AssemblyAI Voice Agent API vs Retell AI
Compare AssemblyAI's managed Voice Agent API with Retell for phone agents: business tools, Twilio setup, human transfers, call history and complete costs.
By Morak · Sources and pricing checked
Choose how your team will build and operate the agent
Our recommendation: evaluate AssemblyAI when your developers want to configure a managed voice agent through an API and integrate it into their application. Evaluate Retell when you want to configure phone workflows, transfers and call operations in a platform. Both still need your business rules and working integrations; AssemblyAI's Voice Agent API is more than its separate transcription service.
Your developers own the agent configuration
AssemblyAI's Voice Agent API lets you configure an agent for a browser, application or phone connection. Its managed stack handles the voice conversation. Shortlist it when your team wants to maintain the configuration alongside its own product and backend.
You want configurable phone operations
Retell offers single-prompt and conversation-flow agents, testing and call monitoring. Shortlist it when the team changing the agent also needs a platform for inspecting calls and managing handoffs. A custom business action can still require an endpoint you operate.
Nobody will maintain the workflow
Start with a packaged receptionist or an implementation partner. Neither a managed API nor an agent builder decides your booking permissions, escalation rules and ongoing support ownership for you.
Explore AI receptionistsWhat you still need to build or configure
Scroll the table horizontally to compare both setups.
| Your task | AssemblyAI Voice Agent API | Retell AI |
|---|---|---|
| Voice conversation | Configure the stored agent, instructions, voice and tools; use the managed stack or connect a custom LLM. | Configure a single prompt or a conversation flow, voice, model and tools. |
| Phone connection | The documented Twilio SIP route connects an existing number for inbound calls. Configure the trunk and bind the agent. | Buy a Retell-managed number or configure custom telephony; the route affects transfer support. |
| Booking or CRM write | Configure an HTTP tool to your backend, or execute a client-side tool in your code. | Configure a custom function to your backend or use a suitable available integration. |
| Permissions and safe retries | Your backend checks caller authorization and prevents duplicate changes. | Your backend still checks authorization and prevents duplicate changes. |
| Investigating failures | Use session recordings and transcripts, then connect the session to your backend logs. | Use call history, tool results and detail logs, then connect the call to your backend logs. |
Example: change an appointment without a duplicate booking
Use the same booking service for both evaluations. It should return a saved appointment identifier on success and a specific failure when the slot is unavailable. Do not let the agent infer that a booking succeeded from a fluent conversation.
AssemblyAI: configure an HTTP booking tool
- Create an agent with instructions and a booking tool
- Configure the tool's URL, parameters and authorization
- Return the saved booking or a precise error to the agent
AssemblyAI can make the HTTP request itself, so this route does not require your own WebSocket tool dispatcher. Your team supplies the endpoint, credentials, argument schema and business validation. Client-side tools are an alternative when the action must run in your code.
Retell: configure a custom function
- Choose a prompt or flow and define when the tool runs
- Configure the function's endpoint, arguments and authentication
- Use the backend result to confirm the change or recover
Retell sends the function arguments to your endpoint and supplies its response to the agent. Configure the timeout and speech while waiting. Retell documents that interrupting the agent does not cancel a request already running, so the backend must reconcile the actual booking state.
How human transfers work
AssemblyAI: plan the carrier action
AssemblyAI's transfer example has your backend handle a tool call and initiate the transfer through the telephony provider, passing a summary. Treat that as integration work to implement and test, rather than assuming the inbound SIP quickstart also supplies your whole human-handoff flow.
Retell: check the chosen telephony method
Retell supplies cold, warm and agentic warm transfer modes. Its dial-to-SIP-URI method cannot use the built-in transfer feature; your system must initiate the transfer. Choose the phone architecture before counting on a platform handoff setting.
A budget worksheet for 1,000 inbound minutes
Assume 1,000 connected conversation minutes, one phone number and no negotiated discount. AssemblyAI uses its included default models. Retell's price depends on the configured model, voice and options; its advertised range is not a matched configuration. USD before taxes.
AssemblyAI's published agent charge would be $75 for this workload before carrier and other external costs. Retell's advertised agent range gives $70–$310 before reconciling telephony and options. Neither is a complete monthly quote or evidence of a cheaper equivalent setup.
Scroll the table horizontally to compare both setups.
| Cost component | AssemblyAI Voice Agent API | Retell AI |
|---|---|---|
| Managed agent usage | 1,000 × $0.075/min = $75; published default STT, LLM and TTS are included. | 1,000 × advertised $0.07–$0.31/min = $70–$310; select an actual configuration before comparing. |
| Phone service and number | Add the Twilio number and inbound calling bill for the documented SIP route. | Add the selected number and calling charges. A zero custom-telephony line in the estimator does not remove your carrier's bill. |
| Model or feature changes | If you bring a model endpoint, include its provider bill and confirm the AssemblyAI rate that applies. | Check whether the selected voice, model and add-ons are already included in your estimate; avoid adding them twice. |
| After a human transfer | Estimate the carrier's transferred call legs and confirm when the agent session ends. | The AI fee stops after the agent drops off; telephony continues for the remaining call. |
| Implementation and support | Estimate tool endpoints, carrier integration, testing and ongoing support. | Estimate configuration, custom endpoints, testing and ongoing support. |
AssemblyAI publishes per-second billing for connected conversation time. Retell also bills connected time, including silence while its agent remains active. Use measured session duration rather than only the time somebody speaks.
Compare the same task, call route and model requirements. A broad range and a bundled default are different pricing scopes; they do not identify a universal winner.
Record setup hours and successful task counts alongside usage. A lower agent bill can be outweighed by the work required to support your integration.
What to test before committing
Give each route the same synthetic appointment-change task. Keep a score sheet for correct saved changes, false confirmations, human escalations, setup hours and complete usage costs. Include someone who will operate the agent after launch in the review.
Slot was taken
The agent offers another available slot instead of claiming the rejected change succeeded.
Caller changes their mind mid-request
Inspect the backend's final state. An interrupted spoken answer must not hide a booking action that already completed.
Human transfer fails
Check what the caller hears and whether the agent resumes or records a callback request. Verify it on the actual carrier route.
Operator investigates a complaint
The person maintaining the agent can find the call, inspect the tool result and identify the matching backend request.
Common questions
Is AssemblyAI only a transcription API?
No. This comparison covers its Voice Agent API, which configures and runs speaking agents. Its standalone speech-to-text API is a separate product and price scope.
Must I build a media server for AssemblyAI inbound calls?
Not for its documented Twilio SIP route. Twilio forwards the call to AssemblyAI. You still configure the number and agent binding, and provide the business tools your workflow requires. The cited guide covers inbound calls only.
Can I use my existing carrier with Retell?
Retell documents custom telephony routes. Confirm your provider's compatibility and transfer support; dial-to-SIP-URI has a different handoff responsibility from the managed-number route.
Sources and pricing references (14)
Official documentation and pricing used for this comparison.
- AssemblyAI Voice Agent API quickstart
- Retell build, deploy and monitoring overview
- Retell custom business functions
- AssemblyAI server-side HTTP tools
- AssemblyAI's human-transfer implementation example
- Retell custom telephony and transfer restrictions
- AssemblyAI inbound Twilio SIP route
- AssemblyAI agent rate and included services
- Retell rates, estimator and transfer billing
- AssemblyAI custom model connection
- AssemblyAI session recordings and transcripts
- Retell call history and debugging
- Retell human-transfer modes
- Retell-managed phone numbers
Voice Agent Platform
AssemblyAI
Voice AI infrastructure and managed Voice Agent API for production real-time voice agents.
Voice Agent Platform
Retell AI
Voice AI platform for building low-latency phone agents across support, scheduling, and sales workflows.