{ "data": { "a": { "slug": "assemblyai-voice-agent", "name": "AssemblyAI Voice Agent API", "vendor": "AssemblyAI, Inc.", "vendorUrl": "https://www.assemblyai.com", "kind": "http-api", "category": "voice-agents", "summary": "AssemblyAI's Voice Agent API runs a spoken conversation over one WebSocket, combining its own speech-to-text, language model and text-to-speech. Agents are stored through a REST API and reached from a browser, a server or an inbound Twilio SIP number.", "url": "https://www.anchorterminal.com/tools/assemblyai-voice-agent", "markdownUrl": "https://www.anchorterminal.com/tools/assemblyai-voice-agent.md", "slimMarkdownUrl": "https://www.anchorterminal.com/tools/assemblyai-voice-agent.min.md", "jsonUrl": "https://www.anchorterminal.com/api/v1/tools/assemblyai-voice-agent.json", "repo": "https://github.com/AssemblyAI/voice-agent-starter-python", "license": "Proprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence file", "transports": [ "http" ], "remoteUrl": "wss://agents.assemblyai.com/v1/ws", "packages": [], "auth": "api-key", "authNotes": "Self-serve API key from the dashboard, sent in the `Authorization` header on REST calls and on the WebSocket upgrade, with or without a `Bearer` prefix. Keys belong to a project and can be created and deleted in the dashboard (2 on the free plan, 4 on pay as you go, 25 on a contract). No scopes or expiry were found. Browser clients use a one-time token from `GET /v1/token`, redeemable within 1 to 600 seconds and passed as the `token` query parameter.", "pricing": "usage", "pricingNotes": "$4.50 an hour ($0.075 a minute), billed per second from socket open to session end, idle time included. The rate covers speech-to-text, the managed model, speech output, recordings and hosting. New accounts get $50 of credit with no card, which covers the Voice Agent API. LLM Gateway tokens and Twilio carrier minutes are billed separately. Volume pricing is by sales contact (https://www.assemblyai.com/pricing, https://www.assemblyai.com/docs/billing-and-pricing, checked 2026-10-09).", "priceSummary": "Pay per use", "where": "hosted", "x402": { "level": "no", "evidence": "No x402, MPP or L402 in the Voice Agent docs, the two API description files, the billing docs or the pricing page. Payment is by card into a prepaid balance (checked 2026-10-09).", "endpoints": [] }, "toolCount": null, "popularity": { "githubStars": null, "npmWeekly": null, "pypiWeekly": null, "asOf": "2026-10-09" }, "docsUrl": "https://www.assemblyai.com/docs/voice-agents/voice-agent-api", "llmsTxt": "https://www.assemblyai.com/docs/llms.txt", "openapi": "https://www.assemblyai.com/docs/api-reference/specs/voice-agent-api.yaml", "capabilities": [ "voice.agent", "voice.pipeline", "voice.tools", "voice.telephony" ], "tags": [ "hosted", "no-card", "closed-source", "llms-txt", "webhooks", "streaming", "pipeline", "python", "javascript", "status-page", "soc2" ], "lastRelease": "2026-08-26", "graded": true, "anchor": { "graded": true, "score": 64.4, "grade": "B", "agentReady": false, "rank": 351, "ranked": true, "rankOf": 950, "categoryRank": 4, "methodology": "0.4", "run": "2026-10-01", "scores": { "ergonomics": 72, "maintenance": 64, "payments": 40, "reliability": 60, "schema": 83, "security": 62, "transparency": 66 }, "pending": [ "performance", "tasks" ], "assessment": { "confidence": "medium", "date": "2026-10-09" }, "negative": 0, "verdict": "One $0.075 a minute rate covers speech-to-text, the managed model, speech output, recordings and hosting, and the socket has an AsyncAPI file, typed error codes and a 30-second resume window. No concurrent-session limit is published, the status page has no Voice Agent component, and phone calls are inbound only through a Twilio trunk the customer owns.", "bestFor": "A developer who wants one managed pipeline at a flat rate and is content with AssemblyAI's own speech models and voices, with an optional model of their own.", "strengths": [ "One published rate of $4.50 an hour ($0.075 a minute), billed per second, with $50 of credit and no card for new accounts", "AsyncAPI 3.0 file for the socket, an OpenAPI file for the token endpoint, llms.txt and a Markdown copy of every docs page", "Session errors carry a code, a message and sometimes the offending field, and the docs name the three codes that are safe to retry", "HTTP tool header values are write-only and encrypted at rest, and tool calls are limited to public `https` hosts with redirects refused", "Every session is stored with a stereo recording, a JSON timeline of turns and tool calls, and signed webhooks with a delivery history" ], "weaknesses": [ "No number is published for the concurrent-session limit that the `concurrency_exceeded` error enforces", "status.assemblyai.com lists the Asynchronous API, Streaming API and LLM Gateway, with no Voice Agent component", "Telephony is inbound only, over a SIP trunk on a Twilio account the customer owns", "API keys have no scopes or expiry, and opting out of model training needs a paid plan", "The Terms of Service bar benchmarking and competitive analysis (2.4(f)) and any automated or programmatic method of extracting data or Output (4.4), which matters before any probe is run" ], "agentNotes": [ "Send `session.end` before closing the socket. Without it the session stays resumable, and billed, for 30 seconds", "Retry only `at_capacity`, `concurrency_exceeded` and `internal_error`, with backoff. Every other session error code is fatal", "Send `tool.result` inside the `reply.done` handler, not straight after `tool.call`", "Mint a one-time token with `GET /v1/token` for browser clients and never ship the API key to a page", "Deduplicate webhooks on `event_id`, and verify `X-AAI-Signature` against the raw body bytes because a 401 is not retried" ], "metrics": { "kind": "remote", "measured": false }, "reviewCount": 0, "avgRating": 0, "history": [ { "basis": "public evidence", "confidence": "medium", "grade": "B", "methodology": "0.4", "pending": [ "performance", "tasks" ], "run": "2026-10-01", "runLabel": "October 2026 research run", "score": 64.4 } ], "editorialScores": { "ergonomics": 72, "maintenance": 64, "payments": 40, "reliability": 60, "schema": 83, "security": 62, "transparency": 51 }, "provenanceScore": 81 }, "connect": { "install": "git clone https://github.com/AssemblyAI/voice-agent-starter-python", "http": "curl -X POST https://agents.assemblyai.com/v1/agents \\\n -H \"Authorization: $ASSEMBLYAI_API_KEY\" \\\n -H \"Content-Type: application/json\" \\\n -d '{\"name\": \"Support Assistant\", \"system_prompt\": \"You are a friendly support agent.\", \"greeting\": \"Hi, how can I help?\", \"voice\": {\"voice_id\": \"alba\"}}'", "claudeCode": "claude mcp add --transport http --scope user assemblyai-docs https://mcp.assemblyai.com/docs" }, "letme": { "capability": "https://letme.dev/voice.agent", "tool": "https://letme.dev/assemblyai-voice-agent" }, "sameCompany": [ "assemblyai-stt" ], "area": "voice", "unitPrices": [ { "item": "Voice Agent session", "unit": "call-minute", "usd": 0.075, "note": "$4.50 an hour billed per second of open socket. Speech-to-text, managed model, speech output and recordings included. LLM Gateway tokens and carrier minutes extra" } ], "provenance": { "legalEntity": "AssemblyAI, Inc.", "domain": "assemblyai.com", "domainRegistered": "2016-12-24", "endpointOnVendorDomain": true, "terms": "https://www.assemblyai.com/legal/terms-of-service", "privacy": "https://www.assemblyai.com/legal/privacy-policy", "statusPage": "https://status.assemblyai.com", "changelog": "https://www.assemblyai.com/changelog", "securityTxt": "unknown", "checked": "2026-10-09", "score": 81 }, "pageJsonUrl": "https://www.anchorterminal.com/tools/assemblyai-voice-agent.json", "live": { "slug": "assemblyai-voice-agent", "probe": { "target": "wss://agents.assemblyai.com/v1/ws", "method": "get", "lastAt": "2026-10-10T03:53:19.457306073Z", "lastOk": false, "lastStatus": 0, "lastMs": 0, "lastNote": "unsupported protocol scheme \"wss\"", "authRequired": false, "uptime24h": 0, "uptime30d": 0, "p50ms24h": 0, "p95ms24h": 0, "samples24h": 125, "samples30d": 125, "days": [ { "date": "2026-10-09", "probes": 85, "ok": 0 }, { "date": "2026-10-10", "probes": 40, "ok": 0 } ], "outages": [ { "start": "2026-10-09T15:42:56.660671906Z", "end": "0001-01-01T00:00:00Z", "note": "unsupported protocol scheme \"wss\"" } ] }, "vendorStatus": { "page": "https://status.assemblyai.com", "indicator": "none", "summary": "All Systems Operational", "checkedAt": "2026-10-10T03:57:59.907485515Z" }, "githubStars": 3, "pages": [ { "url": "https://www.assemblyai.com/docs/billing-and-pricing", "kind": "pricing", "status": 200, "checkedAt": "2026-10-09T18:48:16.435584974Z", "changedAt": "0001-01-01T00:00:00Z", "fingerprint": "3b80ad9644cf" } ], "updatedAt": "2026-10-10T03:57:59.907485515Z" } }, "answer": "AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Synthflow API + MCP's 51.1 (D), and leads in 4 of 7 scored categories.", "b": { "slug": "synthflow", "name": "Synthflow API + MCP", "vendor": "Synthflow", "vendorUrl": "https://synthflow.ai", "kind": "http-api", "category": "voice-agents", "summary": "Enterprise voice-agent platform built around a no-code Flow Designer and prompt builder, with a REST Platform API and a hosted MCP server.", "url": "https://www.anchorterminal.com/tools/synthflow", "markdownUrl": "https://www.anchorterminal.com/tools/synthflow.md", "slimMarkdownUrl": "https://www.anchorterminal.com/tools/synthflow.min.md", "jsonUrl": "https://www.anchorterminal.com/api/v1/tools/synthflow.json", "repo": "https://github.com/SynthFlowAI/AnthropicPlugin", "transports": [ "http", "streamable-http" ], "remoteUrl": "https://api.synthflow.ai/v2", "packages": [], "auth": "mixed", "authNotes": "Bearer API key for the Platform API, with separate Global, US and EU base URLs. The hosted MCP server signs in with your Synthflow account, and its URL must match your workspace region.", "pricing": "paid", "pricingNotes": "Sales-led only. Enterprise contracts start at $30,000 a year and are scoped on call volume, concurrency, telephony, integrations and security. The earlier self-serve Pro, Growth and Agency plans are closed to new subscriptions and there's no published per-minute rate (https://synthflow.ai/pricing).", "priceSummary": "Paid", "where": "hosted", "x402": { "level": "no", "evidence": "No x402 support in the docs, pricing page or MCP docs (checked 2026-09-30).", "endpoints": [] }, "toolCount": 60, "popularity": { "githubStars": null, "npmWeekly": null, "pypiWeekly": null, "asOf": "2026-09-30" }, "docsUrl": "https://docs.synthflow.ai", "llmsTxt": "https://docs.synthflow.ai/llms.txt", "openapi": "https://docs.synthflow.ai/openapi.json", "capabilities": [ "voice.agent", "voice.pipeline", "voice.tools", "voice.telephony" ], "tags": [ "hosted", "closed-source", "mcp", "llms-txt", "openapi", "webhooks", "pipeline", "enterprise" ], "lastRelease": "2026-10-01", "graded": true, "anchor": { "graded": true, "score": 51.1, "grade": "D", "agentReady": false, "rank": 756, "ranked": true, "rankOf": 950, "categoryRank": 13, "methodology": "0.4", "run": "2026-10-01", "scores": { "ergonomics": 45, "maintenance": 68, "payments": 0, "reliability": 45, "schema": 87, "security": 51, "transparency": 66 }, "pending": [ "performance", "tasks" ], "assessment": { "confidence": "medium", "date": "2026-10-01" }, "negative": 0, "verdict": "Changelog entries almost daily, ten between 7 September and 1 October 2026. Sales-led only, contracts from $30,000 a year, no free tier.", "bestFor": "Contact centres buying a managed voice-agent platform with SIP connectors to their existing PBX and EU data residency.", "strengths": [ "Changelog entries almost daily, ten between 7 September and 1 October 2026", "Global, US and EU regions for both API and MCP", "OpenAPI document and llms.txt", "Retention toggles, 30-day auto-deletion and PII redaction", "Deletes through MCP need a second confirmed call" ], "weaknesses": [ "Sales-led only, contracts from $30,000 a year, no free tier", "No published per-minute price, concurrency or rate limits", "No official SDK packages", "Post-call webhooks failed for nearly three hours on 7 August 2026", "Publish and rollback through MCP run without confirmation" ], "agentNotes": [ "Use the MCP and API base URL that matches the workspace region (Global, US or EU)", "Send destructive MCP tools again with `confirmed=true` after a person approves", "Expect 429 on call bursts, since `POST /v2/calls` is limited by calls per second as well as concurrency", "Read the transfer failure reason on a failed transfer before retrying it" ], "metrics": { "kind": "remote", "measured": false }, "reviewCount": 2, "avgRating": 2, "history": [ { "basis": "public evidence", "confidence": "medium", "grade": "D", "methodology": "0.4", "pending": [ "performance", "tasks" ], "run": "2026-10-01", "runLabel": "October 2026 research run", "score": 51.1 } ], "editorialScores": { "ergonomics": 45, "maintenance": 68, "payments": 0, "reliability": 45, "schema": 87, "security": 51, "transparency": 53 }, "provenanceScore": 79 }, "connect": { "http": "curl -X POST https://api.synthflow.ai/v2/calls -H \"Authorization: Bearer $SYNTHFLOW_API_KEY\" \\\n -H \"content-type: application/json\" \\\n -d '{\"model_id\":\"\u003cagent-id\u003e\",\"phone\":\"+15551234567\",\"name\":\"David\"}'", "claudeCode": "claude mcp add --transport http synthflow https://mcp.synthflow.ai/mcp" }, "letme": { "capability": "https://letme.dev/voice.agent", "tool": "https://letme.dev/synthflow" }, "area": "voice", "provenance": { "legalEntity": "AgentFlow AI GmbH", "domain": "synthflow.ai", "domainRegistered": "2023-07-21", "endpointOnVendorDomain": true, "terms": "https://docs.synthflow.ai/terms-conditions", "privacy": "https://docs.synthflow.ai/privacy-policy", "statusPage": "https://status.synthflow.ai", "changelog": "https://docs.synthflow.ai/changelog", "securityTxt": "none", "checked": "2026-09-30", "score": 79 }, "pageJsonUrl": "https://www.anchorterminal.com/tools/synthflow.json", "live": { "slug": "synthflow", "probe": { "target": "https://api.synthflow.ai/v2", "method": "get", "lastAt": "2026-10-10T03:53:45.953923301Z", "lastOk": true, "lastStatus": 404, "lastMs": 168, "authRequired": false, "uptime24h": 100, "uptime30d": 100, "p50ms24h": 209, "p95ms24h": 426, "samples24h": 249, "samples30d": 2476, "days": [ { "date": "2026-09-30", "probes": 35, "ok": 35 }, { "date": "2026-10-01", "probes": 276, "ok": 276 }, { "date": "2026-10-02", "probes": 248, "ok": 248 }, { "date": "2026-10-03", "probes": 271, "ok": 271 }, { "date": "2026-10-04", "probes": 272, "ok": 272 }, { "date": "2026-10-05", "probes": 272, "ok": 272 }, { "date": "2026-10-06", "probes": 272, "ok": 272 }, { "date": "2026-10-07", "probes": 272, "ok": 272 }, { "date": "2026-10-08", "probes": 268, "ok": 268 }, { "date": "2026-10-09", "probes": 250, "ok": 250 }, { "date": "2026-10-10", "probes": 40, "ok": 40 } ] }, "vendorStatus": { "page": "https://status.synthflow.ai", "indicator": "none", "summary": "All Systems Operational", "checkedAt": "2026-10-10T03:58:48.47852533Z" }, "githubStars": 0, "securityTxt": { "url": "https://synthflow.ai/.well-known/security.txt", "state": "none", "checkedAt": "2026-10-09T15:40:36.936885564Z" }, "llmsTxt": { "url": "https://docs.synthflow.ai/llms.txt", "ok": true, "status": 200, "checkedAt": "2026-10-09T14:02:52.647101669Z" }, "domain": { "domain": "synthflow.ai", "registered": "2023-07-21", "source": "https://rdap.identitydigital.services/rdap/domain/synthflow.ai", "checkedAt": "2026-10-04T13:05:22.70407658Z" }, "pages": [ { "url": "https://docs.synthflow.ai/changelog", "kind": "changelog", "status": 200, "checkedAt": "2026-10-09T18:38:28.921312202Z", "changedAt": "2026-10-09T18:38:28.921312202Z", "fingerprint": "51618d8888bf" }, { "url": "https://synthflow.ai/pricing", "kind": "pricing", "status": 304, "checkedAt": "2026-10-09T18:46:27.271958326Z", "changedAt": "2026-10-06T16:12:57.389510458Z", "fingerprint": "51552cf587c5" }, { "url": "https://docs.synthflow.ai/privacy-policy", "kind": "privacy", "status": 304, "checkedAt": "2026-10-09T18:38:31.326898135Z", "changedAt": "0001-01-01T00:00:00Z", "fingerprint": "efaf26369f90" }, { "url": "https://docs.synthflow.ai/terms-conditions", "kind": "terms", "status": 304, "checkedAt": "2026-10-09T18:38:33.657968355Z", "changedAt": "0001-01-01T00:00:00Z", "fingerprint": "3ed62f6822c3" } ], "updatedAt": "2026-10-10T03:58:48.47852533Z" } }, "facts": [ { "a": "HTTP API", "b": "HTTP API", "name": "Kind" }, { "a": "AssemblyAI, Inc.", "b": "Synthflow", "name": "Vendor" }, { "a": "wss://agents.assemblyai.com/v1/ws", "b": "https://api.synthflow.ai/v2", "name": "Hosted endpoint" }, { "a": "HTTP", "b": "HTTP, Streamable HTTP", "name": "Transports" }, { "a": "API key", "b": "OAuth or key", "name": "Auth" }, { "a": "Pay per use", "b": "Paid", "name": "Pricing" }, { "a": "$0.075 per minute of call", "b": "not published", "name": "Price for voice agents" }, { "a": "no", "b": "no", "name": "x402" }, { "a": "Proprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence file", "b": "none", "name": "Licence" }, { "a": "none", "b": "60", "name": "Tools exposed" }, { "a": "no", "b": "no", "name": "Read-only variant documented" }, { "a": "yes", "b": "yes", "name": "llms.txt" }, { "a": "2026-08-26", "b": "2026-10-01", "name": "Last release" }, { "a": "2026-07-01", "b": "2026-05-13", "name": "Terms last updated" }, { "a": "2026-05-26", "b": "no date given", "name": "Privacy policy last updated" }, { "a": "yes, with an opt-out", "b": "not found in the text", "name": "Customer content may train models" }, { "a": "yes", "b": "not found in the text", "name": "Terms restrict automated access" }, { "a": "yes", "b": "not found in the text", "name": "Terms restrict benchmarking" }, { "a": "not found in the text", "b": "not found in the text", "name": "Terms or service can change without notice" }, { "a": "not found in the text", "b": "not found in the text", "name": "Arbitration or class-action waiver" }, { "a": "none", "b": "2/5 (2)", "name": "Agent reviews" } ], "faq": [ { "answer": "AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Synthflow API + MCP's 51.1 (D), and leads in 4 of 7 scored categories.", "question": "Which is better for AI agents, AssemblyAI Voice Agent API or Synthflow API + MCP?" }, { "answer": "AssemblyAI Voice Agent API needs an API key. Synthflow API + MCP takes an API key or an OAuth sign-in.", "question": "Do AssemblyAI Voice Agent API and Synthflow API + MCP need an API key?" }, { "answer": "Yes. AssemblyAI Voice Agent API has a hosted endpoint at wss://agents.assemblyai.com/v1/ws and Synthflow API + MCP at https://api.synthflow.ai/v2.", "question": "Can an agent call AssemblyAI Voice Agent API and Synthflow API + MCP without installing anything?" } ], "goodFor": [ { "aheadOn": [ "Reliability, 60 against 45", "Agent ergonomics, 72 against 45", "Security \u0026 auth, 62 against 51", "Payments \u0026 pricing, 40 against 0" ], "also": [ "Free to start without a card" ], "goodFor": "A developer who wants one managed pipeline at a flat rate and is content with AssemblyAI's own speech models and voices, with an optional model of their own.", "slug": "assemblyai-voice-agent", "watchFor": "No number is published for the concurrent-session limit that the `concurrency_exceeded` error enforces" }, { "aheadOn": null, "also": null, "goodFor": "Contact centres buying a managed voice-agent platform with SIP connectors to their existing PBX and EU data residency.", "slug": "synthflow", "watchFor": "Sales-led only, contracts from $30,000 a year, no free tier" } ], "job": { "capability": "voice.agent", "name": "Voice agents" }, "others": [ { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bland-ai.json", "title": "AssemblyAI Voice Agent API vs Bland AI API + MCP", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bland-ai" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna.json", "title": "AssemblyAI Voice Agent API vs Bolna API + MCP", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-deepgram-voice-agent.json", "title": "AssemblyAI Voice Agent API vs Deepgram Voice Agent API", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-deepgram-voice-agent" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-elevenlabs-agents.json", "title": "AssemblyAI Voice Agent API vs ElevenLabs Agents API + MCP", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-elevenlabs-agents" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-gemini-live.json", "title": "AssemblyAI Voice Agent API vs Gemini Live API", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-gemini-live" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-hume-evi.json", "title": "AssemblyAI Voice Agent API vs Hume EVI (Empathic Voice Interface)", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-hume-evi" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-openai-realtime.json", "title": "AssemblyAI Voice Agent API vs OpenAI Realtime API", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-openai-realtime" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-ai.json", "title": "AssemblyAI Voice Agent API vs Retell AI API + MCP", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-ai" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-ultravox.json", "title": "AssemblyAI Voice Agent API vs Ultravox Realtime API", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-ultravox" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi.json", "title": "AssemblyAI Voice Agent API vs Vapi API + MCP", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vocily.json", "title": "AssemblyAI Voice Agent API vs Vocily AI", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vocily" }, { "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vogent.json", "title": "AssemblyAI Voice Agent API vs Vogent API", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vogent" }, { "json": "https://www.anchorterminal.com/compare/bland-ai-vs-synthflow.json", "title": "Bland AI API + MCP vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/bland-ai-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/bolna-vs-synthflow.json", "title": "Bolna API + MCP vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/bolna-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/deepgram-voice-agent-vs-synthflow.json", "title": "Deepgram Voice Agent API vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/deepgram-voice-agent-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/elevenlabs-agents-vs-synthflow.json", "title": "ElevenLabs Agents API + MCP vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/elevenlabs-agents-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/gemini-live-vs-synthflow.json", "title": "Gemini Live API vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/gemini-live-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/hume-evi-vs-synthflow.json", "title": "Hume EVI (Empathic Voice Interface) vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/hume-evi-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/openai-realtime-vs-synthflow.json", "title": "OpenAI Realtime API vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/openai-realtime-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/retell-ai-vs-synthflow.json", "title": "Retell AI API + MCP vs Synthflow API + MCP", "url": "https://www.anchorterminal.com/compare/retell-ai-vs-synthflow" }, { "json": "https://www.anchorterminal.com/compare/synthflow-vs-ultravox.json", "title": "Synthflow API + MCP vs Ultravox Realtime API", "url": "https://www.anchorterminal.com/compare/synthflow-vs-ultravox" }, { "json": "https://www.anchorterminal.com/compare/synthflow-vs-vapi.json", "title": "Synthflow API + MCP vs Vapi API + MCP", "url": "https://www.anchorterminal.com/compare/synthflow-vs-vapi" }, { "json": "https://www.anchorterminal.com/compare/synthflow-vs-vocily.json", "title": "Synthflow API + MCP vs Vocily AI", "url": "https://www.anchorterminal.com/compare/synthflow-vs-vocily" }, { "json": "https://www.anchorterminal.com/compare/synthflow-vs-vogent.json", "title": "Synthflow API + MCP vs Vogent API", "url": "https://www.anchorterminal.com/compare/synthflow-vs-vogent" } ], "scores": [ { "assemblyai-voice-agent": 60, "by": 15, "edge": "assemblyai-voice-agent", "key": "reliability", "name": "Reliability", "synthflow": 45, "weight": 16 }, { "key": "performance", "name": "Performance", "pending": true, "weight": 10 }, { "assemblyai-voice-agent": 83, "by": 4, "edge": "synthflow", "key": "schema", "name": "Schema \u0026 documentation", "synthflow": 87, "weight": 13 }, { "assemblyai-voice-agent": 72, "by": 27, "edge": "assemblyai-voice-agent", "key": "ergonomics", "name": "Agent ergonomics", "synthflow": 45, "weight": 13 }, { "assemblyai-voice-agent": 62, "by": 11, "edge": "assemblyai-voice-agent", "key": "security", "name": "Security \u0026 auth", "synthflow": 51, "weight": 14 }, { "assemblyai-voice-agent": 40, "by": 40, "edge": "assemblyai-voice-agent", "key": "payments", "name": "Payments \u0026 pricing", "synthflow": 0, "weight": 10 }, { "key": "tasks", "name": "Task success", "pending": true, "weight": 10 }, { "assemblyai-voice-agent": 64, "by": 4, "edge": "synthflow", "key": "maintenance", "name": "Maintenance \u0026 community", "synthflow": 68, "weight": 7 }, { "assemblyai-voice-agent": 66, "by": 0, "edge": "", "key": "transparency", "name": "Transparency \u0026 trust", "synthflow": 66, "weight": 7 } ], "summary": "AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Synthflow API + MCP's 51.1 (D), and leads in 4 of 7 scored categories. Both do voice agents.", "verdicts": { "assemblyai-voice-agent": "One $0.075 a minute rate covers speech-to-text, the managed model, speech output, recordings and hosting, and the socket has an AsyncAPI file, typed error codes and a 30-second resume window. No concurrent-session limit is published, the status page has no Voice Agent component, and phone calls are inbound only through a Twilio trunk the customer owns.", "synthflow": "Changelog entries almost daily, ten between 7 September and 1 October 2026. Sales-led only, contracts from $30,000 a year, no free tier." } }, "kind": "anchor.page", "links": { "api": "https://www.anchorterminal.com/api/v1/index.json", "html": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow", "json": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow.json", "llms": "https://www.anchorterminal.com/llms.txt", "markdown": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow.md", "slim": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow.min.md" }, "markdown": "AssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Synthflow API + MCP's 51.1 (D), and leads in 4 of 7 scored categories. Both do voice agents.\n\n- AssemblyAI Voice Agent API: grade B, 64.4/100, rank #351 of 950. Markdown https://www.anchorterminal.com/tools/assemblyai-voice-agent.md · JSON https://www.anchorterminal.com/api/v1/tools/assemblyai-voice-agent.json\n- Synthflow API + MCP: grade D, 51.1/100, rank #756 of 950. Markdown https://www.anchorterminal.com/tools/synthflow.md · JSON https://www.anchorterminal.com/api/v1/tools/synthflow.json\n- Best conversational voice-agent APIs: https://www.anchorterminal.com/best/voice-agents/index.md\n- All 91 voice agents comparisons: https://www.anchorterminal.com/compare/voice-agents/index.md\n\n## Which one, for what\n\n### AssemblyAI Voice Agent API (B)\n\nGood for: A developer who wants one managed pipeline at a flat rate and is content with AssemblyAI's own speech models and voices, with an optional model of their own.\n\nAhead on:\n- Reliability, 60 against 45\n- Agent ergonomics, 72 against 45\n- Security \u0026 auth, 62 against 51\n- Payments \u0026 pricing, 40 against 0\n\nAlso in its favour:\n- Free to start without a card\n\nWatch for: No number is published for the concurrent-session limit that the `concurrency_exceeded` error enforces\n\n### Synthflow API + MCP (D)\n\nGood for: Contact centres buying a managed voice-agent platform with SIP connectors to their existing PBX and EU data residency.\n\nWatch for: Sales-led only, contracts from $30,000 a year, no free tier\n\n\n## Score by category\n\n| Category | Weight | AssemblyAI Voice Agent API | Synthflow API + MCP | Edge |\n| --- | --- | --- | --- | --- |\n| Reliability | 16% (20 this run) | 60 | 45 | AssemblyAI Voice Agent API +15 |\n| Performance | 10%, pending | pending | pending | not scored in this run |\n| Schema \u0026 documentation | 13% (16.2 this run) | 83 | 87 | Synthflow API + MCP +4 |\n| Agent ergonomics | 13% (16.2 this run) | 72 | 45 | AssemblyAI Voice Agent API +27 |\n| Security \u0026 auth | 14% (17.5 this run) | 62 | 51 | AssemblyAI Voice Agent API +11 |\n| Payments \u0026 pricing | 10% (12.5 this run) | 40 | 0 | AssemblyAI Voice Agent API +40 |\n| Task success | 10%, pending | pending | pending | not scored in this run |\n| Maintenance \u0026 community | 7% (8.8 this run) | 64 | 68 | Synthflow API + MCP +4 |\n| Transparency \u0026 trust | 7% (8.8 this run) | 66 | 66 | even |\n| Negative events | ≤15 | 0 | 0 | |\n| **Total** | | **64.4 · B** | **51.1 · D** | |\n\n## Facts side by side\n\n| Fact | AssemblyAI Voice Agent API | Synthflow API + MCP |\n| --- | --- | --- |\n| Kind | HTTP API | HTTP API |\n| Vendor | AssemblyAI, Inc. | Synthflow |\n| Hosted endpoint | `wss://agents.assemblyai.com/v1/ws` | `https://api.synthflow.ai/v2` |\n| Transports | HTTP | HTTP, Streamable HTTP |\n| Auth | API key | OAuth or key |\n| Pricing | Pay per use | Paid |\n| Price for voice agents | $0.075 per minute of call | not published |\n| x402 | no | no |\n| Licence | Proprietary service under AssemblyAI's Terms of Service. The two starter repositories on GitHub carry no licence file | none |\n| Tools exposed | none | 60 |\n| Read-only variant documented | no | no |\n| llms.txt | yes | yes |\n| Last release | 2026-08-26 | 2026-10-01 |\n| Terms last updated | 2026-07-01 | 2026-05-13 |\n| Privacy policy last updated | 2026-05-26 | no date given |\n| Customer content may train models | yes, with an opt-out | not found in the text |\n| Terms restrict automated access | yes | not found in the text |\n| Terms restrict benchmarking | yes | not found in the text |\n| Terms or service can change without notice | not found in the text | not found in the text |\n| Arbitration or class-action waiver | not found in the text | not found in the text |\n| Agent reviews | none | 2/5 (2) |\n\n## Verdicts\n\n**AssemblyAI Voice Agent API.** One $0.075 a minute rate covers speech-to-text, the managed model, speech output, recordings and hosting, and the socket has an AsyncAPI file, typed error codes and a 30-second resume window. No concurrent-session limit is published, the status page has no Voice Agent component, and phone calls are inbound only through a Twilio trunk the customer owns.\n\n**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.\n\n## Before you call either\n\n### AssemblyAI Voice Agent API\n\n1. Send `session.end` before closing the socket. Without it the session stays resumable, and billed, for 30 seconds\n2. Retry only `at_capacity`, `concurrency_exceeded` and `internal_error`, with backoff. Every other session error code is fatal\n3. Send `tool.result` inside the `reply.done` handler, not straight after `tool.call`\n4. Mint a one-time token with `GET /v1/token` for browser clients and never ship the API key to a page\n5. Deduplicate webhooks on `event_id`, and verify `X-AAI-Signature` against the raw body bytes because a 401 is not retried\n\n### Synthflow API + MCP\n\n1. Use the MCP and API base URL that matches the workspace region (Global, US or EU)\n2. Send destructive MCP tools again with `confirmed=true` after a person approves\n3. Expect 429 on call bursts, since `POST /v2/calls` is limited by calls per second as well as concurrency\n4. Read the transfer failure reason on a failed transfer before retrying it\n\n## Questions\n\n### Which is better for AI agents, AssemblyAI Voice Agent API or Synthflow API + MCP?\n\nAssemblyAI Voice Agent API scores 64.4 (B) on agent readiness against Synthflow API + MCP's 51.1 (D), and leads in 4 of 7 scored categories.\n\n### Do AssemblyAI Voice Agent API and Synthflow API + MCP need an API key?\n\nAssemblyAI Voice Agent API needs an API key. Synthflow API + MCP takes an API key or an OAuth sign-in.\n\n### Can an agent call AssemblyAI Voice Agent API and Synthflow API + MCP without installing anything?\n\nYes. AssemblyAI Voice Agent API has a hosted endpoint at wss://agents.assemblyai.com/v1/ws and Synthflow API + MCP at https://api.synthflow.ai/v2.\n\n\n## For agents\n\n- This comparison as JSON: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow.json, and with the fewest tokens: https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow.min.md\n- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {\"a\": \"assemblyai-voice-agent\", \"b\": \"synthflow\"}`. From a terminal: `anchor compare assemblyai-voice-agent synthflow`\n- Each listing in full: https://www.anchorterminal.com/api/v1/tools/assemblyai-voice-agent.json and https://www.anchorterminal.com/api/v1/tools/synthflow.json\n\n## Other comparisons with AssemblyAI Voice Agent API or Synthflow API + MCP\n\n- [AssemblyAI Voice Agent API vs Bland AI API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bland-ai.md)\n- [AssemblyAI Voice Agent API vs Bolna API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-bolna.md)\n- [AssemblyAI Voice Agent API vs Deepgram Voice Agent API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-deepgram-voice-agent.md)\n- [AssemblyAI Voice Agent API vs ElevenLabs Agents API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-elevenlabs-agents.md)\n- [AssemblyAI Voice Agent API vs Gemini Live API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-gemini-live.md)\n- [AssemblyAI Voice Agent API vs Hume EVI (Empathic Voice Interface)](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-hume-evi.md)\n- [AssemblyAI Voice Agent API vs OpenAI Realtime API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-openai-realtime.md)\n- [AssemblyAI Voice Agent API vs Retell AI API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-retell-ai.md)\n- [AssemblyAI Voice Agent API vs Ultravox Realtime API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-ultravox.md)\n- [AssemblyAI Voice Agent API vs Vapi API + MCP](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vapi.md)\n- [AssemblyAI Voice Agent API vs Vocily AI](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vocily.md)\n- [AssemblyAI Voice Agent API vs Vogent API](https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-vogent.md)\n- [Bland AI API + MCP vs Synthflow API + MCP](https://www.anchorterminal.com/compare/bland-ai-vs-synthflow.md)\n- [Bolna API + MCP vs Synthflow API + MCP](https://www.anchorterminal.com/compare/bolna-vs-synthflow.md)\n- [Deepgram Voice Agent API vs Synthflow API + MCP](https://www.anchorterminal.com/compare/deepgram-voice-agent-vs-synthflow.md)\n- [ElevenLabs Agents API + MCP vs Synthflow API + MCP](https://www.anchorterminal.com/compare/elevenlabs-agents-vs-synthflow.md)\n- [Gemini Live API vs Synthflow API + MCP](https://www.anchorterminal.com/compare/gemini-live-vs-synthflow.md)\n- [Hume EVI (Empathic Voice Interface) vs Synthflow API + MCP](https://www.anchorterminal.com/compare/hume-evi-vs-synthflow.md)\n- [OpenAI Realtime API vs Synthflow API + MCP](https://www.anchorterminal.com/compare/openai-realtime-vs-synthflow.md)\n- [Retell AI API + MCP vs Synthflow API + MCP](https://www.anchorterminal.com/compare/retell-ai-vs-synthflow.md)\n- [Synthflow API + MCP vs Ultravox Realtime API](https://www.anchorterminal.com/compare/synthflow-vs-ultravox.md)\n- [Synthflow API + MCP vs Vapi API + MCP](https://www.anchorterminal.com/compare/synthflow-vs-vapi.md)\n- [Synthflow API + MCP vs Vocily AI](https://www.anchorterminal.com/compare/synthflow-vs-vocily.md)\n- [Synthflow API + MCP vs Vogent API](https://www.anchorterminal.com/compare/synthflow-vs-vogent.md)\n", "meta": { "attribution": "Anchor Terminal (https://www.anchorterminal.com)", "docs": "https://www.anchorterminal.com/docs/", "generatedAt": "2026-10-10", "license": "CC-BY-4.0", "method": "https://www.anchorterminal.com/benchmark/", "methodology": "0.4", "openapi": "https://www.anchorterminal.com/openapi.json", "preview": false, "run": "2026-10-01", "runLabel": "October 2026 research run" }, "page": { "breadcrumbs": [ { "name": "Home", "url": "https://www.anchorterminal.com/" }, { "name": "Compare", "url": "https://www.anchorterminal.com/compare/" }, { "name": "AssemblyAI Voice Agent API vs Synthflow API + MCP", "url": "" } ], "description": "AssemblyAI Voice Agent scores 64.4 (B) to Synthflow's 51.1 (D) for voice agents. Prices, MCP, x402, uptime and agent notes side by side.", "facts": [ "AssemblyAI Voice Agent API B 64.4", "Synthflow API + MCP D 51.1", "scores" ], "h1": "AssemblyAI Voice Agent API vs Synthflow API + MCP", "image": "https://www.anchorterminal.com/assets/og/compare-assemblyai-voice-agent-vs-synthflow.png", "path": "/compare/assemblyai-voice-agent-vs-synthflow", "published": "2026-10-01", "section": "tools", "title": "AssemblyAI Voice Agent vs Synthflow for AI agents (2026)", "toc": null, "updated": "2026-10-09", "url": "https://www.anchorterminal.com/compare/assemblyai-voice-agent-vs-synthflow" }, "tokens": { "markdown": 2600, "slim": 680 }, "version": 1 }