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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context: it discloses the calculation formula, the exact return values (monthly payment, total repaid, total interest), and the currency (GBP). This goes beyond what annotations provide and gives the agent a clear picture of the operation's scope.

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?

The description is two sentences with no filler. The first sentence front-loads the core purpose and inputs, the second lists outputs, currency, and source. Every word adds value, and the structure is clean and scannable for an agent.

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 simple three-parameter calculator with full schema descriptions and an output schema, this description is complete. It covers purpose, methodology, outputs, currency, and provides a source reference. No critical information is missing for an agent to invoke the tool correctly, and the annotations handle safety/idempotency context.

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 each parameter (amount, rate, years) already fully described in the input schema. The description only paraphrases these parameter names without adding new semantic details (e.g., compounding frequency or whether the rate is nominal vs. effective). It does not compensate beyond the schema, but the schema already covers the essentials, so the baseline of 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 clearly states the tool's function: 'Work out the monthly repayment on a loan' using specific inputs (amount, rate, years) and an identified methodology ('standard amortising formula'). It also names the outputs (monthly payment, total repaid, total interest), distinguishing it from sibling calculators like hopi_mortgage_calculator or hopi_compound_interest_calculator.

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 through the description: it is for loan repayment calculations, and the mention of 'amortising formula' gives a hint about its domain. However, there is no explicit guidance on when to use this tool instead of alternatives such as hopi_mortgage_calculator (which might handle similar inputs), nor any exclusions or prerequisites. Context is present but not explicit.

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