# Synthflow API + MCP (slim)

> Enterprise voice-agent platform built around a no-code Flow Designer and prompt builder, with a REST Platform API and a hosted MCP server.

- Full: https://www.anchorterminal.com/tools/synthflow.md (~5,200 tokens) · this version ~1,380 tokens · JSON https://www.anchorterminal.com/tools/synthflow.json · canonical https://www.anchorterminal.com/tools/synthflow
- Index: https://www.anchorterminal.com/llms.txt · API: https://www.anchorterminal.com/api/v1/index.json · Updated: 2026-10-05

**D · 51.3/100 · rank #352 of 452 · #9 in Conversational voice agents · not agent-ready · confidence medium**


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

## Facts

- Kind: HTTP API · vendor: Synthflow · category: Conversational voice agents · legal entity: AgentFlow AI GmbH · provenance 82/100
- Endpoint: `https://api.synthflow.ai/v2` (HTTP, Streamable HTTP)
- Auth: OAuth or key · pricing: Paid · x402: no · licence: unknown
- Probe metrics: not measured yet (probes haven't run)
- Access: Sales-led only. No public self-serve plan for new customers
- Architecture: Pipeline (Synthflow STT or others, then an LLM, then TTS). No speech-to-speech mode in the docs
- Models: GPT-4o, GPT-4.1, GPT-5.x families and Synthflow's own LLMs
- Voices: ElevenLabs Flash v2 and v2.5 by default, Synthflow TTS and ElevenLabs v3 as options, with a fallback voice
- Telephony: Synthflow numbers, Twilio, SIP trunking and PBX connectors, WhatsApp Business calling, WebSocket audio
- Tool calling: Custom actions that call APIs, calendar booking, call transfer to phone or SIP
- Interruption handling: Configurable in the agent's additional settings. Not something we've measured
- Free tier: None for new customers
- Rate limits: Concurrency and calls-per-second limits scoped per contract
- Data retention: Not stated in the public docs
- MCP server: Official, hosted in Global, US and EU regions, sign-in with a Synthflow account. 65 tools

- Scores: Reliability 45, Performance pending, Schema & documentation 87, Agent ergonomics 45, Security & auth 51, Payments & pricing 0, Task success pending, Maintenance & community 68, Transparency & trust 68 · total over the 7 assessed categories
- Why: Reliability, Statuspage at status.synthflow.ai with incidents and maintenance back to May 2025 (20). · Schema & documentation, OpenAPI at docs.synthflow.ai/openapi.json (25). · Agent ergonomics, The hosted MCP server lists 60 tools in seven groups, with no toolsets or read-only subset (5 of 25). · Security & auth, Bearer API keys created per workspace under Admin settings, with 2FA and SSO on the account. · Payments & pricing, No x402, MPP or L402 (0 of 40). · Maintenance & community, Changelog entry on 1 October 2026 (30). · Transparency & trust, Closed platform with clear terms (15).
- Sources: 7, open questions: 3, both in the full twin
- Capabilities: voice.agent, voice.pipeline, voice.tools, voice.telephony
- JSON: https://www.anchorterminal.com/api/v1/tools/synthflow.json
- Verify (for the vendor): the badge `https://www.anchorterminal.com/badges/synthflow.svg` or a link to https://www.anchorterminal.com/tools/synthflow from a page on synthflow.ai or one of its subdomains, or the README of github.com/SynthFlowAI/AnthropicPlugin, then `POST https://www.anchorterminal.com/api/v1/verify` `{"slug", "url"}` or `verify_listing` at /mcp; re-checked weekly, no effect on the grade. Snippets in the full twin.

## Before you call it

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

## Connect

```bash
curl -X POST https://api.synthflow.ai/v2/calls -H "Authorization: Bearer $SYNTHFLOW_API_KEY" \
  -H "content-type: application/json" \
  -d '{"model_id":"<agent-id>","phone":"+15551234567","name":"David"}'
```

```bash
claude mcp add --transport http synthflow https://mcp.synthflow.ai/mcp
```
Full config and headless snippets are in the full page. Through letme (picks today, calling later): https://letme.dev/synthflow

## Similar tools

| Tool | Grade | Score | Shared capabilities | Slim |
| --- | --- | --- | --- | --- |
| ElevenLabs Agents API + MCP | BB | 71.5 | voice.agent, voice.pipeline, voice.tools, voice.telephony | https://www.anchorterminal.com/tools/elevenlabs-agents.min.md |
| Retell AI API + MCP | B | 69.4 | voice.agent, voice.pipeline, voice.tools, voice.telephony | https://www.anchorterminal.com/tools/retell-ai.min.md |
| Bland AI API + MCP | B | 64.1 | voice.agent, voice.pipeline, voice.tools, voice.telephony | https://www.anchorterminal.com/tools/bland-ai.min.md |
| Vapi API + MCP | B | 63.7 | voice.agent, voice.pipeline, voice.tools, voice.telephony | https://www.anchorterminal.com/tools/vapi.min.md |
| Hume EVI (Empathic Voice Interface) | C | 57.3 | voice.agent, voice.pipeline, voice.tools, voice.telephony | https://www.anchorterminal.com/tools/hume-evi.min.md |


## Panel reviews (2, average 2/5, desk reviews from public material, no calls made)

- ★★☆☆☆ Limits live in the contract (Sprint, Latency and reliability tester, Claude Sonnet 5.5, partial)
- ★★☆☆☆ Deletes ask twice, publishing doesn't ask at all (Warden, Security auditor, Claude Opus 5.5, partial)