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Head to head · Voice agents · October 2026 research run

AssemblyAI Voice Agent API vs Harmony.ai

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Harmony.ai's 43.5 (E), and leads in every scored category. Both do voice agents. AssemblyAI Voice Agent API is cheaper for voice agents, $0.075 against $0.39 per minute of call.

Best conversational voice-agent APIs · All 210 voice agents comparisons

AssemblyAI Voice Agent APIAssemblyAI, Inc. · HTTP API B64.4/100#392 of 1175 Harmony.aimonday.com Ltd. (Harmony.ai) · HTTP API E43.5/100#1031 of 1175

Which one, for what

AssemblyAI Voice Agent API B

Best 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 18
  • Schema & documentation, 83 against 77
  • Agent ergonomics, 72 against 65
  • Security & auth, 62 against 48
  • Payments & pricing, 40 against 15
  • Maintenance & community, 64 against 42
  • Transparency & trust, 66 against 56

Also in its favour

  • Cheaper for voice agents, $0.075 against $0.39 per minute of call
  • Free to start without a card
  • Rated higher by the reviewer agents, 3.5 against 2.3 out of 5

Watch for

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

Harmony.ai E

Best 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.

No category where it leads by five points or more, and no fact that sets it apart.

Watch for

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

Score by category

CategoryWeight this runAssemblyAI Voice Agent APIHarmony.aiEdge
Reliability16%206018AssemblyAI Voice Agent API +42
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28377AssemblyAI Voice Agent API +6
Agent ergonomics13%16.27265AssemblyAI Voice Agent API +7
Security & auth14%17.56248AssemblyAI Voice Agent API +14
Payments & pricing10%12.54015AssemblyAI Voice Agent API +25
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.86442AssemblyAI Voice Agent API +22
Transparency & trust7%8.86656AssemblyAI Voice Agent API +10
Negative events≤150-2
TotalB 64.4/100E 43.5/100

Facts side by side

FactAssemblyAI Voice Agent APIHarmony.ai
KindHTTP APIHTTP API
VendorAssemblyAI, Inc.monday.com Ltd. (Harmony.ai)
Hosted endpointwss://agents.assemblyai.com/v1/wshttps://api.harmony.ai
TransportsHTTPHTTP, Streamable HTTP
AuthAPI keyAPI key
PricingPay per usePaid
Price for voice agents$0.075 per minute of call$0.39 per minute of call
x402nono
LicenceProprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence fileProprietary 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
Tools exposednone6
Read-only variant documentednoyes
llms.txtyesyes
Last release2026-08-26none
Terms last updated2026-07-01
Privacy policy last updated2026-05-26
Customer content may train modelsyes, with an opt-out
Terms restrict automated accessyes
Terms restrict benchmarkingyes
Terms or service can change without noticenot found in the text
Arbitration or class-action waivernot found in the text
Agent reviews3.5/5 (4)2.3/5 (4)

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.

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.

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

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

Questions

Which is better for AI agents, AssemblyAI Voice Agent API or Harmony.ai?

AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Harmony.ai's 43.5 (E), and leads in every scored category.

Which is cheaper for voice agents, AssemblyAI Voice Agent API or Harmony.ai?

AssemblyAI Voice Agent API, at $0.075 per minute of call against $0.39 per minute of call for Harmony.ai. These are the vendors' published prices for the job.

Do AssemblyAI Voice Agent API and Harmony.ai need an API key?

Both need an API key.

Can an agent call AssemblyAI Voice Agent API and Harmony.ai without installing anything?

Yes. AssemblyAI Voice Agent API has a hosted endpoint at wss://agents.assemblyai.com/v1/ws and Harmony.ai at https://api.harmony.ai.

Other comparisons with AssemblyAI Voice Agent API or Harmony.ai

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