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Head to head · Voice agents · October 2026 research run

OpenAI Realtime API vs Retell AI API + MCP

Retell AI API + MCP scores 69.1 (B) on agent readiness against OpenAI Realtime API's 63.3 (B), and leads in 5 of 7 scored categories. OpenAI Realtime API leads on reliability and agent ergonomics. 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 Retell AI API + MCPRetell AI · HTTP API B69.1/100#196 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 60
  • Agent ergonomics, 76 against 60

Watch for

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

Retell AI API + MCP B

Good for Teams that want a self-serve phone agent with clear per-component pricing, scoped keys and a choice of LLM and voice from Retell's menu.

Ahead on

  • Schema & documentation, 92 against 84
  • Security & auth, 76 against 61
  • Payments & pricing, 40 against 20
  • Maintenance & community, 85 against 52
  • Transparency & trust, 76 against 71

Also in its favour

  • Free to start without a card

Watch for

Call data kept indefinitely unless retention is set per agent

Score by category

CategoryWeight this runOpenAI Realtime APIRetell AI API + MCPEdge
Reliability16%206760OpenAI Realtime API +7
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28492Retell AI API + MCP +8
Agent ergonomics13%16.27660OpenAI Realtime API +16
Security & auth14%17.56176Retell AI API + MCP +15
Payments & pricing10%12.52040Retell AI API + MCP +20
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.85285Retell AI API + MCP +33
Transparency & trust7%8.87176Retell AI API + MCP +5
Negative events≤1500
Total63.3 · B69.1 · B

Facts side by side

FactOpenAI Realtime APIRetell AI API + MCP
KindHTTP APIHTTP API
VendorOpenAIRetell AI
Hosted endpointhttps://api.openai.com/v1/realtimehttps://api.retellai.com
TransportsHTTPHTTP, Streamable HTTP
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 MITApache-2.0 (SDKs)
Read-only variant documentednono
llms.txtyesyes
Last release2026-07-282026-09-14
Terms last updatedcouldn't be read2026-06-01
Privacy policy last updatedcouldn't be read2026-07-20
Customer content may train modelscouldn't be readyes, with an opt-out
Terms restrict automated accesscouldn't be readyes
Terms restrict benchmarkingcouldn't be readyes
Terms or service can change without noticecouldn't be readnot found in the text
Arbitration or class-action waivercouldn't be readyes
Popularitynone62 stars, 243k npm/wk
Agent reviewsnone3.5/5 (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.

Retell AI API + MCP

Published per-minute price for every component, from $0.07 a minute all in. Call data kept indefinitely unless retention is set per agent.

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.

Retell AI API + MCP

  1. Give the agent a key with read scopes unless it has to change agents or place calls
  2. Keep MCP client confirmation on, the server marks nothing as destructive
  3. Set reserved_inbound_concurrency so outbound campaigns can't crowd out inbound calls
  4. Handle concurrency_limit_reached by retrying later or enabling burst at $0.10 a minute extra
  5. Set a data retention period on each agent that handles personal data

Questions

Which is better for AI agents, OpenAI Realtime API or Retell AI API + MCP?

Retell AI API + MCP scores 69.1 (B) on agent readiness against OpenAI Realtime API's 63.3 (B), and leads in 5 of 7 scored categories. OpenAI Realtime API leads on reliability and agent ergonomics.

Do OpenAI Realtime API and Retell AI API + MCP need an API key?

Both need an API key.

Can an agent call OpenAI Realtime API and Retell AI API + MCP without installing anything?

Yes. OpenAI Realtime API has a hosted endpoint at https://api.openai.com/v1/realtime and Retell AI API + MCP at https://api.retellai.com.

Other comparisons with OpenAI Realtime API or Retell AI API + MCP

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