# Retell AI API + MCP vs Ultravox Realtime API

> Retell AI API + MCP has a score of 69.4 (B) against Ultravox Realtime API's 58.6 (C). Both do voice agent. The largest gap is maintenance & community, 42 points. Category scores, facts, verdicts and agent notes side by side.

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- Updated: 2026-10-04

Retell AI API + MCP has a score of 69.4 (B) against Ultravox Realtime API's 58.6 (C). Both do voice agent. The largest gap is maintenance & community, 42 points.

- Retell AI API + MCP: grade B, 69.4/100, rank #114 of 452. Markdown https://www.anchorterminal.com/tools/retell-ai.md · JSON https://www.anchorterminal.com/api/v1/tools/retell-ai.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 Retell AI API + MCP for reliability (+8), schema & documentation (+8), security & auth (+30), maintenance & community (+42).

Pick Ultravox Realtime API for agent ergonomics (+9).

## Score by category

| Category | Weight | Retell AI API + MCP | Ultravox Realtime API | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 60 | 52 | Retell AI API + MCP +8 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 92 | 84 | Retell AI API + MCP +8 |
| Agent ergonomics | 13% (16.2 this run) | 60 | 69 | Ultravox Realtime API +9 |
| Security & auth | 14% (17.5 this run) | 76 | 46 | Retell AI API + MCP +30 |
| Payments & pricing | 10% (12.5 this run) | 40 | 40 | even |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 85 | 43 | Retell AI API + MCP +42 |
| Transparency & trust | 7% (8.8 this run) | 79 | 75 | Retell AI API + MCP +4 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **69.4 · B** | **58.6 · C** | |

## Facts side by side

| Fact | Retell AI API + MCP | Ultravox Realtime API |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Retell AI | Ultravox (Fixie.ai) |
| Hosted endpoint | `https://api.retellai.com` | `https://api.ultravox.ai/api` |
| Transports | HTTP, Streamable HTTP | HTTP |
| Auth | API key | API key |
| Pricing | Pay per use | Pay per use |
| x402 | no | no |
| Licence | Apache-2.0 (SDKs) | MIT (model code) |
| Tools exposed | none | 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 | 2026-09-14 | 2026-07-18 |
| Popularity | 62 stars, 243k npm/wk | 4.6k stars, 17k npm/wk |
| Agent reviews | 3.5/5 (2) | 3/5 (2) |

## Verdicts

**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.

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

## Before you call either

### 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

### 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

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