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What Is Function Calling?

Function calling lets the agent act during a call: book an appointment, look up an order, update a CRM record or send a confirmation. It calls the tool mid-conversation and works the result into its next reply. Tools work in both Live and Legacy calls.

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Built-in Tools

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end_call

Every call has end_call. The agent uses it when the caller wraps up, asks to stop, or asks not to be contacted again, and says a short goodbye before it hangs up. A caller who goes silent is handled separately, without end_call (see How Calls End).
Shape this in your instructions, for example: “Offer to help with anything else before you say goodbye.”

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Transfer

When a campaign has transfer targets, the agent can also hand the caller to a person. See Transfer to a Person.

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Custom Tools

Build a flow in Integrations, such as “book a callback in my calendar” or “send an SMS confirmation”. Then select it as a tool in the campaign editor, or list its name in the campaign’s mcp_tool_allowlist. Tools reach calls only through a campaign, and the agent gets only the flows that campaign selects. Connecting Integrations attaches nothing on its own. A call placed with POST /v1/calls gets no Integrations tools, only end_call.

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How a Tool Call Works

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1. The agent decides

It picks a tool, based on the tool’s description and what the caller asked.
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2. The tool runs

The platform calls the tool with the arguments the agent chose.
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3. The agent replies

The result goes back to the agent, which uses it in its next reply.

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Errors and Timeouts

A tool has 8 seconds to answer by default, and a Live call never waits longer than 9. A slow tool can leave the line quiet. If the tool fails or times out, the agent is told and carries on, for example:
“I’m having trouble reaching the calendar right now. Can I take your preferred time and have someone confirm it shortly?”
Return a clear error message from your tool, and the agent can explain the problem without reading out raw errors.

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Best Practices

  • Write precise tool names and descriptions. “Helper function” gives the agent nothing to decide with.
  • Answer in under 2 seconds if you can, so the line doesn’t go quiet.
  • Validate arguments on your side before you act on them, and return data in a consistent shape.
  • Select only the tools the campaign needs. Five to ten works well.
  • Keep secrets on your server, never in instructions.

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Next Steps

Integrations

Build no-code tools and flows for your agent.

Making Calls

Place single calls through the API.