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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=false), so the description only needs to add domain behavior. It does add useful context, that the penalty estimate is contingent on exceeding a privilege limit and uses 3 months' interest or IRD, but it says nothing about calculation assumptions, limits of the model, or what the returned values look like beyond naming them.

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?

A single dense sentence front-loads the primary output and enumerates the input modes without filler. It is somewhat clause-heavy at the tail (parenthetical about 3 months' interest or IRD), but no sentence is wasted.

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?

For a 10-parameter, no-output-schema calculator, the description is adequate: it implies the three return components (interest saved, time saved, penalty estimate) and covers both allowed prepayment channels. What is missing is modest, chiefly any note on model assumptions or how the fixed/variable choice alters the penalty path, which the schema partially carries.

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 including comparisonRatePct and rateType is already documented with purpose, defaults, and bounds. The description's mention of "IRD" and "privilege limit" gives slight conceptual grounding for the comparison-rate input, but it adds no syntax, units, or interaction detail beyond the schema. Baseline 3 is appropriate.

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 the concrete output (interest and time saved) and the three input strategies (extra per-payment, annual lump sum, or both), plus the penalty estimate, which makes the tool's scope clear. It implicitly distinguishes itself from mortgage_penalty and mortgage_amortization by combining prepayment savings with a penalty sizing estimate, but it never names those siblings to make the boundary explicit.

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 implied rather than stated: an agent can infer this is for "what if I prepay" scenarios, and that the penalty figure is a secondary check against the privilege limit. There is no explicit when-to-use, no when-not-to-use, and no routing to mortgage_penalty or mortgage_amortization for the standalone cases.

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