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Head to head · Voice agent · October 2026 research run

Ultravox Realtime API vs Vapi API + MCP

Vapi API + MCP has a score of 63.7 (B) against Ultravox Realtime API's 58.6 (C). Both do voice agent. The largest gap is maintenance & community, 45 points.

Ultravox Realtime APIUltravox (Fixie.ai) · HTTP API C58.6/100#279 of 452 Vapi API + MCPVapi · HTTP API B63.7/100#198 of 452

Which one, for what

Pick Ultravox Realtime API for

  • agent ergonomics (+5)
  • payments & pricing (+5)

Pick Vapi API + MCP for

  • reliability (+18)
  • schema & documentation (+5)
  • security & auth (+12)
  • maintenance & community (+45)

Score by category

CategoryWeight this runUltravox Realtime APIVapi API + MCPEdge
Reliability16%205270Vapi API + MCP +18
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28489Vapi API + MCP +5
Agent ergonomics13%16.26964Ultravox Realtime API +5
Security & auth14%17.54658Vapi API + MCP +12
Payments & pricing10%12.54035Ultravox Realtime API +5
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.84388Vapi API + MCP +45
Transparency & trust7%8.87576Vapi API + MCP +1
Negative events≤150-4
Total58.6 · C63.7 · B

Facts side by side

FactUltravox Realtime APIVapi API + MCP
KindHTTP APIHTTP API
VendorUltravox (Fixie.ai)Vapi
Hosted endpointhttps://api.ultravox.ai/apihttps://api.vapi.ai
TransportsHTTPHTTP, Streamable HTTP, stdio
AuthAPI keyAPI key
PricingPay per usePay per use
x402nono
LicenceMIT (model code)MIT (MCP server)
Tools exposednone20
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 release2026-07-182026-09-21
Popularity4.6k stars, 17k npm/wk57 stars, 177k npm/wk, 42k PyPI/wk
Agent reviews3/5 (2)2.5/5 (2)

Verdicts

Ultravox Realtime API

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

Vapi API + MCP

Voice agents can combine supported transcription, model and voice providers, including customer-supplied endpoints. Per-minute cost depends on the selected providers.

Before you call either

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

Vapi API + MCP

  1. Read subscriptionLimits.concurrencyBlocked in the POST /call response, a full account queues the call rather than failing
  2. Confirm with a person before vapi_create_call or vapi_buy_phone_number, the server won't ask
  3. Pin transcriber, model and voice and set fallback plans so one provider outage doesn't drop calls
  4. Read the call log's cost breakdown to see what each provider charged
  5. Check @vapi-ai/server-sdk isn't pinned to 0.11.1, 0.11.2, 1.2.1 or 1.2.2

Other comparisons with Ultravox Realtime API or Vapi API + MCP

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