Agentic Index
Retell AI vs Synthflow (2026)
Retell AI and Synthflow both build phone agents in visual flow editors, and they differ most in how you buy and where data lives. That verdict is the Agentic Index coverage score, graded from each vendor's own published materials.
Retell is pay as you go with no platform fee: its voice engine costs 7 to 8 cents a minute, the language model and telephony are billed separately, and new accounts get 10 dollars in free credits and 20 concurrent calls. Synthflow prices through sales, with enterprise contracts from 30,000 dollars a year, though agents can be built free from templates. On the grid Synthflow documents US and EU data regions where Retell documents none, and Retell is Full on human oversight where Synthflow is Partial. Choose Retell to start metering today with the model of your choice; choose Synthflow when data region and agency features matter more.
This comparison is published by Agentic Index, an independent agentic AI vendor research platform. Retell AI and Synthflow are each graded against the same 14 capability Agentic Index taxonomy, from the vendor's own public materials under the Agentic Index verification standard, alongside 955 researched vendors. No vendor pays for placement and no vendor has reviewed this page. How this evidence is graded
Choose Retell AI if
- You want to pay per minute from day one, with no platform fee and free credits to test.
- You will bring your own language model, or host one, behind the agent.
- Human oversight controls must be documented in full; Retell is Full on that axis and Synthflow Partial.
Choose Synthflow if
- Call data must stay in a named US or EU region.
- Agencies reselling agents need subaccounts, custom domains and rebilling built in.
- A contract scoped with sales on volume, telephony and support suits your procurement.
| Feature | R Retell AI |
S Synthflow |
|---|---|---|
| Action & orchestration | ||
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Integrations & Tool Calling Ability to connect agents to real systems through native integrations, OAuth-authenticated actions, custom tools, APIs, webhooks, or MCP-compatible tools. |
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Retell AIIntegrations & Tool Calling An Apps framework holds credentials for each external system, with agent functions across CRM (Salesforce, HubSpot, Dynamics 365, Zoho, GoHighLevel), helpdesk (Zendesk), scheduling (Calendly, Cal.com) and file storage (Google Drive, OneDrive), plus custom functions calling any HTTP API mid-call, a code tool running JavaScript, and MCP tools and nodes reaching remote MCP servers during a live conversation. Contact data also syncs both ways with the CRM. Sourcedocs.retellai.com/integrations/overviewread 2026-09-05 |
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SynthflowIntegrations & Tool Calling Custom actions call the customer's APIs during a call, with several chained per turn, real time booking writes to calendars, and integrations cover Salesforce, HubSpot, Zendesk, Freshworks, Stripe, Cal.com, GoHighLevel, Make, Zapier and SIP connections to Five9, Genesys Cloud, RingCentral, Talkdesk and others. SourceSynthflow, docs.synthflow.ai (Custom Actions, Real-Time Booking, Integrations overview)read 2026-09-28 |
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Workflow Orchestration Ability to sequence, branch, retry, route, and combine deterministic workflow nodes with autonomous agent steps. |
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Retell AIWorkflow Orchestration Conversation flow agents are node-and-edge graphs with conversation, subagent, function, code, logic split, extract-variable, SMS, transfer and global nodes, transitions gated by either LLM-evaluated prompt conditions or deterministic equations, reusable subflows shared across flows, and Flex Mode compiling the graph into a runtime prompt for varied caller behavior. Multi-prompt state agents and agent-to-agent transfer cover the same ground for prompt-built agents, with versions and environment tags controlling what is live. Sourcedocs.retellai.com/build/conversation-flow/overviewread 2026-09-05 |
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SynthflowWorkflow Orchestration In a visual flow designer, flows combine conversation, message, branch, jump, custom action, transfer and booking nodes, a multi agent system passes calls between agents, workflows automate steps around calls, and version control governs publishing. SourceSynthflow, docs.synthflow.ai (Flow Designer, Branch Node, Multi-Agent System, Version Control, Workflows)read 2026-09-28 |
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Triggers & Channel Coverage How agents wake up and where they work: schedules, webhooks, message events, CRM events, inbox events, chat, email, voice, and collaboration tools. |
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Retell AITriggers & Channel Coverage Agents take inbound and outbound calls on Retell-managed numbers or imported ones over elastic SIP trunking, with setup for Twilio, Telnyx, Vonage, Avaya Aura, Genesys Cloud, Five9 and Amazon Connect. They also run browser web calls, a website widget in chat, voice and callback modes, two-way SMS and MMS, and scheduled batch call campaigns, and an inbound call and SMS webhook picks the agent and passes per-call context at ring time. Sourcedocs.retellai.com/deploy/custom-telephonyread 2026-09-05 |
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SynthflowTriggers & Channel Coverage Agents answer inbound calls on Synthflow numbers, forwarded lines or SIP trunks, inbound call webhooks fire at the start of calls, campaigns dial contact lists and can be created, paused and resumed through the API, and agents also run over chat and WhatsApp Business calling. SourceSynthflow, docs.synthflow.ai (Campaigns, Inbound Call Webhooks, Telephony overview, WhatsApp Business Calling)read 2026-09-28 |
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| Knowledge & context | ||
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Knowledge Grounding & RAG Ability to ground agent behavior in company data through document ingestion, retrieval, external knowledge APIs, semantic search, or RAG layers. |
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Retell AIKnowledge Grounding & RAG The platform builds and maintains knowledge bases from sources attached as URLs, documents across roughly twenty file formats, or custom text, chunking, embedding and storing them in a vector database at creation time, with retrieval tuned by query-rewrite instructions that condense recent dialogue into a standalone search query. Google Drive and OneDrive sources re-sync on a 24-hour refresh cycle, and knowledge bases are managed independently of any one agent through their own API. Sourcedocs.retellai.com/build/knowledge-baseread 2026-09-05 |
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SynthflowKnowledge Grounding & RAG Knowledge base sources are PDFs, documents, URLs, website crawls on a sync schedule and imports from connected services, retrieved by semantic search during the conversation (RAG), and knowledge bases and their sources are created, attached and deleted through the API; new knowledge is added as a new source. SourceSynthflow, docs.synthflow.ai (Knowledge Base, Knowledge Bases API)read 2026-09-28 |
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Memory & State Persistence Ability to persist context across a run, conversation, workflow, user, team, or longer-term memory layer. |
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Retell AIMemory & State Persistence Contact records keyed by phone number carry custom fields populated by post-call extraction mappings, so the agent recalls preferences, past issues and commitments across every later conversation, with a merged timeline of that contact's calls and chats and a backfill job that re-applies mappings to historical calls. Scope is per contact and the record is separately deletable. It is implemented as contact field mappings, so retention follows the contact record. Sourcedocs.retellai.com/integrations/build-contact-memoryread 2026-09-05 |
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SynthflowMemory & State Persistence Memory Groups keep summaries of an agent's past interactions with each contact (questions, solutions, preferences) and inject them into each new voice, SMS or WhatsApp conversation keyed on the phone number, with a retention window setting; memory stores are created, attached to agents and have contacts attached or detached through the API. Synthflow does not state the retention window values. SourceSynthflow, docs.synthflow.ai (Memory of Past Calls, Memory Stores API)read 2026-09-28 |
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| Control & trust | ||
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Human Oversight & Guardrails Approval steps, consent checkpoints, escalation rules, structured guardrails, policy constraints, and pause/resume controls. |
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Retell AIHuman Oversight & Guardrails Guardrails detect prohibited topics in both agent responses and user input and replace flagged content with a safe placeholder before it is spoken, and live monitoring lets a person follow the transcript in real time, listen in silently, take over from the agent mid-call or end the call, with an Update Live Call endpoint to inject context or trigger a response on a running session. Sourcedocs.retellai.com/features/live-monitoringread 2026-09-05 |
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SynthflowHuman Oversight & Guardrails Calls transfer to people by phone number, SIP or dynamic rules, business hours bound when the agent works, and PII redaction limits what is kept. Synthflow does not describe a step that holds an agent action for a person's sign off before it commits. SourceSynthflow, docs.synthflow.ai (Call Transfers, Business Hours, Security and Compliance)read 2026-09-28 |
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Security, Identity & Governance RBAC, SSO, auditability, encryption, least-privilege tool access, compliance posture, and data handling policy. |
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Retell AISecurity, Identity & Governance Retell is SOC 2 Type 1 and Type 2 certified and HIPAA and GDPR compliant, with a BAA, a DPA and a GDPR erasure process. Customer controls include role-based access control with Admin, Developer, Analyst and Viewer roles, API keys scoped read or edit per area, a webhook signing key with signature verification and IP allowlisting, expiring signed URLs for recordings and logs, per-agent data storage settings and a per-agent retention policy. Sourcedocs.retellai.com/general/complianceread 2026-09-05 |
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SynthflowSecurity, Identity & Governance Super Admin, Admin and Member roles separate access, with admins controlling SSO, allowed email domains and two factor authentication, plus PII redaction and a 30 day retention option. Synthflow says it is "certified across SOC 2, HIPAA, PCI DSS, and GDPR" and lists ISO 27001, and the Trust Vault at security.synthflow.ai holds the report types and dates. SourceSynthflow, docs.synthflow.ai (Team members, Security and Compliance) and synthflow.airead 2026-09-28 |
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Observability & Auditability Traces, logs, execution histories, metrics, audit events, and debugging detail for production agent behavior. |
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Retell AIObservability & Auditability A monitor-call WebSocket streams transcript, tool call and node events from a live session, so the path the agent took through the flow is reconstructable turn by turn, backed by session history with transcripts and CSV export, per-call P50, P90 and P99 latency, disconnection reason codes, call_started, call_ended and call_analyzed webhooks, and AI QA scoring calls on hallucination rate, knowledge base accuracy, tool usage, latency and sentiment against customer-defined cohorts. Sourcedocs.retellai.com/api-references/monitor-call-websocketread 2026-09-05 |
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SynthflowObservability & Auditability Each call's details show the transcript and metadata, an Actions tab with the steps triggered and their results, and every outbound API request an agent made with request and response and when in the call it fired, alongside status and end call reasons; retention follows the plan and logs export. SourceSynthflow, docs.synthflow.ai (Logs, Export Call Data)read 2026-09-28 |
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Deployment & Data Residency Deployment modes and options, including SaaS, dedicated cloud, VPC, on-prem, hybrid, local runtime, and self-hosting. |
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Retell AIDeployment & Data Residency Data storage settings and a retention policy set per agent, plus expiring signed URLs, control what is stored and for how long, but not where data lives. Retell offers no region selection, VPC, single tenant, self hosted or on premises option, and its reliability page describes shared enterprise infrastructure without naming a region. Sourcedocs.retellai.com/accounts/data-retentionread 2026-09-05 |
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SynthflowDeployment & Data Residency A workspace is created on the US cluster (storage and compute in the United States), the EU cluster (all data and processing in the European Union) or the Global cluster (data stored in the EU, live processing in the US or EU), chosen at creation and fixed afterward. Synthflow does not describe an on premises deployment. SourceSynthflow, docs.synthflow.ai (Clusters and Regions)read 2026-09-28 |
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| Solution readiness | ||
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Prebuilt Agents, Templates & Packs Ready-made workflows, packaged employees, templates, blueprints, industry solutions, and role-specific agents that reduce time-to-value. |
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Retell AIPrebuilt Agents, Templates & Packs The quick start has the customer pick a template as the first build step, Agent Handbook ships toggleable preset prompt packs for personality, accuracy and safety, and reusable subflows package parts of a conversation flow as shared components adopted across agents. The template catalog sits inside the dashboard, and Retell does not publish how broad it is. Sourcedocs.retellai.com/get-started/quick-startread 2026-09-05 |
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SynthflowPrebuilt Agents, Templates & Packs The marketplace offers ready made agents, among them a real estate buying appointment scheduler, an insurance customer support agent, a hospitality event demand generator, an insurance claims receptionist, a recruitment appointment reminder and a buyer lead qualifier, free to deploy, and lets builders publish and sell their own templates. SourceSynthflow, synthflow.ai/marketplaceread 2026-09-28 |
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| Platform extensibility | ||
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Model Flexibility & Routing Ability to work across multiple foundation models, route tasks to different models, or let buyers bring their own providers and keys. |
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Retell AIModel Flexibility & Routing The customer picks the language model in agent settings and tunes temperature, fast tier, structured output, retries and timeouts, choosing across providers including OpenAI, Anthropic and Google, or runs their own model entirely through the custom LLM WebSocket protocol while Retell handles telephony, transcription and turn-taking. Speech recognition and text-to-speech providers are separately selectable. Sourcedocs.retellai.com/build/llm-optionsread 2026-09-05 |
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SynthflowModel Flexibility & Routing The AI Model selector offers OpenAI models (GPT-4o, GPT-4.1, GPT-5, 5.1, 5.2 and 5.4 families) beside Synthflow's own LLMs so each agent matches its latency, cost and quality needs, and voices can come from ElevenLabs. OpenAI is the only third party LLM provider listed, and Synthflow does not describe a bring your own keys option. SourceSynthflow, docs.synthflow.ai (General Configuration, ElevenLabs)read 2026-09-28 |
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APIs, SDKs & MCP Extensibility Composability layer: stable APIs, SDKs, MCP tool consumption/serving, custom tools, and integration into internal systems. |
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Retell AIAPIs, SDKs & MCP Extensibility Retell publishes a REST API at api.retellai.com with an OpenAPI specification and a Postman collection, covering calls, chats, agents, response engines, knowledge bases, contacts, apps, tests and phone numbers. Developers also get official TypeScript and Python SDKs, a browser Web SDK, a command line interface, a hosted MCP server at mcp.retellai.com for Cursor, Claude Desktop and Claude Code, signed webhooks, and an LLM WebSocket protocol for bringing your own model. A published deprecation feed with dated breaking-change notices sits alongside it. Sourcedocs.retellai.com/api-references/overviewread 2026-09-05 |
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SynthflowAPIs, SDKs & MCP Extensibility Synthflow's Platform API creates, updates and deletes assistants, places calls and batch campaigns, manages chats, simulation cases and suites, actions, knowledge bases, memory stores, contacts, phone numbers and subaccounts, with webhook logs and analytics export, and a Synthflow MCP server sits beside it. SourceSynthflow, docs.synthflow.ai (Platform API introduction, API reference, Synthflow MCP server)read 2026-09-28 |
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Testing, Debugging & Optimization Testing, debugging, scoring, retries, fallbacks, quality gates, and optimization loops for improving agent workflows before and after deployment. |
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Retell AITesting, Debugging & Optimization Retell ships five testing surfaces with readable results. An LLM Playground inspects tool calls and transitions and replays any turn, LLM simulation testing has an AI-simulated user run a scenario while success criteria grade each run, and batch tests over many cases report pass rate and per-case results in a testing history. Web and phone call testing and production A/B testing, which splits live traffic between agent versions, complete the set. Test case definitions, batch tests and test runs each have their own API. Sourcedocs.retellai.com/test/test-overviewread 2026-09-05 |
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SynthflowTesting, Debugging & Optimization Simulation suites generate test cases from the agent's configuration, behavior and actions, judge each against success criteria (all or any), and are rerun after every update as a regression suite before real customers are reached; custom evaluations score live calls, and simulations run through the API. SourceSynthflow, docs.synthflow.ai (Simulations, Custom Evaluations, Simulations API)read 2026-09-28 |
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| Specialist automation | ||
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Browser & Computer Use Browser, desktop, or remote/local computer control for workflows that cannot be handled through stable APIs alone. |
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Retell AIBrowser & Computer Use Agents cannot control a browser, a desktop session or a remote computer. The Press Digit tool and press digit node let the agent infer and press keypad digits to walk a third party's DTMF input IVR menu on an outbound call, and voicemail handling detects and responds to another system's automated flow. Sourcedocs.retellai.com/build/single-multi-prompt/press-digitread 2026-09-05 |
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SynthflowBrowser & Computer Use IVR handling navigates a phone menu by keypad, the only interface the agent operates, and the website and app integration embeds calls in the customer's own product. Synthflow does not describe browser, desktop or computer control. SourceSynthflow, docs.synthflow.ai (IVR Handling, Integrate into Website and Apps)read 2026-09-28 |
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Pricing snapshot
Sourced from the Index pricing dataset · open each vendor's profile for full detail.
| Pricing | R Retell AI |
S Synthflow |
|---|---|---|
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Entry price Lowest public entry point |
Pay as you go from $0 with $10 in free credits, then $0.07 to $0.31 a minute for voice agents. Voice infrastructure is $0.055 a minute, with the voice, LLM and telephony billed separately on top. Enterprise is custom. | Enterprise contracts from $30,000 a year, priced through sales. |
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Pricing confidence How public the numbers are |
Public, partial | Public, partial |
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Billing Primary billing axis |
Usage, billed per connected minute across separately priced components for voice infrastructure, the voice, the LLM and telephony. | A contract scoped on call volume, concurrency, telephony, integrations and support. |
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Variable cost Workload / overage exposure |
High variable cost | High variable cost |
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Free tier / trial Try before you buy |
Free tierTrial
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No free tier
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Buying motion Self-serve vs sales call |
Mixed | Sales call |
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