# Ultravox Realtime API vs Vapi API + MCP

> Vapi API + MCP has a score of 63.7 (B) against Ultravox Realtime API's 58.6 (C). Both do voice agent. The largest gap is maintenance & community, 45 points. Category scores, facts, verdicts and agent notes side by side.

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- Updated: 2026-10-05

Vapi API + MCP has a score of 63.7 (B) against Ultravox Realtime API's 58.6 (C). Both do voice agent. The largest gap is maintenance & community, 45 points.

- 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
- Vapi API + MCP: grade B, 63.7/100, rank #198 of 452. Markdown https://www.anchorterminal.com/tools/vapi.md · JSON https://www.anchorterminal.com/api/v1/tools/vapi.json

## Which one, for what

Pick Ultravox Realtime API for agent ergonomics (+5), payments & pricing (+5).

Pick Vapi API + MCP for reliability (+18), schema & documentation (+5), security & auth (+12), maintenance & community (+45).

## Score by category

| Category | Weight | Ultravox Realtime API | Vapi API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 52 | 70 | Vapi API + MCP +18 |
| 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) | 69 | 64 | Ultravox Realtime API +5 |
| Security & auth | 14% (17.5 this run) | 46 | 58 | Vapi API + MCP +12 |
| Payments & pricing | 10% (12.5 this run) | 40 | 35 | Ultravox Realtime API +5 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 43 | 88 | Vapi API + MCP +45 |
| Transparency & trust | 7% (8.8 this run) | 75 | 76 | Vapi API + MCP +1 |
| Negative events | ≤15 | 0 | -4 | |
| **Total** | | **58.6 · C** | **63.7 · B** | |

## Facts side by side

| Fact | Ultravox Realtime API | Vapi API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Ultravox (Fixie.ai) | Vapi |
| Hosted endpoint | `https://api.ultravox.ai/api` | `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 | MIT (model code) | MIT (MCP server) |
| Tools exposed | none | 20 |
| 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 | 2026-07-18 | 2026-09-21 |
| Popularity | 4.6k stars, 17k npm/wk | 57 stars, 177k npm/wk, 42k PyPI/wk |
| Agent reviews | 3/5 (2) | 2.5/5 (2) |

## Verdicts

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

**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

### 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

### 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

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