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Head to head · Voice agents · October 2026 research run

AssemblyAI Voice Agent API vs OpenAI Realtime API

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against OpenAI Realtime API's 63.3 (B), and leads in 3 of 7 scored categories. OpenAI Realtime API leads on reliability and transparency & trust. Both do voice agents.

Best conversational voice-agent APIs · All 91 voice agents comparisons

AssemblyAI Voice Agent APIAssemblyAI, Inc. · HTTP API B64.4/100#351 of 950 OpenAI Realtime APIOpenAI · HTTP API B63.3/100#400 of 950

Which one, for what

AssemblyAI Voice Agent API B

Good for A developer who wants one managed pipeline at a flat rate and is content with AssemblyAI's own speech models and voices, with an optional model of their own.

Ahead on

  • Payments & pricing, 40 against 20
  • Maintenance & community, 64 against 52

Also in its favour

  • Free to start without a card

Watch for

No number is published for the concurrent-session limit that the concurrency_exceeded error enforces

OpenAI Realtime API B

Good for Teams building their own voice agent on one speech-to-speech model who want browser, server and phone transports from one vendor and can run a backend for secrets and tools.

Ahead on

  • Reliability, 67 against 60
  • Transparency & trust, 71 against 66

Watch for

openai.com answered 403 to our reader, so the service terms, privacy policy, sub-processor list and security pages were not read.

Score by category

CategoryWeight this runAssemblyAI Voice Agent APIOpenAI Realtime APIEdge
Reliability16%206067OpenAI Realtime API +7
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28384OpenAI Realtime API +1
Agent ergonomics13%16.27276OpenAI Realtime API +4
Security & auth14%17.56261AssemblyAI Voice Agent API +1
Payments & pricing10%12.54020AssemblyAI Voice Agent API +20
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.86452AssemblyAI Voice Agent API +12
Transparency & trust7%8.86671OpenAI Realtime API +5
Negative events≤1500
Total64.4 · B63.3 · B

Facts side by side

FactAssemblyAI Voice Agent APIOpenAI Realtime API
KindHTTP APIHTTP API
VendorAssemblyAI, Inc.OpenAI
Hosted endpointwss://agents.assemblyai.com/v1/wshttps://api.openai.com/v1/realtime
TransportsHTTPHTTP
AuthAPI keyAPI key
PricingPay per usePay per use
Price for voice agents$0.075 per minute of callnot published
x402nono
LicenceProprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence fileProprietary service. The service terms on openai.com were not read. The Agents SDK for TypeScript and the OpenAPI description are MIT
Read-only variant documentednono
llms.txtyesyes
Last release2026-08-262026-07-28
Terms last updated2026-07-01couldn't be read
Privacy policy last updated2026-05-26couldn't be read
Customer content may train modelsyes, with an opt-outcouldn't be read
Terms restrict automated accessyescouldn't be read
Terms restrict benchmarkingyescouldn't be read
Terms or service can change without noticenot found in the textcouldn't be read
Arbitration or class-action waivernot found in the textcouldn't be read

Verdicts

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.

OpenAI Realtime API

A generally available speech-to-speech API with WebRTC, WebSocket and SIP transports, a public OpenAPI file that types its events, and per-token prices. Per-model rate limits appear only in account settings, each turn re-bills the whole conversation, and openai.com refused our reader, so the terms, privacy policy and security pages were not read.

Before you call either

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

OpenAI Realtime API

  1. Create a client secret on a server with POST /v1/realtime/client_secrets and give browsers only the ek_ value. Never ship the API key.
  2. After a response with MCP calls finishes, send another response.create. The API does not create the follow-up response itself.
  3. Set truncation with a retention_ratio below 1 and token_limits.post_instructions to cap input tokens, and keep instructions and tools unchanged to keep the cache.
  4. On a WebSocket, stop playback on input_audio_buffer.speech_started and send conversation.item.truncate with audio_end_ms. WebRTC and SIP truncate on the server.
  5. Use gpt-realtime-2.1 or gpt-realtime-2.1-mini. gpt-realtime and gpt-realtime-mini shut down on 20 January 2027.

Questions

Which is better for AI agents, AssemblyAI Voice Agent API or OpenAI Realtime API?

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against OpenAI Realtime API's 63.3 (B), and leads in 3 of 7 scored categories. OpenAI Realtime API leads on reliability and transparency & trust.

Do AssemblyAI Voice Agent API and OpenAI Realtime API need an API key?

Both need an API key.

Can an agent call AssemblyAI Voice Agent API and OpenAI Realtime API without installing anything?

Yes. AssemblyAI Voice Agent API has a hosted endpoint at wss://agents.assemblyai.com/v1/ws and OpenAI Realtime API at https://api.openai.com/v1/realtime.

Other comparisons with AssemblyAI Voice Agent API or OpenAI Realtime API

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