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

Annotations already declare readOnlyHint, idempotentHint, and closed-world behavior, so the safety profile is covered. The description usefully adds output content (schedule breakdown, payoff date, extra-payment savings, CSV export) but, with no output schema, it still does not say how the CSV is delivered or structured, leaving the return contract underspecified.

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 compact sentences with the capability list front-loaded and no filler; the trailing web-page URL is human-oriented noise for an agent but costs little. Structure is appropriate for a zero-required-parameter calculator.

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 7 fully documented params and no output schema, the burden falls on the description to convey return content; it partially does (schedule, payoff date, CSV) but omits delivery format and any sibling differentiation in a crowded calculator family, leaving real gaps for an agent.

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 all seven parameters (rate, term, unit, extra, start, amount, currency) are already documented in the schema. The description adds no parameter-level meaning such as default behavior, currency scope, or how 'extra' interacts with the schedule, 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 specific outputs: splits each payment into principal and interest, shows payoff date, quantifies extra-payment savings, and exports CSV. That is a concrete verb+resource picture, but it never distinguishes itself from the very similar siblings (mortgage_calculator, loan_calculator, mortgage_payoff_calculator), so an agent cannot tell from the text alone which of these to pick.

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 capability (you want a per-payment amortization schedule with extra payments and a CSV). There is no when-to-use/when-not guidance and no named alternative among the numerous loan/mortgage siblings, which is exactly the situation where routing guidance matters most.

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