Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already cover readOnly, idempotent, openWorld, and non-destructive behavior. The description adds useful context beyond that by revealing that the tool is lightweight and cheaper than phone_validate, and clarifies the returned scope (country, region, primary timezone).

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Two concise sentences front-load the core purpose and then provide a cost-based comparison with a sibling. Every sentence earns its place, with no redundant detail.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple two-parameter lookup with rich annotations and no output schema, the description is largely complete: it states the input kind, the output scope, and the key alternative. The only notable gap is the unaddressed default_region parameter, but it is understandable from its name and default value.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0%, so the description must compensate for parameter meaning. It clarifies that the tool operates on a phone number, but it does not explain the purpose or effect of default_region, nor any formatting expectations for phone.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description names a specific operation: a lightweight country + region + primary timezone lookup for a phone number. It also differentiates from the sibling phone_validate by noting it is cheaper when only the country header is needed.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives clear context for when to use this tool: when only the country header is needed, and explicitly references phone_validate as the costlier alternative. However, it does not explicitly state when to choose phone_validate over this tool, so the guidance is clear but not fully explicit.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.