Tool Manifest

What it contains

• Name: a stable, meaningful identifier.
• Description: what the tool does, when to use it, and any side effects or limits — the model's main basis for choosing it.
• Parameter schema: each parameter's type, whether required, allowed range, default — usually a piece of JSON Schema.

The runtime uses this declaration to present the tool to the model as an available action and to validate arguments after the model proposes a call.

Getting it right

Descriptions should state boundaries and side effects ("actually sends email," "read-only"), parameter names should be plain, and enum values complete. In practice, the wording of a tool manifest often affects agent success more than swapping in a stronger model.

Common misconceptions

• A tool manifest is not a doc comment — it enters the model's context and affects token cost and selection accuracy.
• Type constraints alone are not enough — without a clear description the model still misjudges *whether* to use it.

Related terms

Sources

1. JSON Schema — specification
2. Anthropic — Claude Docs: Tool use, defining tools