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

Annotations already declare readOnly, idempotent, non-destructive and open-world, so the safety profile is covered. The description adds genuinely new behavior: the rate defaults to the latest Bank of England average for the deal type, the output includes total interest and the post-fix reversion payment, and it discloses a cost of 1 credit.

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 front-loaded sentence states the primary output first, then the secondary outputs, then the cost. It is dense but every clause carries information; only the parenthetical cost note slightly interrupts the flow.

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 seven optional parameters and no output schema, the description does the necessary work by naming the three returned figures and the rate-sourcing rule. What remains thin is how repaymentType (interest_only) interacts with the 'total interest' output and how loan versus propertyPrice+deposit are reconciled, but nothing critical for calling it is missing.

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 all seven parameters are already documented, including the BoE default rate, the product enum values, and the loan-versus-propertyPrice+deposit alternative. The description reinforces the default-rate source and the optional custom rate but adds no syntax, ranges, or edge-case meaning beyond the schema, so the baseline 3 applies.

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 concrete verb and resource ('Monthly mortgage payment for a UK home loan') and enumerates what it returns: payment amount, total interest, and the post-fixed-deal payment. It clearly reads as a UK mortgage calculator, distinct from the generic sibling loan_repayment, though it never says so explicitly.

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: the '(or your own rate)' and 'if you do not remortgage' phrasing hints at scenarios, but there is no explicit when-to-use guidance and no reference to loan_repayment or other siblings as alternatives. An agent must infer the selection rule between this and the generic loan calculator.

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