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

Annotations already establish read-only, idempotent, non-destructive behavior, so the description is free to add the substantive details: returns are based on a real (after-inflation) return of 7% by default, and the math matches a named external calculator. It doesn't cover edge cases (zero savings, unreachable targets) but adds genuine context beyond the annotation set.

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 user-question triggers, then the return shape, then the modeling assumption. Three dense sentences, no filler, and each one carries distinct information.

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?

Seven all-optional parameters and no output schema could easily leave an agent guessing, but the description explains what comes back (FIRE number, months/years, age) and which inputs are alternatives. Minor gap: it doesn't state what happens if neither annual_spending nor fire_number is supplied.

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 100% schema coverage the baseline is 3, and the description earns more by explaining the relationship between inputs: annual_spending vs. fire_number are alternates, and current_savings plus monthly_contribution drive the time-to-goal calculation. It also clarifies that current_age only affects the reported age, which is more than the schema's terse "Optional age now" conveys.

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 resource (FIRE number) and exactly what it computes: the target nest egg, the months/years to reach it, and the resulting age. It also gives the formula (yearly spending / withdrawal rate, 25x at 4%), so the agent knows the output precisely. It does not, however, distinguish itself from the close sibling calculate_coast_fire, which an agent may easily confuse with this tool.

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 when they can retire or how much they need to retire" plus three quoted example phrasings give a concrete trigger condition. No when-not guidance or named alternative (e.g., coast FIRE vs. full FIRE) is provided, so the sibling-routing burden remains on the agent.

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