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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds the method disclosure (4% rule) and the secondary output (years of saving), but says nothing about language handling, formatting side effects, or determinism beyond what annotations imply.

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, purpose first, concrete example second — zero filler and nothing repeated from the schema. Well front-loaded for a quick selection decision.

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 a full output schema and 100% parameter documentation, the description only needs to frame the calculation, which it does. It is close to complete; the only omission is routing guidance relative to the many sibling 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?

Schema coverage is 100%, so every parameter including lang, rate and withdrawalRate is already documented in the schema. The example illustrates expenses/saving/current meaningfully but adds no syntax or format detail beyond the schema, so the baseline 3 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 (calculate) and a well-defined resource (FIRE target) and spells out the two outputs: the target and years of saving. The FIRE concept is distinct enough to separate it from siblings like compound_interest or emergency_fund, though it never names an alternative.

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 only implied by the worked example ({expenses, saving, current} → fireNumber), which shows the intended call shape but not when to pick this over cost_of_waiting or compound_interest. No when-not conditions or prerequisites are given.

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