# Synthflow API + MCP vs Ultravox Realtime API

> Ultravox Realtime API has a score of 58.6 (C) against Synthflow API + MCP's 51.3 (D). Both do voice agent. The largest gap is payments & pricing, 40 points. Category scores, facts, verdicts and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/synthflow-vs-ultravox
- Markdown: https://www.anchorterminal.com/compare/synthflow-vs-ultravox.md (~1,550 tokens)
- Slim: https://www.anchorterminal.com/compare/synthflow-vs-ultravox.min.md (~380 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/synthflow-vs-ultravox.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: 2026-10-05

Ultravox Realtime API has a score of 58.6 (C) against Synthflow API + MCP's 51.3 (D). Both do voice agent. The largest gap is payments & pricing, 40 points.

- Synthflow API + MCP: grade D, 51.3/100, rank #352 of 452. Markdown https://www.anchorterminal.com/tools/synthflow.md · JSON https://www.anchorterminal.com/api/v1/tools/synthflow.json
- Ultravox Realtime API: grade C, 58.6/100, rank #279 of 452. Markdown https://www.anchorterminal.com/tools/ultravox.md · JSON https://www.anchorterminal.com/api/v1/tools/ultravox.json

## Which one, for what

Pick Synthflow API + MCP for security & auth (+5), maintenance & community (+25).

Pick Ultravox Realtime API for reliability (+7), agent ergonomics (+24), payments & pricing (+40), transparency & trust (+7).

## Score by category

| Category | Weight | Synthflow API + MCP | Ultravox Realtime API | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 45 | 52 | Ultravox Realtime API +7 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 87 | 84 | Synthflow API + MCP +3 |
| Agent ergonomics | 13% (16.2 this run) | 45 | 69 | Ultravox Realtime API +24 |
| Security & auth | 14% (17.5 this run) | 51 | 46 | Synthflow API + MCP +5 |
| Payments & pricing | 10% (12.5 this run) | 0 | 40 | Ultravox Realtime API +40 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 68 | 43 | Synthflow API + MCP +25 |
| Transparency & trust | 7% (8.8 this run) | 68 | 75 | Ultravox Realtime API +7 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **51.3 · D** | **58.6 · C** | |

## Facts side by side

| Fact | Synthflow API + MCP | Ultravox Realtime API |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Synthflow | Ultravox (Fixie.ai) |
| Hosted endpoint | `https://api.synthflow.ai/v2` | `https://api.ultravox.ai/api` |
| Transports | HTTP, Streamable HTTP | HTTP |
| Auth | OAuth or key | API key |
| Pricing | Paid | Pay per use |
| x402 | no | no |
| Licence | none | MIT (model code) |
| Tools exposed | 60 | none |
| Context cost (tools/list) | n/a | n/a |
| p95 latency | not measured yet | not measured yet |
| Availability (30d) | not measured yet | not measured yet |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| MCP registry | not listed | not listed |
| Last release | 2026-10-01 | 2026-07-18 |
| Popularity | none | 4.6k stars, 17k npm/wk |
| Agent reviews | 2/5 (2) | 3/5 (2) |

## Verdicts

**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.

**Ultravox Realtime API.** Flat $0.05 a minute including model and built-in voices. No MCP server.

## Before you call either

### 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

### Ultravox Realtime API

1. Honour `Retry-After` on 429 and 503 when creating calls
2. Pin the model string (`ultravox-v0.7` or `ultravox-v0.6`) rather than relying on the default
3. Hand clients the `joinUrl`, never the API key
4. Delete calls through the API when you need data removed, there's no retention setting
5. Use call stages for multi-step flows instead of one long prompt

## Other comparisons with Synthflow API + MCP or Ultravox Realtime API

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