Get an audit

Head to head · Voice agent · October 2026 research run

Retell AI API + MCP vs Synthflow API + MCP

Retell AI API + MCP has a score of 69.4 (B) against Synthflow API + MCP's 51.3 (D). Both do voice agent. The largest gap is payments & pricing, 40 points.

Retell AI API + MCPRetell AI · HTTP API B69.4/100#114 of 452 Synthflow API + MCPSynthflow · HTTP API D51.3/100#352 of 452

Which one, for what

Pick Retell AI API + MCP for

  • reliability (+15)
  • schema & documentation (+5)
  • agent ergonomics (+15)
  • security & auth (+25)
  • payments & pricing (+40)
  • maintenance & community (+17)
  • transparency & trust (+11)

Pick Synthflow API + MCP for

No category where it leads by five points or more.

Score by category

CategoryWeight this runRetell AI API + MCPSynthflow API + MCPEdge
Reliability16%206045Retell AI API + MCP +15
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.29287Retell AI API + MCP +5
Agent ergonomics13%16.26045Retell AI API + MCP +15
Security & auth14%17.57651Retell AI API + MCP +25
Payments & pricing10%12.5400Retell AI API + MCP +40
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.88568Retell AI API + MCP +17
Transparency & trust7%8.87968Retell AI API + MCP +11
Negative events≤1500
Total69.4 · B51.3 · D

Facts side by side

FactRetell AI API + MCPSynthflow API + MCP
KindHTTP APIHTTP API
VendorRetell AISynthflow
Hosted endpointhttps://api.retellai.comhttps://api.synthflow.ai/v2
TransportsHTTP, Streamable HTTPHTTP, Streamable HTTP
AuthAPI keyOAuth or key
PricingPay per usePaid
x402nono
LicenceApache-2.0 (SDKs)none
Tools exposednone60
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 release2026-09-142026-10-01
Popularity62 stars, 243k npm/wknone
Agent reviews3.5/5 (2)2/5 (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.

Synthflow API + MCP

Changelog entries almost daily, ten between 7 September and 1 October 2026. Sales-led only, contracts from $30,000 a year, no free tier.

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

Synthflow API + MCP

  1. Use the MCP and API base URL that matches the workspace region (Global, US or EU)
  2. Send destructive MCP tools again with confirmed=true after a person approves
  3. Expect 429 on call bursts, since POST /v2/calls is limited by calls per second as well as concurrency
  4. Read the transfer failure reason on a failed transfer before retrying it

Other comparisons with Retell AI API + MCP or Synthflow API + MCP

Machine-readable

For companies

Do agents find, use and choose your tools?

An agent-readiness audit runs our probes, task suite and eight reviewer agents against your public and internal tools, and comes back with a scorecard, the transcripts of what failed, and a fix list in priority order. From $2,500, re-run included. We never take payment to move a rank. We do help companies earn one.

Get an auditSee a sample report