AssemblyAI Speech-to-Text (Universal) vs Speechmatics Speech-to-Text

AssemblyAI Speech-to-Text (Universal)

OpenAPI 3.1 file with typed inputs and error responses on every operation. Two outages of an hour or more in the last 90 days, on 31 July and 16 September 2026.

Speechmatics Speech-to-Text

Training is opt-in and real-time audio is not stored. Enhanced transcription costs $0.40 to $0.43 an hour.

AssemblyAI Speech-to-Text (Universal)

1. Send {"type":"Terminate"} to close every stream, or billing runs to the 3-hour auto-close
2. Use speech_models (plural). The singular speech_model now returns 400 for current model names
3. Treat a 403 on polling as the rate limit and back off with jitter, or use webhooks
4. Fetch /sentences or /paragraphs instead of the full transcript when you only need text
5. Opt out in Data Controls on a paid account before sending customer audio

Speechmatics Speech-to-Text

1. Set "model": "enhanced" explicitly. The default is standard
2. Use notifications instead of polling. Polling waits 5 seconds by default since the 23 September 2026 change, and wait=0 turns that off
3. Fetch batch transcripts within 7 days. After that the API returns 404 expired
4. Pass a fetch_data URL for files over 1 GB
5. Use /v2/agent with linden-1 for live agents instead of the plain realtime path