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

The description does not contradict the readOnlyHint annotation. However, it adds no behavioral context beyond the annotation—such as return format or handling of zero impressions. Given the annotation covers safety, a neutral score is appropriate.

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 one sentence and front-loaded, making it extremely concise. However, it borders on under-specification; still, for a simple calculator, it's appropriately brief.

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?

The tool is simple with full schema coverage and a read-only annotation. However, no output schema exists, and the description doesn't mention the return format (e.g., decimal or percentage). This leaves a minor gap, so it's not fully complete.

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 already describes both parameters ('Clicks' and 'Impressions') with 100% coverage. The description adds no additional parameter semantics, so the baseline of 3 is applied.

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 uses the specific verb 'Calculate' and identifies the resource as 'Click-through rate', making the tool's purpose clear. It distinguishes from sibling calculators by naming a distinct metric, though it doesn't explicitly contrast with them.

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 provides no guidance on when to use this tool versus the many sibling calculators (e.g., CPC, CPM, ROAS). It doesn't state when this metric is appropriate or exclude other cases, leaving usage implied.

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