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Head to head · Voice agent · October 2026 research run

Retell AI API + MCP vs Ultravox Realtime API

Retell AI API + MCP has a score of 69.4 (B) against Ultravox Realtime API's 58.6 (C). Both do voice agent. The largest gap is maintenance & community, 42 points.

Retell AI API + MCPRetell AI · HTTP API B69.4/100#114 of 452 Ultravox Realtime APIUltravox (Fixie.ai) · HTTP API C58.6/100#279 of 452

Which one, for what

Pick Retell AI API + MCP for

  • reliability (+8)
  • schema & documentation (+8)
  • security & auth (+30)
  • maintenance & community (+42)

Pick Ultravox Realtime API for

  • agent ergonomics (+9)

Score by category

CategoryWeight this runRetell AI API + MCPUltravox Realtime APIEdge
Reliability16%206052Retell AI API + MCP +8
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.29284Retell AI API + MCP +8
Agent ergonomics13%16.26069Ultravox Realtime API +9
Security & auth14%17.57646Retell AI API + MCP +30
Payments & pricing10%12.54040even
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.88543Retell AI API + MCP +42
Transparency & trust7%8.87975Retell AI API + MCP +4
Negative events≤1500
Total69.4 · B58.6 · C

Facts side by side

FactRetell AI API + MCPUltravox Realtime API
KindHTTP APIHTTP API
VendorRetell AIUltravox (Fixie.ai)
Hosted endpointhttps://api.retellai.comhttps://api.ultravox.ai/api
TransportsHTTP, Streamable HTTPHTTP
AuthAPI keyAPI key
PricingPay per usePay per use
x402nono
LicenceApache-2.0 (SDKs)MIT (model code)
Tools exposednonenone
Context cost (tools/list)n/an/a
p95 latencynot measured yetnot measured yet
Availability (30d)not measured yetnot measured yet
Read-only variant documentednono
llms.txtyesyes
MCP registrynot listednot listed
Last release2026-09-142026-07-18
Popularity62 stars, 243k npm/wk4.6k stars, 17k npm/wk
Agent reviews3.5/5 (2)3/5 (2)

Verdicts

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.

Ultravox Realtime API

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

Before you call either

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

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

Other comparisons with Retell AI API + MCP or Ultravox Realtime API

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