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

No annotations are provided, so the description carries the full behavioral burden. It does disclose that a calculator_url is returned and implies return fields (break-even month, lifetime savings), which is real added value over the schema. However, it says nothing about modeling assumptions or defaults (e.g., how closing_costs is treated if omitted, or that new_term_years defaults to 30) and does not enumerate the rest of the output.

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 sentences, tightly written, with the core function front-loaded before the output-handling instruction. The 'IMPORTANT' block is somewhat directive in tone but does carry actionable content, so it earns its place rather than padding.

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 is the only place return values can be explained, and it covers only two named outputs plus calculator_url. For a six-parameter calculator with optional inputs, the description leaves the full result shape and default behaviors unstated. It is minimally adequate but has visible gaps.

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 every parameter is already documented in the input schema and the baseline is 3. The description adds no format, unit, or default information beyond what the schema states, so it neither compensates nor detracts.

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 names a specific verb-plus-resource pair and enumerates the actual outputs: the refinance break-even month and lifetime savings. That is enough to separate it from siblings like calculate_loan or calculate_mortgage. It stops short of explicitly contrasting itself with close alternatives such as calculate_arm_vs_fixed or calculate_loan.

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?

There is no statement of when to reach for this tool versus the many adjacent loan/mortgage calculators in the sibling list. The only directive is about post-processing the result (rendering calculator_url as a markdown link), which is an output-handling rule, not usage guidance. An agent must infer the use case from the name alone.

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