# AssemblyAI Voice Agent API vs Vapi API + MCP

> AssemblyAI Voice Agent and Vapi score within a point of each other for voice agents, 64.4 and 63.5 out of 100. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi
- Markdown: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi.md (~2,650 tokens)
- Slim: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi.min.md (~780 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi.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 and Vapi API + MCP score within a point of each other on agent readiness, 64.4 (B) and 63.5 (B). Vapi API + MCP leads on reliability, schema & documentation, maintenance & community and transparency & trust. 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
- Vapi API + MCP: grade B, 63.5/100, rank #390 of 950. Markdown https://www.anchorterminal.com/tools/vapi.md · JSON https://www.anchorterminal.com/api/v1/tools/vapi.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 64
- Payments & pricing, 40 against 35

Also in its favour:
- No incidents deducted, where Vapi API + MCP loses 4 points for them

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

### Vapi API + MCP (B)

Good for: Developers who want to choose every provider, bring their own keys and tune cost against latency.

Ahead on:
- Reliability, 70 against 60
- Schema & documentation, 89 against 83
- Maintenance & community, 88 against 64
- Transparency & trust, 73 against 66

Also in its favour:
- Runs on your own machine

Watch for: Real per-minute cost depends on provider choices


## Score by category

| Category | Weight | AssemblyAI Voice Agent API | Vapi API + MCP | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 60 | 70 | Vapi API + MCP +10 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 83 | 89 | Vapi API + MCP +6 |
| Agent ergonomics | 13% (16.2 this run) | 72 | 64 | AssemblyAI Voice Agent API +8 |
| Security & auth | 14% (17.5 this run) | 62 | 58 | AssemblyAI Voice Agent API +4 |
| Payments & pricing | 10% (12.5 this run) | 40 | 35 | AssemblyAI Voice Agent API +5 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 64 | 88 | Vapi API + MCP +24 |
| Transparency & trust | 7% (8.8 this run) | 66 | 73 | Vapi API + MCP +7 |
| Negative events | ≤15 | 0 | -4 | |
| **Total** | | **64.4 · B** | **63.5 · B** | |

## Facts side by side

| Fact | AssemblyAI Voice Agent API | Vapi API + MCP |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | AssemblyAI, Inc. | Vapi |
| Hosted endpoint | `wss://agents.assemblyai.com/v1/ws` | `https://api.vapi.ai` |
| Transports | HTTP | HTTP, Streamable HTTP, stdio |
| 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 (MCP server) |
| Tools exposed | none | 20 |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-08-26 | 2026-09-21 |
| Terms last updated | 2026-07-01 | 2026-09-14 |
| Privacy policy last updated | 2026-05-26 | 2023-09-09 |
| Customer content may train models | yes, with an opt-out | yes |
| Terms restrict automated access | yes | yes |
| Terms restrict benchmarking | yes | not found in the text |
| Terms or service can change without notice | not found in the text | yes |
| Arbitration or class-action waiver | not found in the text | yes |
| Popularity | none | 57 stars, 177k npm/wk, 42k 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.

**Vapi API + MCP.** Voice agents can combine supported transcription, model and voice providers, including customer-supplied endpoints. Per-minute cost depends on the selected providers.

## 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

### Vapi API + MCP

1. Read `subscriptionLimits.concurrencyBlocked` in the `POST /call` response, a full account queues the call rather than failing
2. Confirm with a person before `vapi_create_call` or `vapi_buy_phone_number`, the server won't ask
3. Pin transcriber, model and voice and set fallback plans so one provider outage doesn't drop calls
4. Read the call log's cost breakdown to see what each provider charged
5. Check `@vapi-ai/server-sdk` isn't pinned to 0.11.1, 0.11.2, 1.2.1 or 1.2.2

## Questions

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

AssemblyAI Voice Agent API and Vapi API + MCP score within a point of each other on agent readiness, 64.4 (B) and 63.5 (B). Vapi API + MCP leads on reliability, schema & documentation, maintenance & community and transparency & trust.

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

Both need an API key.

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

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


## For agents

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

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