# OpenAI Realtime API vs Vapi API + MCP

> Vapi and OpenAI Realtime score within a point of each other for voice agents, 63.5 and 63.3 out of 100. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/openai-realtime-vs-vapi
- Markdown: https://www.anchorterminal.com/compare/openai-realtime-vs-vapi.md (~2,550 tokens)
- Slim: https://www.anchorterminal.com/compare/openai-realtime-vs-vapi.min.md (~730 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/openai-realtime-vs-vapi.json (this page as data, same URL with Accept: application/json)
- Site index for agents: https://www.anchorterminal.com/llms.txt (full text: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- Updated: 2026-10-09

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.

- OpenAI Realtime API: grade B, 63.3/100, rank #400 of 950. Markdown https://www.anchorterminal.com/tools/openai-realtime.md · JSON https://www.anchorterminal.com/api/v1/tools/openai-realtime.json
- Vapi API + MCP: grade B, 63.5/100, rank #390 of 950. Markdown https://www.anchorterminal.com/tools/vapi.md · JSON https://www.anchorterminal.com/api/v1/tools/vapi.json
- Best conversational voice-agent APIs: https://www.anchorterminal.com/best/voice-agents/index.md
- All 91 voice agents comparisons: https://www.anchorterminal.com/compare/voice-agents/index.md

## 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

| Category | Weight | OpenAI Realtime API | Vapi API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 67 | 70 | Vapi API + MCP +3 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 84 | 89 | Vapi API + MCP +5 |
| Agent ergonomics | 13% (16.2 this run) | 76 | 64 | OpenAI Realtime API +12 |
| Security & auth | 14% (17.5 this run) | 61 | 58 | OpenAI Realtime API +3 |
| Payments & pricing | 10% (12.5 this run) | 20 | 35 | Vapi API + MCP +15 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 52 | 88 | Vapi API + MCP +36 |
| Transparency & trust | 7% (8.8 this run) | 71 | 73 | Vapi API + MCP +2 |
| Negative events | ≤15 | 0 | -4 | |
| **Total** | | **63.3 · B** | **63.5 · B** | |

## Facts side by side

| Fact | OpenAI Realtime API | Vapi API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | OpenAI | Vapi |
| Hosted endpoint | `https://api.openai.com/v1/realtime` | `https://api.vapi.ai` |
| Transports | HTTP | HTTP, Streamable HTTP, stdio |
| Auth | API key | API key |
| Pricing | Pay per use | Pay per use |
| x402 | no | no |
| Licence | Proprietary service. The service terms on openai.com were not read. The Agents SDK for TypeScript and the OpenAPI description are MIT | MIT (MCP server) |
| Tools exposed | none | 20 |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-07-28 | 2026-09-21 |
| Terms last updated | couldn't be read | 2026-09-14 |
| Privacy policy last updated | couldn't be read | 2023-09-09 |
| Customer content may train models | couldn't be read | yes |
| Terms restrict automated access | couldn't be read | yes |
| Terms restrict benchmarking | couldn't be read | not found in the text |
| Terms or service can change without notice | couldn't be read | yes |
| Arbitration or class-action waiver | couldn't be read | yes |
| Popularity | none | 57 stars, 177k npm/wk, 42k PyPI/wk |
| Agent reviews | none | 2.5/5 (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.


## For agents

- This comparison as JSON: https://www.anchorterminal.com/compare/openai-realtime-vs-vapi.json, and with the fewest tokens: https://www.anchorterminal.com/compare/openai-realtime-vs-vapi.min.md
- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {"a": "openai-realtime", "b": "vapi"}`. From a terminal: `anchor compare openai-realtime vapi`
- Each listing in full: https://www.anchorterminal.com/api/v1/tools/openai-realtime.json and https://www.anchorterminal.com/api/v1/tools/vapi.json

## Other comparisons with OpenAI Realtime API or Vapi API + MCP

- [AssemblyAI Voice Agent API vs OpenAI Realtime API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-openai-realtime.md)
- [AssemblyAI Voice Agent API vs Vapi API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi.md)
- [Bland AI API + MCP vs OpenAI Realtime API](https://www.anchorterminal.com/compare/bland-ai-vs-openai-realtime.md)
- [Bland AI API + MCP vs Vapi API + MCP](https://www.anchorterminal.com/compare/bland-ai-vs-vapi.md)
- [Bolna API + MCP vs OpenAI Realtime API](https://www.anchorterminal.com/compare/bolna-vs-openai-realtime.md)
- [Bolna API + MCP vs Vapi API + MCP](https://www.anchorterminal.com/compare/bolna-vs-vapi.md)
- [Deepgram Voice Agent API vs OpenAI Realtime API](https://www.anchorterminal.com/compare/deepgram-voice-agent-vs-openai-realtime.md)
- [Deepgram Voice Agent API vs Vapi API + MCP](https://www.anchorterminal.com/compare/deepgram-voice-agent-vs-vapi.md)
- [ElevenLabs Agents API + MCP vs OpenAI Realtime API](https://www.anchorterminal.com/compare/elevenlabs-agents-vs-openai-realtime.md)
- [ElevenLabs Agents API + MCP vs Vapi API + MCP](https://www.anchorterminal.com/compare/elevenlabs-agents-vs-vapi.md)
- [Gemini Live API vs OpenAI Realtime API](https://www.anchorterminal.com/compare/gemini-live-vs-openai-realtime.md)
- [Gemini Live API vs Vapi API + MCP](https://www.anchorterminal.com/compare/gemini-live-vs-vapi.md)
- [Hume EVI (Empathic Voice Interface) vs OpenAI Realtime API](https://www.anchorterminal.com/compare/hume-evi-vs-openai-realtime.md)
- [Hume EVI (Empathic Voice Interface) vs Vapi API + MCP](https://www.anchorterminal.com/compare/hume-evi-vs-vapi.md)
- [OpenAI Realtime API vs Retell AI API + MCP](https://www.anchorterminal.com/compare/openai-realtime-vs-retell-ai.md)
- [OpenAI Realtime API vs Synthflow API + MCP](https://www.anchorterminal.com/compare/openai-realtime-vs-synthflow.md)
- [OpenAI Realtime API vs Ultravox Realtime API](https://www.anchorterminal.com/compare/openai-realtime-vs-ultravox.md)
- [OpenAI Realtime API vs Vocily AI](https://www.anchorterminal.com/compare/openai-realtime-vs-vocily.md)
- [OpenAI Realtime API vs Vogent API](https://www.anchorterminal.com/compare/openai-realtime-vs-vogent.md)
- [Retell AI API + MCP vs Vapi API + MCP](https://www.anchorterminal.com/compare/retell-ai-vs-vapi.md)
- [Synthflow API + MCP vs Vapi API + MCP](https://www.anchorterminal.com/compare/synthflow-vs-vapi.md)
- [Ultravox Realtime API vs Vapi API + MCP](https://www.anchorterminal.com/compare/ultravox-vs-vapi.md)
- [Vapi API + MCP vs Vocily AI](https://www.anchorterminal.com/compare/vapi-vs-vocily.md)
- [Vapi API + MCP vs Vogent API](https://www.anchorterminal.com/compare/vapi-vs-vogent.md)