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

Beond the readOnlyHint annotation, the description discloses the exact return shape, the meaning of the dimension value (ISO alpha-2 code), and the distinction between the code and display label. This is high-value behavioral context, especially since there is no output schema.

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?

The description is short, front-loaded with the purpose, and the included JSON example earns its place by compensating for the missing output schema. There is no filler or repetition.

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?

With all three optional parameters fully documented in the schema, the only material gap is the return contract, which the description supplies completely and precisely. An agent can invoke this tool and interpret the result without additional inference.

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?

The input schema covers 100% of parameter descriptions, including defaults, patterns, and site_id vs domain behavior. The tool description adds no further parameter semantics, so the baseline of 3 applies.

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 phrase 'Top countries by visitors for a site' clearly names the resource (countries) and metric (visitors). It is readily distinguishable from sibling tools like top_pages, top_sources, and top_custom_events without needing to open the schema.

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?

The description makes the use-case clear: a site's visitor rankings by country. It does not explicitly name alternative tools or say when not to use it, but the context is unambiguous enough that an agent can select it correctly.

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