# Tool Calling

Source: https://customlabs.io/glossary/tool-calling/
Updated: 2026-09-13

Agents

# Tool Calling

Also known as function calling

Tool calling lets a model request a structured action instead of only generating text.

The calling application executes the action and returns the result to the model.

It is the primitive underneath every agent, which is a model given tools and a loop to call them in.

Reliability comes down to tight tool schemas and validating what the model asks for before executing it.

[Capabilities](https://customlabs.io/capabilities/)[AI Integrations](https://customlabs.io/services/ai-integrations/)[Field guide: Failure modes](https://customlabs.io/failure-modes/)

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## Related terms

[Agent (Agentic AI) An agent is an LLM given a loop, memory, and tools it can call.](https://customlabs.io/glossary/agent/)[Tool Contract A tool contract is the schema a tool exposes to a model.](https://customlabs.io/glossary/tool-contract/)[Structured Output Structured output constrains a model's response to a defined schema instead of free-form prose.](https://customlabs.io/glossary/structured-output/)[Idempotency An idempotent operation produces the same result no matter how many times it runs.](https://customlabs.io/glossary/idempotency/)

## More in Agents

[Model Context Protocol (MCP) MCP is an open standard for connecting LLM applications to tools and data sources.](https://customlabs.io/glossary/model-context-protocol/)[Context Engineering Context engineering decides what occupies a model's context window at every step, and in what order.](https://customlabs.io/glossary/context-engineering/)[Guardrails Guardrails are the checks that keep an LLM or agent inside acceptable bounds in production.](https://customlabs.io/glossary/guardrails/)[Golden Path A golden path is the single sanctioned way to do a common piece of work.](https://customlabs.io/glossary/golden-path/)