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

The annotations already establish the safe read-only, idempotent, non-open-world profile, so the remaining burden is low. The description adds genuinely useful behavioral context beyond the structured fields: it identifies Statbel as the data source and explains that registration duties are set regionally while the notary tariff is federal, which tells the agent why results differ across Regions.

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 sentences: the country tag is front-loaded, the output enumeration follows immediately, and the closing clause about regional vs federal competences earns its place by explaining result variation. No filler.

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 present, the description carries the return-value burden and does so by listing every output field, and all three inputs are fully documented in the schema. It stops short of noting whether figures are indicative estimates or how current the Statbel price data is, a minor gap for a cost calculator.

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% (price default 300,000 EUR, sole_home default true, flemish_core_city scoped to Flanders/periphery), so the schema already carries all parameter meaning. The description adds no formats, ranges, or interactions between parameters, making the baseline of 3 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?

Names a specific verb (compare) and resource (the same purchase across Brussels-Capital, Wallonia and Flanders), then enumerates the exact comparison outputs: registration duty, notary, other costs, total, share of price, and each Region's median house/apartment price. This distinguishes it from single-jurisdiction siblings such as purchase_costs_be and house_prices_be without ambiguity.

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 by the comparative framing, but the description never states when to reach for this versus purchase_costs_be (single-region cost detail) or house_prices_be (price data only), nor does it state any exclusions. An agent can infer the fit but is given no explicit routing rule.

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