AssemblyAI Voice Agent API vs Deepgram Voice Agent API

AssemblyAI Voice Agent API

One $0.075 a minute rate covers speech-to-text, the managed model, speech output, recordings and hosting, and the socket has an AsyncAPI file, typed error codes and a 30-second resume window. No concurrent-session limit is published, the status page has no Voice Agent component, and phone calls are inbound only through a Twilio trunk the customer owns.

Deepgram Voice Agent API

$0.075 a minute all in on Standard, $0.050 with your own LLM and TTS, $200 of credit with no card. No phone numbers or SIP, so you bridge Twilio or another carrier yourself.

AssemblyAI Voice Agent API

1. Send session.end before closing the socket. Without it the session stays resumable, and billed, for 30 seconds
2. Retry only at_capacity, concurrency_exceeded and internal_error, with backoff. Every other session error code is fatal
3. Send tool.result inside the reply.done handler, not straight after tool.call
4. Mint a one-time token with GET /v1/token for browser clients and never ship the API key to a page
5. Deduplicate webhooks on event_id, and verify X-AAI-Signature against the raw body bytes because a 401 is not retried

Deepgram Voice Agent API

1. Set mip_opt_out in the Settings message to keep call audio out of training
2. Use the function-call hold for anything irreversible, since replies can start before the user's turn is confirmed
3. Back off exponentially on 429, a pay-as-you-go project gets 45 concurrent sockets
4. Pin the LLM version, an unpinned Gemini route failed for 1.5 hours on 21 July 2026
5. Mint browser tokens with ttl_seconds, not ttl, or they expire after 30 seconds