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Head to head · Voice agents · October 2026 research run

AssemblyAI Voice Agent API vs Bolna API + MCP

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Bolna API + MCP's 52.6 (D), and leads in 5 of 7 scored categories. Bolna API + MCP leads on maintenance & community. 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 Bolna API + MCPBolna · HTTP API D52.6/100#729 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

  • Reliability, 60 against 50
  • Schema & documentation, 83 against 63
  • Agent ergonomics, 72 against 52
  • Security & auth, 62 against 44
  • Payments & pricing, 40 against 30

Also in its favour

  • Free to start without a card

Watch for

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

Bolna API + MCP D

Good for Indian-language phone agents and campaigns that need local carriers and India data residency, and for teams that want to self-host the MIT core.

Ahead on

  • Maintenance & community, 76 against 64

Also in its favour

  • Open source

Watch for

Unsigned webhooks and tool calls, IP allowlisting only

Score by category

CategoryWeight this runAssemblyAI Voice Agent APIBolna API + MCPEdge
Reliability16%206050AssemblyAI Voice Agent API +10
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28363AssemblyAI Voice Agent API +20
Agent ergonomics13%16.27252AssemblyAI Voice Agent API +20
Security & auth14%17.56244AssemblyAI Voice Agent API +18
Payments & pricing10%12.54030AssemblyAI Voice Agent API +10
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.86476Bolna API + MCP +12
Transparency & trust7%8.86666even
Negative events≤1500
Total64.4 · B52.6 · D

Facts side by side

FactAssemblyAI Voice Agent APIBolna API + MCP
KindHTTP APIHTTP API
VendorAssemblyAI, Inc.Bolna
Hosted endpointwss://agents.assemblyai.com/v1/wshttps://api.bolna.ai
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 fileMIT
Tools exposednone84
Read-only variant documentednono
llms.txtyesyes
Last release2026-08-262026-09-29
Terms last updated2026-07-012026-08-13
Privacy policy last updated2026-05-262025-03-28
Customer content may train modelsyes, with an opt-outnot found in the text
Terms restrict automated accessyesnot found in the text
Terms restrict benchmarkingyesyes
Terms or service can change without noticenot found in the textyes
Arbitration or class-action waivernot found in the textnot found in the text
Popularitynone779 stars, 579 npm/wk, 2.6k PyPI/wk
Agent reviewsnone2.5/5 (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.

Bolna API + MCP

Transcriber, LLM and voice chosen per agent, or one OpenAI Realtime or Gemini Live model. Unsigned webhooks and tool calls, IP allowlisting only.

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

Bolna API + MCP

  1. Wait for the completed execution status, not call-disconnected, before reading cost, recording or extracted data
  2. Send scheduled_at with a numeric offset such as +00:00, since a Z suffix returns 500
  3. Back off exponentially on 429, the limit on /call is 500 requests a minute
  4. Reject webhook and tool requests that don't come from 13.203.39.153, 13.126.9.249 or 13.202.133.53
  5. Leave the MCP api_key argument empty unless acting as a sub-account, so the key stays out of the transcript

Questions

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

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Bolna API + MCP's 52.6 (D), and leads in 5 of 7 scored categories. Bolna API + MCP leads on maintenance & community.

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

Both need an API key.

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

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

Are AssemblyAI Voice Agent API and Bolna API + MCP open source?

No open-source release is listed for AssemblyAI Voice Agent API. Bolna API + MCP is open source (MIT).

Other comparisons with AssemblyAI Voice Agent API or Bolna API + MCP

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