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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so safety is covered. The description usefully adds that the calculation supports two input shapes and emits a full-year run rate, but it does not say what happens if both input modes are supplied, if monthly arrays are uneven, or what the response contains.

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?

A single front-loaded sentence that states the comparison, the accepted inputs, and the extra output. Nothing redundant and nothing buried.

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 no output schema, the description should say what the tool actually returns beyond 'a simple full-year run rate' – the growth figure and its shape are unstated. All five parameters are optional, and the description does not explain which combinations are valid, leaving a gap for a calculator with mutually exclusive input modes.

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%, so the baseline is 3. The description does add a little meaning by framing this_ytd_total/last_ytd_total as one alternative and the monthly arrays as another, but it does not clarify constraints such as matching lengths or that months_elapsed is what enables the run rate.

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?

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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?

Usage is implied by the mention of YTD comparison and the two accepted input modes (monthly values or YTD totals), so an agent can infer the scenario. There is no explicit when-to-use or when-not, and no pointer to growth_rate or period_change for non-YTD comparisons.

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