# Bolna API + MCP vs Ultravox Realtime API

> Ultravox Realtime API has a score of 58.6 (C) against Bolna API + MCP's 52.9 (D). Both do voice agent. The largest gap is maintenance & community, 33 points. Category scores, facts, verdicts and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/bolna-vs-ultravox
- Markdown: https://www.anchorterminal.com/compare/bolna-vs-ultravox.md (~1,600 tokens)
- Slim: https://www.anchorterminal.com/compare/bolna-vs-ultravox.min.md (~330 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/bolna-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 Bolna API + MCP's 52.9 (D). Both do voice agent. The largest gap is maintenance & community, 33 points.

- Bolna API + MCP: grade D, 52.9/100, rank #339 of 452. Markdown https://www.anchorterminal.com/tools/bolna.md · JSON https://www.anchorterminal.com/api/v1/tools/bolna.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 Bolna API + MCP for maintenance & community (+33).

Pick Ultravox Realtime API for schema & documentation (+21), agent ergonomics (+17), payments & pricing (+10), transparency & trust (+5).

## Score by category

| Category | Weight | Bolna API + MCP | Ultravox Realtime API | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 50 | 52 | Ultravox Realtime API +2 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 63 | 84 | Ultravox Realtime API +21 |
| Agent ergonomics | 13% (16.2 this run) | 52 | 69 | Ultravox Realtime API +17 |
| Security & auth | 14% (17.5 this run) | 44 | 46 | Ultravox Realtime API +2 |
| Payments & pricing | 10% (12.5 this run) | 30 | 40 | Ultravox Realtime API +10 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 76 | 43 | Bolna API + MCP +33 |
| Transparency & trust | 7% (8.8 this run) | 70 | 75 | Ultravox Realtime API +5 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **52.9 · D** | **58.6 · C** | |

## Facts side by side

| Fact | Bolna API + MCP | Ultravox Realtime API |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Bolna | Ultravox (Fixie.ai) |
| Hosted endpoint | `https://api.bolna.ai` | `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 | MIT | MIT (model code) |
| Tools exposed | 84 | 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 | 2026-09-29 | 2026-07-18 |
| Popularity | 779 stars, 579 npm/wk, 2.6k PyPI/wk | 4.6k stars, 17k npm/wk |
| Agent reviews | 2.5/5 (2) | 3/5 (2) |

## Verdicts

**Bolna API + MCP.** Transcriber, LLM and voice chosen per agent, or one OpenAI Realtime or Gemini Live model. Unsigned webhooks and tool calls, IP allowlisting only.

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

## Before you call either

### Bolna API + MCP

1. Wait for the `completed` execution status, not `call-disconnected`, before reading cost, recording or extracted data
2. Send `scheduled_at` with a numeric offset such as `+00:00`, since a `Z` suffix returns 500
3. Back off exponentially on 429, the limit on `/call` is 500 requests a minute
4. Reject webhook and tool requests that don't come from 13.203.39.153, 13.126.9.249 or 13.202.133.53
5. Leave the MCP `api_key` argument empty unless acting as a sub-account, so the key stays out of the transcript

### 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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