, STT, LLMs, TTS, tool calls, and call observability in a managed voice runtime. Providers are swappable, but Vapi's realtime orchestration is not portable. In May 2026, the company reported one million developers and announced a $50 million Series B.