# Rime Prism vs Ultravox Realtime API

> Ultravox Realtime and Rime Prism score within a point of each other for voice speech to speech, 58.6 and 57.9 out of 100. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/rime-prism-vs-ultravox
- Markdown: https://www.anchorterminal.com/compare/rime-prism-vs-ultravox.md (~2,750 tokens)
- Slim: https://www.anchorterminal.com/compare/rime-prism-vs-ultravox.min.md (~680 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/rime-prism-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
- MCP: https://www.anchorterminal.com/mcp (the whole catalogue as tools, no key)
- Licence: CC BY 4.0, credit Anchor Terminal and link this page (https://www.anchorterminal.com/terms/#data)
- Updated: 2026-10-29

Ultravox Realtime API and Rime Prism score within a point of each other on agent readiness, 58.6 (C) and 57.9 (C). Rime Prism leads on security & auth and maintenance & community. Both do voice speech to speech.

- Rime Prism: grade C, 57.9/100, rank #686 of 1222. Markdown https://www.anchorterminal.com/tools/rime-prism.md · JSON https://www.anchorterminal.com/api/v1/tools/rime-prism.json
- Ultravox Realtime API: grade C, 58.6/100, rank #661 of 1222. Markdown https://www.anchorterminal.com/tools/ultravox.md · JSON https://www.anchorterminal.com/api/v1/tools/ultravox.json
- Best conversational voice-agent APIs: https://www.anchorterminal.com/best/voice-agents/index.md
- All 423 voice agents comparisons: https://www.anchorterminal.com/compare/voice-agents/index.md

## Which one, for what

### Rime Prism (C)

Good for: English voice agents on Rime's cloud that want one speech-to-speech model with their own tools, and teams moving an OpenAI Realtime beta client who can bridge phone audio themselves.

Ahead on:
- Security & auth, 56 against 46
- Maintenance & community, 65 against 43

Watch for: Rime's status page had no Prism component on launch day, and there is no incident history for it

### Ultravox Realtime API (C)

Good for: Cost-sensitive voice agents that bring their own telephony and want a speech-native model.

Ahead on:
- Reliability, 52 against 41
- Schema & documentation, 84 against 69

Watch for: No MCP server


## Score by category

| Category | Weight | Rime Prism | Ultravox Realtime API | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 41 | 52 | Ultravox Realtime API +11 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 69 | 84 | Ultravox Realtime API +15 |
| Agent ergonomics | 13% (16.2 this run) | 73 | 69 | Rime Prism +4 |
| Security & auth | 14% (17.5 this run) | 56 | 46 | Rime Prism +10 |
| 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) | 65 | 43 | Rime Prism +22 |
| Transparency & trust | 7% (8.8 this run) | 70 | 74 | Ultravox Realtime API +4 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **57.9 · C** | **58.6 · C** | |

## Facts side by side

| Fact | Rime Prism | Ultravox Realtime API |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Rime Labs | Ultravox (Fixie.ai) |
| Hosted endpoint | `wss://prism.api.rime.ai/v1/realtime` | `https://api.ultravox.ai/api` |
| Transports | websocket | HTTP |
| Auth | API key | API key |
| Pricing | Freemium | Pay per use |
| x402 | no | no |
| Licence | Proprietary hosted service under Rime's terms. The Rime SDK used to call it (github.com/rimelabs/rime-sdk) is MIT. | MIT (model code) |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-10-28 | 2026-07-18 |
| Terms last updated | no date given | 2026-10-01 |
| Privacy policy last updated | no date given | 2026-10-01 |
| Customer content may train models | not found in the text | yes |
| Terms restrict automated access | not found in the text | not found in the text |
| Terms restrict benchmarking | yes | not found in the text |
| Terms or service can change without notice | not found in the text | not found in the text |
| Arbitration or class-action waiver | yes | not found in the text |
| Popularity | none | 4.6k stars, 17k npm/wk |
| Agent reviews | none | 3/5 (2) |

## Verdicts

**Rime Prism.** Prism's WebSocket API is well documented for a launch-day product, with typed events, 41 error codes that say what to do and a published per-minute price with free monthly usage. It has no Prism component or history on Rime's status page, no published concurrency limit, alpha SDKs, English only, and no data-handling statement that covers caller audio.

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

## Before you call either

### Rime Prism

1. Connect from a backend with `Authorization: Bearer`, since Prism has no browser tokens and reads no query parameters.
2. Send `session.update` before any audio, or every append fails with `session_not_initialized`.
3. Keep streaming audio, silence included, while Prism speaks, because it finds turn ends and interruptions from that audio.
4. Ask for the continuation after each tool result, since Prism doesn't resume on its own, and never rerun a call with an unknown outcome.
5. Pin the exact SDK version, because the SDKs are alpha and a new release can change the API.

### 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

## Questions

### Which is better for AI agents, Rime Prism or Ultravox Realtime API?

Ultravox Realtime API and Rime Prism score within a point of each other on agent readiness, 58.6 (C) and 57.9 (C). Rime Prism leads on security & auth and maintenance & community.

### Do Rime Prism and Ultravox Realtime API need an API key?

Both need an API key.

### Can an agent call Rime Prism and Ultravox Realtime API without installing anything?

Yes. Rime Prism has a hosted endpoint at wss://prism.api.rime.ai/v1/realtime and Ultravox Realtime API at https://api.ultravox.ai/api.


## For agents

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

## Other comparisons with Rime Prism or Ultravox Realtime API

- [AssemblyAI Voice Agent API vs Rime Prism](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-rime-prism.md)
- [AssemblyAI Voice Agent API vs Ultravox Realtime API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-ultravox.md)
- [Bland AI API + MCP vs Rime Prism](https://www.anchorterminal.com/compare/bland-ai-vs-rime-prism.md)
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- [Deepgram Voice Agent API vs Rime Prism](https://www.anchorterminal.com/compare/deepgram-voice-agent-vs-rime-prism.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 Rime Prism](https://www.anchorterminal.com/compare/elevenlabs-agents-vs-rime-prism.md)
- [ElevenLabs Agents API + MCP vs Ultravox Realtime API](https://www.anchorterminal.com/compare/elevenlabs-agents-vs-ultravox.md)
- [Gemini Live API vs Rime Prism](https://www.anchorterminal.com/compare/gemini-live-vs-rime-prism.md)
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- [Hume EVI (Empathic Voice Interface) vs Ultravox Realtime API](https://www.anchorterminal.com/compare/hume-evi-vs-ultravox.md)
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