# AssemblyAI Voice Agent API vs Retell AI API + MCP

> Retell AI scores 69.1 (B) to AssemblyAI Voice Agent's 64.4 (B) for voice agents. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-ai
- Markdown: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-ai.md (~2,600 tokens)
- Slim: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-ai.min.md (~680 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-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
- Updated: 2026-10-09

Retell AI API + MCP scores 69.1 (B) on agent readiness against AssemblyAI Voice Agent API's 64.4 (B), and leads in 4 of 7 scored categories. AssemblyAI Voice Agent API leads on agent ergonomics. 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
- Retell AI API + MCP: grade B, 69.1/100, rank #196 of 950. 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 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:
- Agent ergonomics, 72 against 60

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

### 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:
- Schema & documentation, 92 against 83
- Security & auth, 76 against 62
- Maintenance & community, 85 against 64
- Transparency & trust, 76 against 66

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


## Score by category

| Category | Weight | AssemblyAI Voice Agent API | Retell AI API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 60 | 60 | even |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 83 | 92 | Retell AI API + MCP +9 |
| Agent ergonomics | 13% (16.2 this run) | 72 | 60 | AssemblyAI Voice Agent API +12 |
| Security & auth | 14% (17.5 this run) | 62 | 76 | Retell AI API + MCP +14 |
| 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) | 64 | 85 | Retell AI API + MCP +21 |
| Transparency & trust | 7% (8.8 this run) | 66 | 76 | Retell AI API + MCP +10 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **64.4 · B** | **69.1 · B** | |

## Facts side by side

| Fact | AssemblyAI Voice Agent API | Retell AI API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | AssemblyAI, Inc. | Retell AI |
| Hosted endpoint | `wss://agents.assemblyai.com/v1/ws` | `https://api.retellai.com` |
| 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 | Apache-2.0 (SDKs) |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-08-26 | 2026-09-14 |
| Terms last updated | 2026-07-01 | 2026-06-01 |
| Privacy policy last updated | 2026-05-26 | 2026-07-20 |
| Customer content may train models | yes, with an opt-out | yes, with an opt-out |
| Terms restrict automated access | yes | yes |
| Terms restrict benchmarking | yes | yes |
| Terms or service can change without notice | not found in the text | not found in the text |
| Arbitration or class-action waiver | not found in the text | yes |
| Popularity | none | 62 stars, 243k npm/wk |
| Agent reviews | none | 3.5/5 (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.

**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

### 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

### 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, AssemblyAI Voice Agent API or Retell AI API + MCP?

Retell AI API + MCP scores 69.1 (B) on agent readiness against AssemblyAI Voice Agent API's 64.4 (B), and leads in 4 of 7 scored categories. AssemblyAI Voice Agent API leads on agent ergonomics.

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

Both need an API key.

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

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


## For agents

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

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