GuidesAI agents
Classic and Live engines
How the two engines run a call, which settings each one uses, and how to choose between them.
Every AI agent runs on an engine, which decides how it listens and speaks. Classic gives you exact wording and fine control over turn-taking. Live sounds more natural, but says things in its own words.
- Choose it
- When you create an inbound agent. It can’t be changed later
- Live
- Inbound agents, English voices
- Classic
- Inbound and outbound agents, in English, Spanish and French
How each one works
Classic
Transcribes, thinks, then speaks.
- 01Listens until the caller has finished speaking
- 02Turns what they said into text
- 03Decides the reply, using your instructions, documents and Tools
- 04Speaks it, word for word for your fixed lines
Live
Listens and speaks at the same time.
- 01Listens while it talks, and decides when to reply or stop
- 02Hands lookups and Tools to a second part that does the work
- 03Keeps the conversation going while that runs
- 04Works the answer in, in its own words
Which to choose
| Choose | When |
|---|---|
| Classic | Wording matters: a legal notice, a fixed greeting, an exact farewell. You need another language, an outbound agent, or control over interruptions and pauses |
| Live | A natural, flowing conversation matters most, the agent answers in English, and it’s fine for the agent to put things in its own words |
Not sure? Start with Classic. It’s the default, and every setting works on it.
What’s different
| Setting | Classic | Live |
|---|---|---|
| Greeting | Said word for word | Said in its own words |
| Let the agent generate its own greeting | Available | Not offered |
| Farewell, idle prompt and handoff announcements | Said word for word | Said in its own words |
| Speaking speed, stability, similarity | Available | Not offered: Live voices have a fixed pace |
| Response variability | Available | Not offered |
| Interruption sensitivity and override words | Available | The engine decides when to stop |
| Endpointing | Available | The engine decides when the caller has finished |
| Boosted keywords | Available | Not offered |
| Filler phrases | Available | Not offered |
| Speech while a Tool runs | Your line, or nothing | Its own words, and it keeps talking |
| Voices | English, Spanish and French voices | Eight English voices |
| Outbound agents | Yes | No |
Settings that don’t apply are hidden in the builder of a Live agent.
What’s the same
Both engines use your instructions, Knowledge, Tools, Handoff, End call, background sound, the idle prompt and timeout, the maximum call duration, the date and time, and Acknowledge while the caller is speaking. Testing, publishing and versions work the same way. Both use AI minutes in the same way.
Voices
| Engine | Languages and accents |
|---|---|
| Classic | English (US, UK, Australia, New Zealand), Spanish (Mexico, Spain), French (Canada, France) |
| Live | English: US, UK and Australian voices |
The voice picker only shows voices for the agent’s engine.
Instructions on Live
A Live agent talks with the caller and hands the work, such as Tools and your documents, to a second part. Effective prompt shows what each one gets: What the voice hears and What the backend follows. The voice hears your Role, Tone and the start of your Never rules, so keep Never short and put the most important rules first. See On Live agents.
Live agents are also told not to state addresses, phone numbers, prices or policies unless they came from your instructions, a Tool, your documents or the caller.
Tools on Live
A Live agent tells the caller what it’s checking in its own words and keeps talking while the Tool runs, then works the result in when it arrives. It uses one Tool at a time. Minimum time before answering and Give up after still apply. See Classic and Live agents.
Change engine
You can’t change an agent’s engine. To use the other one, create a new agent with it, copy your instructions across, and attach the same Knowledge, Tools and destinations. Then point your Call Flow at the new agent.
Versions keep their engine too: an agent can’t roll back to a version from the other engine.
If something goes wrong
Live is for inbound agents. An outbound agent always runs on Classic.
Live agents speak English. For another language, create a Classic agent.
Choose a voice from the list: it only shows voices for this agent’s engine.
That’s how Live works. If the exact words matter, use a Classic agent.
On Live agents, the transcript can trail the audio by a few seconds. The call itself isn’t delayed.
Last updated 18 Oct 2026