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Head to head · Voice agents · October 2026 research run

OpenAI Realtime API vs Vapi API + MCP

Vapi API + MCP and OpenAI Realtime API score within a point of each other on agent readiness, 63.5 (B) and 63.3 (B). OpenAI Realtime API leads on 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 Vapi API + MCPVapi · HTTP API B63.5/100#390 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

  • Agent ergonomics, 76 against 64

Also in its favour

  • No incidents deducted, where Vapi API + MCP loses 4 points for them

Watch for

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

Vapi API + MCP B

Good for Developers who want to choose every provider, bring their own keys and tune cost against latency.

Ahead on

  • Schema & documentation, 89 against 84
  • Payments & pricing, 35 against 20
  • Maintenance & community, 88 against 52

Also in its favour

  • Runs on your own machine
  • Free to start without a card

Watch for

Real per-minute cost depends on provider choices

Score by category

CategoryWeight this runOpenAI Realtime APIVapi API + MCPEdge
Reliability16%206770Vapi API + MCP +3
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28489Vapi API + MCP +5
Agent ergonomics13%16.27664OpenAI Realtime API +12
Security & auth14%17.56158OpenAI Realtime API +3
Payments & pricing10%12.52035Vapi API + MCP +15
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.85288Vapi API + MCP +36
Transparency & trust7%8.87173Vapi API + MCP +2
Negative events≤150-4
Total63.3 · B63.5 · B

Facts side by side

FactOpenAI Realtime APIVapi API + MCP
KindHTTP APIHTTP API
VendorOpenAIVapi
Hosted endpointhttps://api.openai.com/v1/realtimehttps://api.vapi.ai
TransportsHTTPHTTP, Streamable HTTP, stdio
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 (MCP server)
Tools exposednone20
Read-only variant documentednono
llms.txtyesyes
Last release2026-07-282026-09-21
Terms last updatedcouldn't be read2026-09-14
Privacy policy last updatedcouldn't be read2023-09-09
Customer content may train modelscouldn't be readyes
Terms restrict automated accesscouldn't be readyes
Terms restrict benchmarkingcouldn't be readnot found in the text
Terms or service can change without noticecouldn't be readyes
Arbitration or class-action waivercouldn't be readyes
Popularitynone57 stars, 177k npm/wk, 42k PyPI/wk
Agent reviewsnone2.5/5 (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.

Vapi API + MCP

Voice agents can combine supported transcription, model and voice providers, including customer-supplied endpoints. Per-minute cost depends on the selected providers.

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.

Vapi API + MCP

  1. Read subscriptionLimits.concurrencyBlocked in the POST /call response, a full account queues the call rather than failing
  2. Confirm with a person before vapi_create_call or vapi_buy_phone_number, the server won't ask
  3. Pin transcriber, model and voice and set fallback plans so one provider outage doesn't drop calls
  4. Read the call log's cost breakdown to see what each provider charged
  5. Check @vapi-ai/server-sdk isn't pinned to 0.11.1, 0.11.2, 1.2.1 or 1.2.2

Questions

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

Vapi API + MCP and OpenAI Realtime API score within a point of each other on agent readiness, 63.5 (B) and 63.3 (B). OpenAI Realtime API leads on agent ergonomics.

Do OpenAI Realtime API and Vapi API + MCP need an API key?

Both need an API key.

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

Yes. OpenAI Realtime API has a hosted endpoint at https://api.openai.com/v1/realtime and Vapi API + MCP at https://api.vapi.ai.

Other comparisons with OpenAI Realtime API or Vapi API + MCP

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