AssistantsTools

Static variables and aliases

Inject server-controlled values into tool calls without LLM involvement, and chain values across tool calls deterministically

Vapi tools support two features that let you move data between tool calls deterministically, without relying on the LLM to interpret or forward values:

  • Static variables (parameters) inject fixed or template-resolved values into every tool call, regardless of what the LLM generates.
  • Variable extraction (aliases) pull specific fields out of a tool’s JSON response and store them for use in subsequent tool calls.

Combined, these features enable deterministic tool chaining — Tool A fetches data and extracts variables, Tool B receives those variables automatically. The LLM orchestrates when tools run, but the data flow between them is fully controlled by you.

The four pages in this section cover each mechanism in turn, then the failure modes that come up most often.

In this section

How tool variables work

Understand the two parameters fields and the trust tiers that classify where a value came from.

Set static parameters

Inject server-controlled values into every tool call without exposing them to the model.

Extract and chain values

Pull fields out of a tool response with aliases and pass them to the next tool.

Troubleshoot tool variables

Diagnose the five common ways static parameters and aliases fail.

Next steps

Now that you understand static variables and aliases: