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The quickstart gets you a working agent. This is how to make one good enough to put on a real phone line.

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Write the prompt in layers

Split what you know across the fields rather than stuffing it all into basePrompt.
Identity in basePrompt, facts in context, rules in customInstructions. Prompts that blend them drift, because the model cannot tell which parts are its character and which are merely true this month.

How these become one prompt

The exact assembly order, what gets omitted when a field is empty, and how firstSpeaker changes what the agent is told.
The single highest-value sentence you can add is a refusal: “If you don’t know something, say so.” Without it, an agent under pressure invents an answer — and on a phone call nobody can see it happening.

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Choose the voice deliberately

Pull voices for your language and actually listen to the previewUrl. A voice that reads well in a list can be wrong at conversational pace.

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Set the call behaviour

For an inbound line, this is usually right:
Why: the agent greets (callers expect it), the greeting cannot be talked over (so a recording notice is actually heard), answers stay consistent, calls cannot run away, and twenty seconds of silence prompts a check-in rather than a hang-up. See Calls for every setting.

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Test before you point a number at it

Create a browser call and talk to it. Then deliberately try to break it:
  • Ask something it cannot know — does it admit that, or invent?
  • Interrupt mid-sentence — does it recover?
  • Go silent — does the inactivity handling feel natural?
  • Say something off-topic — does it come back gracefully?
  • If it has tools, does it call them at the right moment?
Test in the agent’s actual language. An agent that performs well in English can behave differently in Finnish or Spanish — different phrasing, different pacing, different failure modes.

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Then connect it

Give it a number

Answer inbound calls.

Give it tools

Let it do things, not just talk.

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Iterating

Every meaningful update creates a version with an optional note. Change one thing at a time and write down why — six weeks later, “made it friendlier” tells you nothing, but “callers thought it was rushing them” tells you everything.