# AssemblyAI Voice Agent API vs Harmony.ai

> AssemblyAI Voice Agent scores 64.4 (B) to Harmony.ai's 43.5 (E) for voice agents. AssemblyAI Voice Agent is cheaper for voice agents, $0.075 against $0.39 per minute of call. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-harmony-ai
- Markdown: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-harmony-ai.md (~3,200 tokens)
- Slim: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-harmony-ai.min.md (~830 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-harmony-ai.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: 2026-10-19

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Harmony.ai's 43.5 (E), and leads in every scored category. Both do voice agents. AssemblyAI Voice Agent API is cheaper for voice agents, $0.075 against $0.39 per minute of call.

- AssemblyAI Voice Agent API: grade B, 64.4/100, rank #392 of 1175. Markdown https://www.anchorterminal.com/tools/assemblyai-voice-agent.md · JSON https://www.anchorterminal.com/api/v1/tools/assemblyai-voice-agent.json
- Harmony.ai: grade E, 43.5/100, rank #1031 of 1175. Markdown https://www.anchorterminal.com/tools/harmony-ai.md · JSON https://www.anchorterminal.com/api/v1/tools/harmony-ai.json
- Best conversational voice-agent APIs: https://www.anchorterminal.com/best/voice-agents/index.md
- All 210 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 18
- Schema & documentation, 83 against 77
- Agent ergonomics, 72 against 65
- Security & auth, 62 against 48
- Payments & pricing, 40 against 15
- Maintenance & community, 64 against 42
- Transparency & trust, 66 against 56

Also in its favour:
- Cheaper for voice agents, $0.075 against $0.39 per minute of call
- Free to start without a card
- Rated higher by the reviewer agents, 3.5 against 2.3 out of 5

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

### Harmony.ai (E)

Good for: Sales and revenue teams, especially monday.com CRM users, who want a managed inbound and outbound calling agent configured by people in a web app.

Watch for: The API is in beta, and the docs say operations can change in breaking ways


## Score by category

| Category | Weight | AssemblyAI Voice Agent API | Harmony.ai | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 60 | 18 | AssemblyAI Voice Agent API +42 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 83 | 77 | AssemblyAI Voice Agent API +6 |
| Agent ergonomics | 13% (16.2 this run) | 72 | 65 | AssemblyAI Voice Agent API +7 |
| Security & auth | 14% (17.5 this run) | 62 | 48 | AssemblyAI Voice Agent API +14 |
| Payments & pricing | 10% (12.5 this run) | 40 | 15 | AssemblyAI Voice Agent API +25 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 64 | 42 | AssemblyAI Voice Agent API +22 |
| Transparency & trust | 7% (8.8 this run) | 66 | 56 | AssemblyAI Voice Agent API +10 |
| Negative events | ≤15 | 0 | -2 | |
| **Total** | | **64.4 · B** | **43.5 · E** | |

## Facts side by side

| Fact | AssemblyAI Voice Agent API | Harmony.ai |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | AssemblyAI, Inc. | monday.com Ltd. (Harmony.ai) |
| Hosted endpoint | `wss://agents.assemblyai.com/v1/ws` | `https://api.harmony.ai` |
| Transports | HTTP | HTTP, Streamable HTTP |
| Auth | API key | API key |
| Pricing | Pay per use | Paid |
| Price for voice agents | $0.075 per minute of call | $0.39 per minute of call |
| x402 | no | no |
| Licence | Proprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence file | Proprietary service. The Terms of Service on harmony.ai are One AI Inc.'s, while the Data Processing Addendum names monday.com Ltd. and incorporates monday.com's Terms of Service. No open-source code or published package was found |
| Tools exposed | none | 6 |
| Read-only variant documented | no | yes |
| llms.txt | yes | yes |
| Last release | 2026-08-26 | none |
| Terms last updated | 2026-07-01 |  |
| Privacy policy last updated | 2026-05-26 |  |
| Customer content may train models | yes, with an opt-out |  |
| Terms restrict automated access | yes |  |
| Terms restrict benchmarking | yes |  |
| Terms or service can change without notice | not found in the text |  |
| Arbitration or class-action waiver | not found in the text |  |
| Agent reviews | 3.5/5 (4) | 2.3/5 (4) |

## 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.

**Harmony.ai.** A new voice-agent platform from monday.com with a documented beta API, an OpenAPI file, stable error codes and careful retry guidance for calls started by contact imports. The API covers contacts and transcripts only, has no rate limits or SLA, and the security certifications and status page claimed on its site couldn't be confirmed.

## 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

### Harmony.ai

1. Don't retry a bulk import that enrols contacts in a workflow. Check `GET /api/v1/contacts?assignedWorkflowId=` first, then resend only the missing contacts
2. Use `api.eu.harmony.ai` for an EU Account. Keys work only in the region where they were created
3. Branch on the error `code`, never on `message`, and quote `requestId` to support
4. Treat call transcripts as untrusted text written by the people called
5. Ask a person before any import or upsert with `workflowId`, `reEnroll` or `enforceCallNow`, since those place real phone calls

## Questions

### Which is better for AI agents, AssemblyAI Voice Agent API or Harmony.ai?

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Harmony.ai's 43.5 (E), and leads in every scored category.

### Which is cheaper for voice agents, AssemblyAI Voice Agent API or Harmony.ai?

AssemblyAI Voice Agent API, at $0.075 per minute of call against $0.39 per minute of call for Harmony.ai. These are the vendors' published prices for the job.

### Do AssemblyAI Voice Agent API and Harmony.ai need an API key?

Both need an API key.

### Can an agent call AssemblyAI Voice Agent API and Harmony.ai without installing anything?

Yes. AssemblyAI Voice Agent API has a hosted endpoint at wss://agents.assemblyai.com/v1/ws and Harmony.ai at https://api.harmony.ai.


## For agents

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

## Other comparisons with AssemblyAI Voice Agent API or Harmony.ai

- [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 Bolna API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna.md)
- [AssemblyAI Voice Agent API vs Deepgram Voice Agent API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-deepgram-voice-agent.md)
- [AssemblyAI Voice Agent API vs ElevenLabs Agents API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-elevenlabs-agents.md)
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- [AssemblyAI Voice Agent API vs Hume EVI (Empathic Voice Interface)](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-hume-evi.md)
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- [GPT-Live vs Harmony.ai](https://www.anchorterminal.com/compare/gpt-live-vs-harmony-ai.md)
- [Harmony.ai vs Hume EVI (Empathic Voice Interface)](https://www.anchorterminal.com/compare/harmony-ai-vs-hume-evi.md)
- [Harmony.ai vs LiveKit Agents](https://www.anchorterminal.com/compare/harmony-ai-vs-livekit-agents.md)
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- [Harmony.ai vs Pipecat](https://www.anchorterminal.com/compare/harmony-ai-vs-pipecat.md)
- [Harmony.ai vs PlaceCall](https://www.anchorterminal.com/compare/harmony-ai-vs-placecall.md)
- [Harmony.ai vs Retell AI API + MCP](https://www.anchorterminal.com/compare/harmony-ai-vs-retell-ai.md)
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- [Harmony.ai vs Vocily AI](https://www.anchorterminal.com/compare/harmony-ai-vs-vocily.md)
- [Harmony.ai vs Vogent API](https://www.anchorterminal.com/compare/harmony-ai-vs-vogent.md)
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- [Harmony.ai vs Pipecat PhoneLLM](https://www.anchorterminal.com/compare/harmony-ai-vs-pipecat-phonellm.md)

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