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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the pure-calculation, no-side-effect nature is covered. The description adds the return values (repayment, total interest, total repaid), but says nothing about the model's assumptions — no deposit, no fees/insurance, fixed rate for the full term, monthly compounding — which materially change the numbers for a bond calculation.

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 tight, front-loaded sentences: what it computes first, then the rate caveat. The 'because rates change' clause is slightly redundant but earns its place by explaining why the rate must be supplied rather than looked up.

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?

No output schema exists, and the description does exactly what is needed there by naming all three returned figures. With only three simple parameters and safety covered by annotations, the remaining gap is disclosure of calculation assumptions, which is a real but not fatal omission.

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 coverage is 33%; only annual_interest_rate_pct is documented ('e.g. 10.75'). The description compensates meaningfully for that one parameter ('rates change — use prime ± the bank's margin'), but says nothing about loan_amount units or term_years, whose default of 20 years is left entirely to the schema.

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?

States a specific verb ('Calculates') and resource (monthly repayment, total interest and total repaid for a South African home loan), and even lists the three outputs. It never names or contrasts the sibling calculators (transfer_duty, purchase_costs, municipal_rates), so the agent must infer that this is the bond-servicing calculator rather than the up-front-cost ones.

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?

Gives real guidance on sourcing the rate ('use prime ± the bank's margin'), which is the main practical question. It does not say when to prefer this over the sibling calculators, nor any exclusion such as 'do not use for once-off purchase costs — use purchase_costs'.

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