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

Annotations cover only the safety profile (readOnly, idempotent, not destructive, closed-world), and the description adds real behavior beyond that: it returns the FIRE number under a 4% default, a date/days/age, a cautious-base-optimistic range and a year-by-year table, plus extra outputs when a purchase is supplied. It also discloses a notable interaction policy ('ask for any missing number instead of guessing') and an accuracy caveat ('An estimate, not financial advice').

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 primary use case, then return values, then exclusions, so an agent can stop reading early. It is dense but slightly overstuffed: the multi-country parenthetical ('also retire early in the UK, Canada or Australia') and the example lists (car, phone, holiday) add length without adding routing signal.

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?

There is no output schema, so the description must describe returns, and it does: FIRE number, date and days left, age, a three-scenario range, a year-by-year table, and the purchase-specific deltas. Combined with coverage of the nested purchase object and the missing-input behavior, an agent has everything needed to call and interpret this correctly.

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 description coverage is 100%, so the baseline is 3 and the schema already carries parameter meaning. The description goes further by explaining what the optional nested purchase object actually changes in the output (days the date moves, what the money would grow to if invested instead) and that the withdrawal rate is adjustable, which is semantic guidance the schema does not phrase.

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 resource (calculate the FIRE number / financial independence date) and lists the exact user phrasings it answers ('when can I retire', 'FIRE number', 'how long until work is optional'). It also names the sibling it is not (retire_at_age), so the agent can separate it from alternatives without opening either schema.

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?

Gives explicit when-to-use triggers (retirement date, FIRE number, purchase opportunity cost) and an explicit exclusion list: single-retirement-age questions route to retire_at_age, and products, loans, stock picks, tax, pension rules and live market data are all out of scope. The alternative is named rather than left to inference.

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