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The flow builder is a visual canvas where you design a call as a graph: nodes do the work, edges define where the conversation can go next. Under the hood, each agent node is a full conversational agent, and moving along an edge is a handoff between agents.

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The mental model

  • Nodes are steps: greet, converse, decide, call an API, collect data, transfer, end.
  • Edges are possible paths. An agent node with three outgoing edges can hand the conversation to three different next steps.
  • The LLM chooses the path — based on the conversation and, crucially, on your edge labels.

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Why edge and agent labels matter

This is the most important concept in the flow builder:
Edge labels are not decoration. Every outgoing edge of an agent node becomes a handoff tool that the LLM can call — and the edge label becomes that tool’s description. The LLM decides where to route the conversation by reading your labels. Vague labels produce vague routing.
Compare: The same applies to condition nodes: the node’s description tells the LLM what is being decided, and each edge label describes one outcome. The LLM is forced to pick exactly one edge — so the labels must be mutually exclusive and cover all cases you expect. Agent node names matter too: they appear in handoff tooling and logs, so Qualification agent beats Agent 2.

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A minimal useful flow

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Variables

collect and dtmf nodes store results in flow variables (e.g. customer_phone). Variables are included in the call.completed webhook and the call details, so downstream systems get structured data — not just a transcript.

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Fallback behavior

  • An agent node with empty instructions uses the assistant’s system prompt (Advanced prompt) as the base — with node text, that text is appended. The flow never runs an agent without instructions.
  • A collect node that fails after its retry budget takes the outgoing edge labeled failed if one exists, otherwise the normal edge.
  • Broken flow configurations never crash a call: the engine falls back to a safe stack and logs a flow_node_error event.
Continue with the node reference and best practices.