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

Annotations already declare readOnlyHint=true, idempotentHint=true, and openWorldHint=false, so safety is covered. The description adds value beyond that by disclosing the tool's core two-phase behavior (interest-only payment, then a higher amortized payment) and, with no output schema present, embedding concrete sample figures that convey the shape of the result.

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 plus a source URL, with the illustrative numbers front-loaded. Every element earns its place, though leading with a numeric teaser rather than a one-line statement of purpose slightly delays orientation.

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 no output schema, the description must carry return-value burden, and the example partially does so (monthly interest-only cost, post-IO cost, comparative interest). It does not explain how term and ioYears interact (e.g., that repayment amortizes over the remaining years) or what currency affects, leaving gaps for a calculator with five inputs.

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% and all five parameters (loan, rate, term, ioYears, currency) are documented in the schema. The description's example mirrors the schema defaults but adds no semantics for term, ioYears, or currency, so it neither compensates nor detracts; 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?

The worked example ('$300,000 loan at 6.5% costs $1,625 a month interest-only, then $2,236.72') makes it clear this computes interest-only mortgage payments and the subsequent repayment phase. However, it never names or distinguishes itself from close siblings like mortgage_calculator, loan_calculator, or amortization_calculator, so an agent must infer the distinction from the name alone.

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?

'Compare it with a standard mortgage and see the extra interest' describes an output capability, not a selection rule. There is no statement of when to choose this tool over the many mortgage/loan calculators in the sibling list, nor any prerequisite or exclusion guidance.

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