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

Annotations already declare readOnly, idempotent, non-destructive, so the safety profile is covered; the description adds real value by disclosing the exact amortization method and warning that results are a close estimate that lenders may differ on. It does not address edge cases such as a payment too small to ever amortize the balance or a zero balance, which is the main remaining behavioral gap.

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?

Purpose is front-loaded in the first clause, followed by a compact method parenthetical and two short caveat sentences. The parenthetical is slightly dense but earns its place by making the calculation reproducible; nothing is redundant with the annotations.

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, rich annotations, and 100% schema coverage, the description need only explain method and precision, which it does. The only missing piece is behavior for degenerate inputs (payment <= monthly interest), which would matter for a purely computational tool.

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 balance, apr_percent, monthly_payment and extra_payment are already documented with units and examples. The description reinforces what extra_payment does for the outcome but adds no format, range, or interaction detail beyond the schema; 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 and resource: it computes months-to-payoff and total interest for a loan, plus the savings from an extra payment. The mechanics (APR/12 interest added, payment subtracted) and the sibling calculators' distinct domains make it unambiguous which tool 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?

Clear context: this is for amortizing a loan with a fixed monthly payment, optionally with an extra payment, and it names the reference implementation at profitdrawer.com so the agent knows the intended scenario. It does not state exclusions or when a sibling calculator would be preferable, but the other siblings cover different domains (jobs, tax, startup costs).

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