> ## Documentation Index
> Fetch the complete documentation index at: https://docs.agentcallback.com/llms.txt
> Use this file to discover all available pages before exploring further.

# State variables

> Track data that changes while the call is in progress.

State variables hold data your agent captures or updates *during* a call — as opposed to [variables](/build-voice-agents/variables), which are set before the call starts.

<Note>
  Use variables for data you already know before the call. Use state variables for data the agent learns or decides as the conversation happens.
</Note>

## Declare a state variable

A state variable is a `key` and a `defaultValue` — the value it holds until the agent updates it during the call.

## Capture state in Pathways

In a [Pathways](/build-voice-agents/pathways) agent, conversation nodes can declare state captures — data the agent should extract from the caller at that step, with a description to guide it. Transitions can also declare state updates, setting a state variable's value as the agent moves to the next node.

## Update state outside Pathways

On a [Single Prompt](/build-voice-agents/single-prompt) agent, the agent updates state using the `updateCallState` tool. It can only set keys you've declared as allowed for the agent — this keeps the agent from writing arbitrary state during the call.

## Reference state later

Reference a state variable's current value in your prompt with `{{state.variableKey}}`. State variables also feed into other parts of the platform:

* [Call outcome](/build-voice-agents/call-outcomes) classification can take final state into account.
* [Post-call webhooks](/build-voice-agents/post-call-webhooks) can include state values in their payload, and use them as trigger conditions.

## Next step

[Tool calls](/build-voice-agents/tool-calls) to let your agent take actions during the call.