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 adds genuinely non-obvious behavioral context: the exact monthly-cost formula, the ordinary-annuity convention (contributions at month end), and a worked example showing the expected magnitude of output.

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?

Front-loaded with the core purpose, then the formula and a compact example; every sentence carries information. It is dense but not padded, and the modeling assumption is stated rather than 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?

An output schema exists, so return values need not be explained, and the description supplies the remaining essentials: the calculation model, the annuity timing convention, and a sanity-check example. Only the choice of defaults vs siblings is not addressed.

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 baseline is 3, and the description goes further by tying parameters together in a formula (cost × perWeek × 52 / 12) and a concrete example ({cost:5, years:30, rate:8, perWeek:5} → ~108/month). This clarifies how cost and perWeek interact, which the individual schema entries do not.

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?

States a specific verb and subject: calculate the lifetime cost of a small recurring daily expense and the invested alternative, with concrete examples (coffee, cigarettes, takeaway). The purpose is unambiguous, though it never names the closest siblings (cost_of_waiting, compound_interest) to disambiguate scope.

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 scenario described ('small recurring daily expense') implies when the tool fits, but there is no explicit when-to-use guidance and no routing against nearby calculators like compound_interest or cost_of_waiting. Usage is inferable rather than stated.

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