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

Annotations already declare readOnly, idempotent, non-destructive and openWorld=false, so safety is covered; the description adds genuinely useful behavioral context: the applicable rules version and validity window (offers from 1 April 2025 to 31 Dec 2027, checked 15 September 2026) and the fact that it returns derived eligibility outputs such as income tranche, ceiling, deferral/repayment schedule and monthly payments. This goes beyond annotations and compensates for the absence of an output schema.

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?

It is front-loaded with the purpose, and nearly every clause carries domain content, but it is delivered as a single sprawling sentence that interleaves input definitions, eligibility rules and output fields, making it hard to parse quickly. Structuring it into purpose, inputs and outputs would be far more usable.

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 sensibly enumerates the outputs (income vs ceiling, income counted, tranche, share financed, cost ceiling, PTZ amount, deferral and repayment schedule, monthly payments) and states the applicable rule set and eligibility exclusions. An agent has enough to invoke and interpret the result, though the eligibility/priority conditions are dense and the placement vs sibling tools is unclear.

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 the baseline is 3. The description largely restates the schema's parameter meanings (zone ABC, cost TTC including works, RFR of year N-2, person count) rather than adding syntax or format detail the schema lacks; the one genuinely extra element, the 'income counted (the higher of RFR and cost/9)', is a derived computation, not a parameter.

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 states a specific resource and computation: 'What a first-time buyer's PTZ may be, by the rules of offers issued from 1 April 2025 to 31 December 2027', which clearly separates it from sibling tools such as home_loan or appraise_listing. The purpose is recognizable, though it is framed as an open question rather than a direct verb+object statement ('compute PTZ eligibility and terms'), which slightly blurs the action.

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 guidance on when to choose this tool over alternatives like home_loan or property_prices, and no discussion of prerequisites for calling it. The eligibility rule about not having owned a main home in the last 2 years is a domain rule of the calculator, not usage guidance for the agent.

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