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

Annotations already declare readOnlyHint=true and openWorldHint=true. The description adds behavioral context: with no argument it lists all countries; with a country code it lists sub-regions, returning code-and-name pairs. It also clarifies that these codes are acceptable inputs elsewhere, which goes beyond the annotation.

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 exactly two sentences, front-loaded with the core purpose, and every clause adds value. No filler or repetition of schema content.

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 read-only reference tool with one optional parameter and no output schema, the description fully explains the input/output behavior (list of code/name pairs) and its integration with sibling tools. No critical information is missing.

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 coverage is 100% with a detailed description for the `country` parameter. The description adds semantic meaning by explaining the effect of omitting the argument (list all countries) versus supplying a country code (list sub-regions). This complements the schema's format-level details.

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 states a specific verb and resource: 'Returns the geo codes Google Trends accepts.' It clearly distinguishes itself from trend-analysis siblings by explaining it provides region codes, not trend data. The behavior with and without an argument is also described.

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 gives practical usage context: 'Any code returned here can be passed as the `region` argument of the other tools.' This tells the agent when to use this tool (to obtain valid region codes for sibling tools), but it does not explicitly state exclusions or alternatives beyond that implicit guidance.

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