# Tool Contract

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

Agents

# Tool Contract

A tool contract is the schema a tool exposes to a model.

It lists parameter names, types, required fields, and any enums or patterns that constrain them.

A loose contract lets a model fill an ambiguous field with a plausible-looking guess.

A tight contract, validated server-side, stops a hallucinated argument from reaching execution.

It is the one piece of documentation a model reads before every call.

[The Agent Tool Interface](https://customlabs.io/tool-design/)[Pattern: Typed tool contract](https://customlabs.io/patterns/typed-tool-contract/)

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## Related terms

[Tool Calling Tool calling lets a model request a structured action instead of only generating text.](https://customlabs.io/glossary/tool-calling/)[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/)[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

[Agent (Agentic AI) An agent is an LLM given a loop, memory, and tools it can call.](https://customlabs.io/glossary/agent/)[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/)