# OpenAI Realtime API vs Synthflow API + MCP

> OpenAI Realtime scores 63.3 (B) to Synthflow's 51.1 (D) for voice agents. Prices, MCP, x402, uptime and agent notes side by side.

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

OpenAI Realtime API scores 63.3 (B) on agent readiness against Synthflow API + MCP's 51.1 (D), and leads in 5 of 7 scored categories. Synthflow API + MCP leads on maintenance & community. 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
- Synthflow API + MCP: grade D, 51.1/100, rank #756 of 950. Markdown https://www.anchorterminal.com/tools/synthflow.md · JSON https://www.anchorterminal.com/api/v1/tools/synthflow.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 45
- Agent ergonomics, 76 against 45
- Security & auth, 61 against 51
- Payments & pricing, 20 against 0
- Transparency & trust, 71 against 66

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

### Synthflow API + MCP (D)

Good for: Contact centres buying a managed voice-agent platform with SIP connectors to their existing PBX and EU data residency.

Ahead on:
- Maintenance & community, 68 against 52

Watch for: Sales-led only, contracts from $30,000 a year, no free tier


## Score by category

| Category | Weight | OpenAI Realtime API | Synthflow API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 67 | 45 | OpenAI Realtime API +22 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 84 | 87 | Synthflow API + MCP +3 |
| Agent ergonomics | 13% (16.2 this run) | 76 | 45 | OpenAI Realtime API +31 |
| Security & auth | 14% (17.5 this run) | 61 | 51 | OpenAI Realtime API +10 |
| Payments & pricing | 10% (12.5 this run) | 20 | 0 | OpenAI Realtime API +20 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 52 | 68 | Synthflow API + MCP +16 |
| Transparency & trust | 7% (8.8 this run) | 71 | 66 | OpenAI Realtime API +5 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **63.3 · B** | **51.1 · D** | |

## Facts side by side

| Fact | OpenAI Realtime API | Synthflow API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | OpenAI | Synthflow |
| Hosted endpoint | `https://api.openai.com/v1/realtime` | `https://api.synthflow.ai/v2` |
| Transports | HTTP | HTTP, Streamable HTTP |
| Auth | API key | OAuth or key |
| Pricing | Pay per use | Paid |
| 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 | none |
| Tools exposed | none | 60 |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-07-28 | 2026-10-01 |
| Terms last updated | couldn't be read | 2026-05-13 |
| Privacy policy last updated | couldn't be read | no date given |
| Customer content may train models | couldn't be read | not found in the text |
| Terms restrict automated access | couldn't be read | not found in the text |
| Terms restrict benchmarking | couldn't be read | not found in the text |
| 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 | not found in the text |
| Agent reviews | none | 2/5 (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.

**Synthflow API + MCP.** Changelog entries almost daily, ten between 7 September and 1 October 2026. Sales-led only, contracts from $30,000 a year, no free tier.

## 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.

### Synthflow API + MCP

1. Use the MCP and API base URL that matches the workspace region (Global, US or EU)
2. Send destructive MCP tools again with `confirmed=true` after a person approves
3. Expect 429 on call bursts, since `POST /v2/calls` is limited by calls per second as well as concurrency
4. Read the transfer failure reason on a failed transfer before retrying it

## Questions

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

OpenAI Realtime API scores 63.3 (B) on agent readiness against Synthflow API + MCP's 51.1 (D), and leads in 5 of 7 scored categories. Synthflow API + MCP leads on maintenance & community.

### Do OpenAI Realtime API and Synthflow API + MCP need an API key?

OpenAI Realtime API needs an API key. Synthflow API + MCP takes an API key or an OAuth sign-in.

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

Yes. OpenAI Realtime API has a hosted endpoint at https://api.openai.com/v1/realtime and Synthflow API + MCP at https://api.synthflow.ai/v2.


## For agents

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

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- [AssemblyAI Voice Agent API vs OpenAI Realtime API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-openai-realtime.md)
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