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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is fully covered. The description's listing of return fields is largely redundant with the output schema, and it adds no behavioral detail such as input-domain limits (annual_rate must be > 0) or rounding behavior. The rule72-vs-precise distinction is a minor value add.

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?

Two sentences, no filler, with the core purpose front-loaded before the return shape and the discovery pointer. Every sentence carries information and nothing is padded.

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 one-parameter, idempotent, read-only calculator with an output schema and full annotation coverage, the description is essentially complete; the return shape is even restated unnecessarily. The only shortfall is the absence of any routing hint to sibling finance calculators.

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?

With a single parameter at 100% schema description coverage, the schema already fully documents annual_rate as an annual return rate percent. The description adds no format, range, or unit nuance beyond that, so the baseline 3 for schema-covered parameters 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?

States a specific verb and outcome ('Estimate years to double an investment using the Rule of 72'), which is unambiguous and distinct from generic finance siblings like calculate_compound_interest. It does not, however, explicitly contrast itself with those siblings, so an agent must infer when the Rule of 72 beats a full compound-interest calculation.

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 only guidance is 'See list_bundles for related finance-universal calculators,' which is a discovery pointer rather than when-to-use or when-not-to-use direction. No alternative tool is named for the case where the agent wants exact compounding over N periods rather than a doubling estimate.

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