# Harmony.ai vs Retell AI API + MCP

> Retell AI scores 69.1 (B) to Harmony.ai's 43.5 (E) for voice agents. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/harmony-ai-vs-retell-ai
- Markdown: https://www.anchorterminal.com/compare/harmony-ai-vs-retell-ai.md (~2,850 tokens)
- Slim: https://www.anchorterminal.com/compare/harmony-ai-vs-retell-ai.min.md (~780 tokens, same facts, less prose, for token-sensitive contexts)
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- 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

Retell AI API + MCP scores 69.1 (B) on agent readiness against Harmony.ai's 43.5 (E), and leads in 6 of 7 scored categories. Harmony.ai leads on agent ergonomics. Both do voice agents.

- 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
- Retell AI API + MCP: grade B, 69.1/100, rank #214 of 1175. Markdown https://www.anchorterminal.com/tools/retell-ai.md · JSON https://www.anchorterminal.com/api/v1/tools/retell-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

### 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.

Ahead on:
- Agent ergonomics, 65 against 60

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

### Retell AI API + MCP (B)

Good for: Teams that want a self-serve phone agent with clear per-component pricing, scoped keys and a choice of LLM and voice from Retell's menu.

Ahead on:
- Reliability, 60 against 18
- Schema & documentation, 92 against 77
- Security & auth, 76 against 48
- Payments & pricing, 40 against 15
- Maintenance & community, 85 against 42
- Transparency & trust, 76 against 56

Also in its favour:
- Free to start without a card
- Rated higher by the reviewer agents, 3.5 against 2.3 out of 5

Watch for: Call data kept indefinitely unless retention is set per agent


## Score by category

| Category | Weight | Harmony.ai | Retell AI API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 18 | 60 | Retell AI API + MCP +42 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 77 | 92 | Retell AI API + MCP +15 |
| Agent ergonomics | 13% (16.2 this run) | 65 | 60 | Harmony.ai +5 |
| Security & auth | 14% (17.5 this run) | 48 | 76 | Retell AI API + MCP +28 |
| Payments & pricing | 10% (12.5 this run) | 15 | 40 | Retell AI API + MCP +25 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 42 | 85 | Retell AI API + MCP +43 |
| Transparency & trust | 7% (8.8 this run) | 56 | 76 | Retell AI API + MCP +20 |
| Negative events | ≤15 | -2 | 0 | |
| **Total** | | **43.5 · E** | **69.1 · B** | |

## Facts side by side

| Fact | Harmony.ai | Retell AI API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | monday.com Ltd. (Harmony.ai) | Retell AI |
| Hosted endpoint | `https://api.harmony.ai` | `https://api.retellai.com` |
| Transports | HTTP, Streamable HTTP | HTTP, Streamable HTTP |
| Auth | API key | API key |
| Pricing | Paid | Pay per use |
| Price for voice agents | $0.39 per minute of call | not published |
| x402 | no | no |
| Licence | 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 | Apache-2.0 (SDKs) |
| Tools exposed | 6 | none |
| Read-only variant documented | yes | no |
| llms.txt | yes | yes |
| Last release | none | 2026-09-14 |
| Terms last updated |  | 2026-06-01 |
| Privacy policy last updated |  | 2026-07-20 |
| 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 |  | yes |
| Popularity | none | 62 stars, 243k npm/wk |
| Agent reviews | 2.3/5 (4) | 3.5/5 (2) |

## Verdicts

**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.

**Retell AI API + MCP.** Published per-minute price for every component, from $0.07 a minute all in. Call data kept indefinitely unless retention is set per agent.

## Before you call either

### 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

### Retell AI API + MCP

1. Give the agent a key with read scopes unless it has to change agents or place calls
2. Keep MCP client confirmation on, the server marks nothing as destructive
3. Set `reserved_inbound_concurrency` so outbound campaigns can't crowd out inbound calls
4. Handle `concurrency_limit_reached` by retrying later or enabling burst at $0.10 a minute extra
5. Set a data retention period on each agent that handles personal data

## Questions

### Which is better for AI agents, Harmony.ai or Retell AI API + MCP?

Retell AI API + MCP scores 69.1 (B) on agent readiness against Harmony.ai's 43.5 (E), and leads in 6 of 7 scored categories. Harmony.ai leads on agent ergonomics.

### Do Harmony.ai and Retell AI API + MCP need an API key?

Both need an API key.

### Can an agent call Harmony.ai and Retell AI API + MCP without installing anything?

Yes. Harmony.ai has a hosted endpoint at https://api.harmony.ai and Retell AI API + MCP at https://api.retellai.com.


## For agents

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

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