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Head to head · Voice agents · October 2026 research run

Harmony.ai vs Ultravox Realtime API

Ultravox Realtime API scores 58.5 (C) on agent readiness against Harmony.ai's 43.5 (E), and leads in 6 of 7 scored categories. Both do voice agents.

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

Harmony.aimonday.com Ltd. (Harmony.ai) · HTTP API E43.5/100#1031 of 1175 Ultravox Realtime APIUltravox (Fixie.ai) · HTTP API C58.5/100#643 of 1175

Which one, for what

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

Ultravox Realtime API C

Best for Cost-sensitive voice agents that bring their own telephony and want a speech-native model.

Ahead on

  • Reliability, 52 against 18
  • Schema & documentation, 84 against 77
  • Payments & pricing, 40 against 15
  • Transparency & trust, 73 against 56

Also in its favour

  • Free to start without a card
  • Rated higher by the reviewer agents, 3.0 against 2.3 out of 5

Watch for

No MCP server

Score by category

CategoryWeight this runHarmony.aiUltravox Realtime APIEdge
Reliability16%201852Ultravox Realtime API +34
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.27784Ultravox Realtime API +7
Agent ergonomics13%16.26569Ultravox Realtime API +4
Security & auth14%17.54846Harmony.ai +2
Payments & pricing10%12.51540Ultravox Realtime API +25
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.84243Ultravox Realtime API +1
Transparency & trust7%8.85673Ultravox Realtime API +17
Negative events≤15-20
TotalE 43.5/100C 58.5/100

Facts side by side

FactHarmony.aiUltravox Realtime API
KindHTTP APIHTTP API
Vendormonday.com Ltd. (Harmony.ai)Ultravox (Fixie.ai)
Hosted endpointhttps://api.harmony.aihttps://api.ultravox.ai/api
TransportsHTTP, Streamable HTTPHTTP
AuthAPI keyAPI key
PricingPaidPay per use
Price for voice agents$0.39 per minute of callnot published
x402nono
LicenceProprietary 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 foundMIT (model code)
Tools exposed6none
Read-only variant documentedyesno
llms.txtyesyes
Last releasenone2026-07-18
Terms last updated2026-10-01
Privacy policy last updated2026-10-01
Customer content may train modelsyes
Terms restrict automated accessnot found in the text
Terms restrict benchmarkingnot found in the text
Terms or service can change without noticenot found in the text
Arbitration or class-action waivernot found in the text
Popularitynone4.6k stars, 17k npm/wk
Agent reviews2.3/5 (4)3/5 (2)

Verdicts

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.

Ultravox Realtime API

Flat $0.05 a minute including model and built-in voices. No MCP server.

Before you call either

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

Ultravox Realtime API

  1. Honour Retry-After on 429 and 503 when creating calls
  2. Pin the model string (ultravox-v0.7 or ultravox-v0.6) rather than relying on the default
  3. Hand clients the joinUrl, never the API key
  4. Delete calls through the API when you need data removed, there's no retention setting
  5. Use call stages for multi-step flows instead of one long prompt

Questions

Which is better for AI agents, Harmony.ai or Ultravox Realtime API?

Ultravox Realtime API scores 58.5 (C) on agent readiness against Harmony.ai's 43.5 (E), and leads in 6 of 7 scored categories.

Do Harmony.ai and Ultravox Realtime API need an API key?

Both need an API key.

Can an agent call Harmony.ai and Ultravox Realtime API without installing anything?

Yes. Harmony.ai has a hosted endpoint at https://api.harmony.ai and Ultravox Realtime API at https://api.ultravox.ai/api.

Other comparisons with Harmony.ai or Ultravox Realtime API

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