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Head to head · Voice agents · October 2026 research run

OpenAI Realtime API vs Ultravox Realtime API

OpenAI Realtime API scores 63.3 (B) on agent readiness against Ultravox Realtime API's 58.5 (C), and leads in 4 of 7 scored categories. Ultravox Realtime API leads on payments & pricing. Both do voice agents.

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

OpenAI Realtime APIOpenAI · HTTP API B63.3/100#400 of 950 Ultravox Realtime APIUltravox (Fixie.ai) · HTTP API C58.5/100#576 of 950

Which one, for what

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 52
  • Agent ergonomics, 76 against 69
  • Security & auth, 61 against 46
  • Maintenance & community, 52 against 43

Watch for

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

Ultravox Realtime API C

Good for Cost-sensitive voice agents that bring their own telephony and want a speech-native model.

Ahead on

  • Payments & pricing, 40 against 20

Also in its favour

  • Free to start without a card

Watch for

No MCP server

Score by category

CategoryWeight this runOpenAI Realtime APIUltravox Realtime APIEdge
Reliability16%206752OpenAI Realtime API +15
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28484even
Agent ergonomics13%16.27669OpenAI Realtime API +7
Security & auth14%17.56146OpenAI Realtime API +15
Payments & pricing10%12.52040Ultravox Realtime API +20
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.85243OpenAI Realtime API +9
Transparency & trust7%8.87173Ultravox Realtime API +2
Negative events≤1500
Total63.3 · B58.5 · C

Facts side by side

FactOpenAI Realtime APIUltravox Realtime API
KindHTTP APIHTTP API
VendorOpenAIUltravox (Fixie.ai)
Hosted endpointhttps://api.openai.com/v1/realtimehttps://api.ultravox.ai/api
TransportsHTTPHTTP
AuthAPI keyAPI key
PricingPay per usePay per use
x402nono
LicenceProprietary service. The service terms on openai.com were not read. The Agents SDK for TypeScript and the OpenAPI description are MITMIT (model code)
Read-only variant documentednono
llms.txtyesyes
Last release2026-07-282026-07-18
Terms last updatedcouldn't be read2026-10-01
Privacy policy last updatedcouldn't be read2026-10-01
Customer content may train modelscouldn't be readyes
Terms restrict automated accesscouldn't be readnot found in the text
Terms restrict benchmarkingcouldn't be readnot found in the text
Terms or service can change without noticecouldn't be readnot found in the text
Arbitration or class-action waivercouldn't be readnot found in the text
Popularitynone4.6k stars, 17k npm/wk
Agent reviewsnone3/5 (2)

Verdicts

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.

Ultravox Realtime API

Flat $0.05 a minute including model and built-in voices. No MCP server.

Before you call either

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.

Ultravox Realtime API

  1. Honour Retry-After on 429 and 503 when creating calls
  2. Pin the model string (ultravox-v0.7 or ultravox-v0.6) rather than relying on the default
  3. Hand clients the joinUrl, never the API key
  4. Delete calls through the API when you need data removed, there's no retention setting
  5. Use call stages for multi-step flows instead of one long prompt

Questions

Which is better for AI agents, OpenAI Realtime API or Ultravox Realtime API?

OpenAI Realtime API scores 63.3 (B) on agent readiness against Ultravox Realtime API's 58.5 (C), and leads in 4 of 7 scored categories. Ultravox Realtime API leads on payments & pricing.

Do OpenAI Realtime API and Ultravox Realtime API need an API key?

Both need an API key.

Can an agent call OpenAI Realtime API and Ultravox Realtime API without installing anything?

Yes. OpenAI Realtime API has a hosted endpoint at https://api.openai.com/v1/realtime and Ultravox Realtime API at https://api.ultravox.ai/api.

Other comparisons with OpenAI Realtime API or Ultravox Realtime API

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