Function calling

Function calling lets a model propose a structured function name and arguments for software to validate and optionally execute.

Plan for this page

Understand one concept in plain language and connect it to nearby ideas.

You will leave with: The model usually does not execute the function by itself; an application receives the proposed call, checks schema and policy, runs approved code, and may return an observation. Output structure improves integration but does not guarantee valid intent or arguments.
Time: 3–8 min
Before you begin: No prerequisite. Keep one real example in mind.
Do this now: Read the explanation, then compare the example with the common misunderstanding.

Prepared next stepLearn how agents act through software

Function callingFunction calling lets a model propose a structured function name and arguments for software to validate and optionally execute.

Tool use
Guardrail
API

Plain explanation

The model usually does not execute the function by itself; an application receives the proposed call, checks schema and policy, runs approved code, and may return an observation. Output structure improves integration but does not guarantee valid intent or arguments.

Example

The model emits `get_weather` with a city and date, and the application validates those fields before calling a weather service.

Why it matters

Structured calls connect probabilistic generation to deterministic software. Validation, permissions, idempotency, timeouts, and error handling are needed at that boundary.

Common misunderstanding

Misconception: a valid JSON function call is automatically safe to run. Correction: syntactic validity does not establish authorization, purpose, factual inputs, or acceptable consequence.

Related concepts

Follow relationships, not only the alphabet

Tool useTool use is a system’s ability to invoke an external function, service, interface, or device and incorporate the result.GuardrailA guardrail is a preventive, detective, or corrective control intended to keep system behavior within defined boundaries.APIAn API, or application programming interface, is a defined way for software components to request functions or exchange data.

Learn how agents act through software

Sources and scope

What supports this object

Hugging Face Agents Course — IntroductionHugging Face

Shows structured tool selection and argument generation in agent implementations; naming and execution behavior differ by platform.

Open source

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