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Tool use and automation concept

What is tool calling?

Reference

What is tool calling? It is a way for a language model to request an operation, such as looking up information or changing a record, through an application that connects it to tools. The model produces a structured request with a tool name and arguments; depending on the setup, the application or a service runs it and can return the result for the model to use.

Reviewed 2026-10-09

A request that connects a model to an operation

A tool is an operation a model can ask to use, such as retrieving information or carrying out an action. Tool calling is the exchange in which the model selects an available tool and returns a structured request naming it and supplying arguments. The application or service that connects the model to the tool handles what happens next. For example, a weather tool might receive a city name and return current conditions. The model’s request is a handoff; by itself, it does not show that the operation ran or that its result is correct.

The request, execution and result

First, the application makes descriptions of available tools and their expected inputs available to the model. The model can then reply with an ordinary answer or request a tool by returning its name and arguments. For a custom tool, the application receives that request, checks it, and decides whether to run the operation. It sends the result back to the model, which can use it in a reply or request another tool. This back-and-forth may repeat. Some products also provide built-in tools that the service runs for the application, so the exact handoff depends on the setup.

A way to connect model output to action

Tool calling sits between a model’s response and the software that can do something with it. It is useful when an answer needs information or an operation outside the model’s conversation, such as querying a database or submitting a request. It is different from asking for structured output alone: structured output shapes the model’s answer, while tool calling requests an intermediate operation and may return its result to the model. Tool calling can be part of an agent or workflow, but the call itself is only one mechanism within that larger system.

Names, permissions and what a call means

“Function calling” and “tool calling” often refer to the same kind of model-to-application handoff. The names vary: some documentation calls the feature function calling, while other documentation calls it tool use or tool calling. A tool call is also not the same as executing a function: it is the model’s request, and execution depends on the application or service handling it. Nor does a call automatically mean the model has permission to perform every requested action. The application controls which tools are available and should check requests before consequential actions.

Questions people ask

Does the model run the tool itself?

For a custom tool, the model returns a request and the application runs the operation. Some services also offer built-in tools that they execute on the application’s behalf.

What is the difference between function calling and tool calling?

Often, there is no practical difference: providers use these names for a model requesting an operation through an application. Terminology varies, and some use “tool” as the broader label.

How is tool calling different from structured output?

Structured output shapes the model’s answer to fit a format or schema. Tool calling requests an operation, and the result can be sent back to the model for a later reply.

Does a tool call mean the requested action has happened?

No. A call is a structured request. The application or service still has to handle it, and custom-tool execution is done by the application.

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What changed

Sources

Part of Tool use and automation: How a model calls a toolValidate requests before acting

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