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Overview

Variables are the data layer of Flow Builder. They let you store, transform, and inject dynamic values throughout your flow — in prompts, URLs, headers, request bodies, and assignments. The syntax: wrap any variable name in double curly braces like {{variable_name}}.

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Substitution Syntax

{{variable_name}} works everywhere text is accepted:
WhereExample
Conversation promptsYou are speaking with {{caller_name}}
Request URLshttps://api.example.com/users/{{user_id}}
Request headersAuthorization: Bearer {{api_token}}
Request body{"email": "{{email}}"}
Variable assignments{{first_name}} {{last_name}}
Query parametersfilter={{status}}&limit=10

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Variable Categories

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Right Now

Date/time variables that resolve at runtime:
  • {{current_date}} — Today’s date
  • {{current_time}} — Current time
  • {{current_day}} — Day of the week

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System

Session-level variables populated automatically:
  • {{session_key}} — Unique session identifier
  • {{flow_id}} — ID of the running flow

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Flow Variables

User-defined variables set via Variable nodes, Extraction nodes, or initialized via webhook.

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Node Outputs

Automatically created by specific nodes:
  • Extraction nodes — Each extraction creates a variable
  • Request nodes — Creates {{response}}, {{response_status}}, {{response_error}}
  • Tool/Function nodes — Results via {{_last_function_result}}

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Special Variables

VariableDescription
{{_last_function_result}}Return value of the most recently executed Tool/Function node
{{responseVariable_status}}HTTP status from a Request node
{{responseVariable_error}}Error message from a failed Request node

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Variables Webhook (Initialization)

Configure a variables webhook at the flow level. When a new session starts, the engine calls this webhook before the first node executes. The response should return JSON key-value pairs: 
{
  "caller_name": "Sarah Johnson",
  "account_id": "ACC-4821",
  "plan": "premium",
  "language": "English"
}
These variables are immediately available in your start node’s prompt and throughout the flow.

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The VariablePicker UI

Click the {x} icon in any field to open the VariablePicker — browse all available variables organized by category with search.

Extraction & Variable Nodes

Set and extract variable values

Logic Node

Branch based on variable values

Request Node

Use variables in API calls

Conversation Node

Use variables in prompts