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 contributes beyond them by disclosing the return content — both the Rule of 72 estimate and the exact figure — which matters since no output schema exists.

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 triggering question and closed with a concrete example, which is good structure. The second sentence largely restates the two modes from the first, though it does earn its place by adding the parameter mapping.

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 two-parameter, fully documented, read-only calculator with no output schema, the description covers the trigger, both modes, and the returned values. The one gap is unspecified behavior when neither optional parameter is provided.

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, but the description adds real value by mapping each input to a mode ("send annual_rate" vs "send years") and stating both parameters are optional alternatives. It does not specify behavior if both or neither are supplied.

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?

Names a specific computation (Rule of 72 doubling time / required rate) and frames it as the answer to two concrete user questions. It is unmistakable against siblings like calculate_compound_interest or calculate_fire_number, which address different financial questions.

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?

"Use when a user asks how long it takes to double money... or what return doubles it in a number of years" gives explicit trigger conditions for both modes. It stops short of saying when a sibling calculator (e.g. calculate_compound_interest) is the better choice.

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