An agent is a prompt, a voice, and its tools
Define a voice agent as a small object and manage it in code: create it, refine the prompt, swap the voice, register tools, all over a REST API. Define the behaviour once and reuse the agent across unlimited numbers. Flat $0.08 a minute.
The prompt is where the agent lives
Most of an agent's behaviour comes from its prompt. It sets the persona, the scope of what the agent handles, the rules it follows, and how it responds when a caller says something unexpected. The rest of the object configures the surface: which voice speaks, which language it opens in, and which tools it can reach for. Get the prompt right and the agent is production-ready; refine it and every future call improves.
Manage agents over four verbs
Create an agent with a prompt, voice, language and tools. You get back an id to attach numbers to.
List your agents and read any one back, so your own dashboard can show and edit them.
Update an agent by sending the changed fields. Refining a prompt is one call, live on the next call.
Remove an agent you no longer need. Numbers can be moved to another agent first.
Define once, reuse everywhere
An agent is a template, not a single deployment. One agent carries one personality and prompt, and it attaches to as many numbers as you have customers. Change the template and every number behind it changes with it. That is how a single agent answers across two hundred separate numbers without two hundred configurations to keep in sync.
Give the agent real actions
An agent is more useful when it can do things, not just talk. Register custom functions and the agent calls your API mid-conversation, using the response to continue. Booking works the same way, so the agent can check availability and confirm an appointment inside the call. Anything that must be exact is supplied by the engine, so the agent acts on facts rather than guesses.
See the Function calling, Tools and Multi-tenant pages, or the full docs. Coming from another platform? Nixflex is a modern alternative to Retell, Vapi and Bland. Compare them →
Developer FAQ
What actually defines an agent?
An agent is a small object: a name, a prompt that sets its persona and rules, a voice, a language, and the tools it can use. The prompt does most of the work, since it decides how the agent talks, what it is allowed to do, and how it behaves when it is unsure. Everything else configures the surface around that.
How do I create and update agents?
POST to /v1/agents to create one and you get back an id. GET lists your agents, PUT updates one, and DELETE removes it. Updating is just sending the changed fields, so refining a prompt is a single call, not a redeploy.
Can one agent serve many numbers?
Yes, and this is the core idea. An agent is a template with one personality, and it attaches to unlimited phone numbers. You define the behaviour once and reuse it across every number and every customer, instead of maintaining a separate agent per line.
How do I give an agent tools?
Register custom functions on the number the agent runs on, and the agent can call your API mid-conversation, using the JSON you return to keep talking. Booking integrations like Cal.com and Google Calendar work the same way, so the agent can check availability and book inside a call.
How fast can I iterate on behaviour?
Fast, because behaviour is configuration, not code. Change the prompt or swap the voice and the next call uses it, with no build step. You can hear the change yourself on a web call or a test call before any customer does.
Is there an SDK for this?
Not yet. The agent endpoints are plain REST, so you manage agents from any language with your existing HTTP client, and typed client libraries are on the roadmap. The fields and verbs in the docs are exactly what you send.
What does it cost to run an agent?
A flat $0.08 per minute of voice while an agent is on a call, premium model included, with your own carrier underneath. Creating and updating agents costs nothing; you pay for talk time, not for the agents themselves.
Define your first agent today
Write a prompt, pick a voice, attach a number. Iterate in minutes, not deploys.