AssemblyAI and Cartesia are competing with broader stacks for voice agents rather than single text-to-speech models, with the comparison turning on speech synthesis, speech recognition and agent orchestration, along with pricing, language coverage and published benchmark results. According to the companies' pricing pages and docs as of October 28, 2026, AssemblyAI lists AssemblyAI-TTS-1 for text-to-speech, Universal-3.6 Pro Realtime for streaming speech-to-text and LLM Gateway for model access, while Cartesia lists Sonic 3.6 for text-to-speech, Ink 2 for speech-to-text and Managed Agents as a hosted agent product billed per minute. AssemblyAI's components are designed to run inside LiveKit, Pipecat or a user's own orchestrator, while Cartesia includes hosted agent tooling. AssemblyAI says AssemblyAI-TTS-1 was trained on millions of hours of audio, including real conversational data, and offers thousands of voices across six languages: English, Spanish, French, German, Italian and Portuguese, with instant voice cloning in the dashboard, three interfaces and unlimited concurrency. Cartesia's Sonic 3.6 is described as generally available with 44 languages, and Cartesia publishes dated snapshots such as sonic-3.6-2026-08-27 alongside a moving sonic-3.6 alias. Cartesia's docs and tooling include pronunciation dictionaries, while AssemblyAI's TTS connection parameters do not include speed