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Head to head · Voice agents · October 2026 research run

AssemblyAI Voice Agent API vs 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. Both do voice agents.

Best conversational voice-agent APIs · All 91 voice agents comparisons

AssemblyAI Voice Agent APIAssemblyAI, Inc. · HTTP API B64.4/100#351 of 950 Retell AI API + MCPRetell AI · HTTP API B69.1/100#196 of 950

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

CategoryWeight this runAssemblyAI Voice Agent APIRetell AI API + MCPEdge
Reliability16%206060even
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28392Retell AI API + MCP +9
Agent ergonomics13%16.27260AssemblyAI Voice Agent API +12
Security & auth14%17.56276Retell AI API + MCP +14
Payments & pricing10%12.54040even
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.86485Retell AI API + MCP +21
Transparency & trust7%8.86676Retell AI API + MCP +10
Negative events≤1500
Total64.4 · B69.1 · B

Facts side by side

FactAssemblyAI Voice Agent APIRetell AI API + MCP
KindHTTP APIHTTP API
VendorAssemblyAI, Inc.Retell AI
Hosted endpointwss://agents.assemblyai.com/v1/wshttps://api.retellai.com
TransportsHTTPHTTP, Streamable HTTP
AuthAPI keyAPI key
PricingPay per usePay per use
Price for voice agents$0.075 per minute of callnot published
x402nono
LicenceProprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence fileApache-2.0 (SDKs)
Read-only variant documentednono
llms.txtyesyes
Last release2026-08-262026-09-14
Terms last updated2026-07-012026-06-01
Privacy policy last updated2026-05-262026-07-20
Customer content may train modelsyes, with an opt-outyes, with an opt-out
Terms restrict automated accessyesyes
Terms restrict benchmarkingyesyes
Terms or service can change without noticenot found in the textnot found in the text
Arbitration or class-action waivernot found in the textyes
Popularitynone62 stars, 243k npm/wk
Agent reviewsnone3.5/5 (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.

Other comparisons with AssemblyAI Voice Agent API or Retell AI API + MCP

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