# AssemblyAI Voice Agent API vs Bolna API + MCP

> AssemblyAI Voice Agent scores 64.4 (B) to Bolna's 52.6 (D) for voice agents. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna
- Markdown: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna.md (~2,650 tokens)
- Slim: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna.min.md (~730 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna.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: 2026-10-09

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Bolna API + MCP's 52.6 (D), and leads in 5 of 7 scored categories. Bolna API + MCP leads on maintenance & community. Both do voice agents.

- AssemblyAI Voice Agent API: grade B, 64.4/100, rank #351 of 950. Markdown https://www.anchorterminal.com/tools/assemblyai-voice-agent.md · JSON https://www.anchorterminal.com/api/v1/tools/assemblyai-voice-agent.json
- Bolna API + MCP: grade D, 52.6/100, rank #729 of 950. Markdown https://www.anchorterminal.com/tools/bolna.md · JSON https://www.anchorterminal.com/api/v1/tools/bolna.json
- Best conversational voice-agent APIs: https://www.anchorterminal.com/best/voice-agents/index.md
- All 91 voice agents comparisons: https://www.anchorterminal.com/compare/voice-agents/index.md

## Which one, for what

### AssemblyAI Voice Agent API (B)

Good for: A developer who wants one managed pipeline at a flat rate and is content with AssemblyAI's own speech models and voices, with an optional model of their own.

Ahead on:
- Reliability, 60 against 50
- Schema & documentation, 83 against 63
- Agent ergonomics, 72 against 52
- Security & auth, 62 against 44
- Payments & pricing, 40 against 30

Also in its favour:
- Free to start without a card

Watch for: No number is published for the concurrent-session limit that the `concurrency_exceeded` error enforces

### Bolna API + MCP (D)

Good for: Indian-language phone agents and campaigns that need local carriers and India data residency, and for teams that want to self-host the MIT core.

Ahead on:
- Maintenance & community, 76 against 64

Also in its favour:
- Open source

Watch for: Unsigned webhooks and tool calls, IP allowlisting only


## Score by category

| Category | Weight | AssemblyAI Voice Agent API | Bolna API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 60 | 50 | AssemblyAI Voice Agent API +10 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 83 | 63 | AssemblyAI Voice Agent API +20 |
| Agent ergonomics | 13% (16.2 this run) | 72 | 52 | AssemblyAI Voice Agent API +20 |
| Security & auth | 14% (17.5 this run) | 62 | 44 | AssemblyAI Voice Agent API +18 |
| Payments & pricing | 10% (12.5 this run) | 40 | 30 | AssemblyAI Voice Agent API +10 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 64 | 76 | Bolna API + MCP +12 |
| Transparency & trust | 7% (8.8 this run) | 66 | 66 | even |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **64.4 · B** | **52.6 · D** | |

## Facts side by side

| Fact | AssemblyAI Voice Agent API | Bolna API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | AssemblyAI, Inc. | Bolna |
| Hosted endpoint | `wss://agents.assemblyai.com/v1/ws` | `https://api.bolna.ai` |
| Transports | HTTP | HTTP, Streamable HTTP |
| Auth | API key | API key |
| Pricing | Pay per use | Pay per use |
| Price for voice agents | $0.075 per minute of call | not published |
| x402 | no | no |
| Licence | Proprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence file | MIT |
| Tools exposed | none | 84 |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-08-26 | 2026-09-29 |
| Terms last updated | 2026-07-01 | 2026-08-13 |
| Privacy policy last updated | 2026-05-26 | 2025-03-28 |
| Customer content may train models | yes, with an opt-out | not found in the text |
| Terms restrict automated access | yes | not found in the text |
| Terms restrict benchmarking | yes | yes |
| Terms or service can change without notice | not found in the text | yes |
| Arbitration or class-action waiver | not found in the text | not found in the text |
| Popularity | none | 779 stars, 579 npm/wk, 2.6k PyPI/wk |
| Agent reviews | none | 2.5/5 (2) |

## Verdicts

**AssemblyAI Voice Agent API.** One $0.075 a minute rate covers speech-to-text, the managed model, speech output, recordings and hosting, and the socket has an AsyncAPI file, typed error codes and a 30-second resume window. No concurrent-session limit is published, the status page has no Voice Agent component, and phone calls are inbound only through a Twilio trunk the customer owns.

**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.

## Before you call either

### AssemblyAI Voice Agent API

1. Send `session.end` before closing the socket. Without it the session stays resumable, and billed, for 30 seconds
2. Retry only `at_capacity`, `concurrency_exceeded` and `internal_error`, with backoff. Every other session error code is fatal
3. Send `tool.result` inside the `reply.done` handler, not straight after `tool.call`
4. Mint a one-time token with `GET /v1/token` for browser clients and never ship the API key to a page
5. Deduplicate webhooks on `event_id`, and verify `X-AAI-Signature` against the raw body bytes because a 401 is not retried

### 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

## Questions

### Which is better for AI agents, AssemblyAI Voice Agent API or Bolna API + MCP?

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Bolna API + MCP's 52.6 (D), and leads in 5 of 7 scored categories. Bolna API + MCP leads on maintenance & community.

### Do AssemblyAI Voice Agent API and Bolna API + MCP need an API key?

Both need an API key.

### Can an agent call AssemblyAI Voice Agent API and Bolna API + MCP without installing anything?

Yes. AssemblyAI Voice Agent API has a hosted endpoint at wss://agents.assemblyai.com/v1/ws and Bolna API + MCP at https://api.bolna.ai.

### Are AssemblyAI Voice Agent API and Bolna API + MCP open source?

No open-source release is listed for AssemblyAI Voice Agent API. Bolna API + MCP is open source (MIT).


## For agents

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

## Other comparisons with AssemblyAI Voice Agent API or Bolna API + MCP

- [AssemblyAI Voice Agent API vs Bland AI API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bland-ai.md)
- [AssemblyAI Voice Agent API vs Deepgram Voice Agent API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-deepgram-voice-agent.md)
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- [AssemblyAI Voice Agent API vs Gemini Live API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-gemini-live.md)
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- [AssemblyAI Voice Agent API vs OpenAI Realtime API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-openai-realtime.md)
- [AssemblyAI Voice Agent API vs Retell AI API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-ai.md)
- [AssemblyAI Voice Agent API vs Synthflow API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow.md)
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- [Bolna API + MCP vs Deepgram Voice Agent API](https://www.anchorterminal.com/compare/bolna-vs-deepgram-voice-agent.md)
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- [Bolna API + MCP vs Synthflow API + MCP](https://www.anchorterminal.com/compare/bolna-vs-synthflow.md)
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