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The Extract Dynamic Variable tool captures values from the conversation while a call is in progress and stores them as dynamic variables your agent can reuse in later turns. The agent decides when to call it from the tool’s name, the variables you define, and your agent prompt. For example, a booking agent can extract the caller’s name and email as soon as they give them, then reference {{user_name}} in its confirmation and pass {{user_email}} to a later custom function that creates the appointment. If you use a conversation flow agent, see Extract Dynamic Variable node.

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When to use it

Use this tool when the agent needs a value again during the same call — to personalize a later response, branch on it, or send it to a function or transfer. Because the agent chooses when to call the tool, extraction isn’t guaranteed to run at an exact point. When a value must be captured in a strict order, use a conversation flow instead.

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Create the tool

1

Add an Extract Dynamic Variable tool

In the agent’s Functions section, select + Add, then select Extract Dynamic Variable.
Function-type menu with Extract Dynamic Variable listed above Custom Function.
2

Name the tool

Give the tool a Function Name with 1–64 letters, numbers, underscores, or dashes. Spaces aren’t allowed, and the name must be unique among the agent’s tools.Add a short Description summarizing what the tool captures. What actually steers extraction is the per-variable descriptions (next step) and your agent prompt, so keep this field brief.
  • Function Name: extract_user_details
  • Description: Captures the caller's name and email.
Extract Dynamic Variable dialog with Function Name, Description, and a Variables list.
3

Add variables

Select + Add under Variables and fill in the fields for each value you want to capture:
  • Variable Name – the name you’ll reference as {{variable_name}}. Keep it short and unique within the tool.
  • Variable Description – what the value is. The agent uses this to decide what to pull from the conversation, so a clear description is the main driver of extraction accuracy.
  • Variable Type – Text, Number, Boolean, or Enum. Optional; defaults to Text.
  • Enum Options – the allowed values, added one per row. Shown only when the type is Enum. At least one option is required.
Variable editor with Variable Name, Variable Description, and a Variable Type dropdown set to Text.
Select Save, then add more variables as needed.
4

Save the tool

Select Save to add the tool to the agent. Each variable appears in the tool’s Variables list.
Extract Dynamic Variable dialog showing the saved user_name variable in the Variables list.
5

Tell the agent when to extract

Add an instruction to the agent prompt so the agent calls the tool at the right moment.

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Variable types

Pick the type that matches the value. The type guides how the agent formats what it extracts. All extracted values are stored as strings, matching how dynamic variables work everywhere else in Retell. A Number is stored as "42" and a Boolean as "true". In a Code Tool, convert them before use, for example Number(dv.order_count).

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Use the extracted values

Once extracted, a value behaves like any other dynamic variable: If the caller never provides a value, the tool doesn’t set that variable, and {{variable_name}} stays in its raw form. See handling missing variables for how to write prompts that tolerate an unset value.

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Play typing sound

Turn on Play typing sound in the tool to play a subtle typing sound on the agent’s audio track while it extracts. Extraction is usually fast, so most agents leave this off.

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FAQ

Yes. Add as many variables as you need under one tool, and the agent fills the ones the conversation supports in a single call. Group related values (name and email, for example) so they’re captured together.
The tool only sets variables it can extract. A value the caller never provides isn’t stored, so {{variable_name}} stays literal until it’s set. Design prompts to handle an unset variable, or set a default at the agent level.
Yes. If the tool extracts the same variable again, the new value replaces the old one for the rest of the call.
No. This tool captures values during the call so the agent can use them in later turns. Post-call analysis extracts data after the call ends for reporting and webhooks, and those values aren’t available mid-call.
Spell out in your agent prompt when to call the tool (for example, “call extract_user_details once, right after the caller gives both their name and email”). Since the agent prompt and variable descriptions drive the timing, vague instructions lead the agent to call the tool repeatedly.