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

Annotations already declare readOnlyHint, idempotentHint and closed-world, so the safety profile is covered. The description adds that it computes break-even and lifetime interest, which is useful context about output, but discloses no rate limits, auth, or side-effect detail beyond that. Adequate given the annotation coverage, but not rich.

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 short sentences plus a reference URL, front-loaded with the decision question and then the concrete outputs. Nothing is redundant, though the trailing URL is auxiliary rather than essential content.

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 compensates by naming the key results (break-even month, lifetime interest), and annotations cover safety. All inputs are documented in the schema. An agent has enough to call it correctly, though more explicit routing to sibling mortgage tools would round it out.

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%, with all seven parameters (rate, balance, closing, newRate, newTerm, remaining, currency) fully documented and defaults supplied. The description adds no parameter-level meaning (e.g. that closing costs are paid upfront), so the baseline of 3 is correct.

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 description gives specific verbs and outputs: it compares current vs new mortgage payments and produces the break-even month and lifetime interest. That clearly separates it from a plain mortgage_calculator. It stops short of naming any sibling or stating the differentiator explicitly, so it lands at 4 rather than 5.

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?

The framing question 'Will a lower rate pay off?' implies the use case (deciding whether refinancing is worthwhile), so usage is implied. There is no explicit when-to-use, when-not-to-use, prerequisites, or named alternative (e.g. mortgage_payoff_calculator, mortgage_calculator), leaving guidance at the minimum viable level.

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