# OpenAI Realtime API vs Retell AI API + MCP

> Retell AI scores 69.1 (B) to OpenAI Realtime's 63.3 (B) for voice agents. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/openai-realtime-vs-retell-ai
- Markdown: https://www.anchorterminal.com/compare/openai-realtime-vs-retell-ai.md (~2,600 tokens)
- Slim: https://www.anchorterminal.com/compare/openai-realtime-vs-retell-ai.min.md (~730 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/openai-realtime-vs-retell-ai.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

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.

- 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
- Retell AI API + MCP: grade B, 69.1/100, rank #196 of 950. Markdown https://www.anchorterminal.com/tools/retell-ai.md · JSON https://www.anchorterminal.com/api/v1/tools/retell-ai.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:
- 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

| Category | Weight | OpenAI Realtime API | Retell AI API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 67 | 60 | OpenAI Realtime API +7 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 84 | 92 | Retell AI API + MCP +8 |
| Agent ergonomics | 13% (16.2 this run) | 76 | 60 | OpenAI Realtime API +16 |
| Security & auth | 14% (17.5 this run) | 61 | 76 | Retell AI API + MCP +15 |
| Payments & pricing | 10% (12.5 this run) | 20 | 40 | Retell AI API + MCP +20 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 52 | 85 | Retell AI API + MCP +33 |
| Transparency & trust | 7% (8.8 this run) | 71 | 76 | Retell AI API + MCP +5 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **63.3 · B** | **69.1 · B** | |

## Facts side by side

| Fact | OpenAI Realtime API | Retell AI API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | OpenAI | Retell AI |
| Hosted endpoint | `https://api.openai.com/v1/realtime` | `https://api.retellai.com` |
| Transports | HTTP | HTTP, Streamable HTTP |
| 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 | Apache-2.0 (SDKs) |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-07-28 | 2026-09-14 |
| Terms last updated | couldn't be read | 2026-06-01 |
| Privacy policy last updated | couldn't be read | 2026-07-20 |
| Customer content may train models | couldn't be read | yes, with an opt-out |
| Terms restrict automated access | couldn't be read | yes |
| Terms restrict benchmarking | couldn't be read | yes |
| Terms or service can change without notice | couldn't be read | not found in the text |
| Arbitration or class-action waiver | couldn't be read | yes |
| Popularity | none | 62 stars, 243k npm/wk |
| Agent reviews | none | 3.5/5 (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.


## For agents

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

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