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

Annotations already declare this a safe, idempotent, read-only, closed-world calculation, so the description's burden is lighter. It still adds real behavioral content: the actual duty rates (12.5% / 12%), the sole-home reduced-rate and abatement logic, notary tariff plus VAT, and the adjective 'estimated' on other costs, which warns the output is approximate rather than authoritative.

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?

Front-loaded with the country and the verb, and the component list is dense without filler. The trailing payotte.com URL/multilingual-reference sentence earns its place only as a fidelity anchor; it is the one expendable element.

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 adequately pre-announces the returned cost components and that the result is an estimate, and the schema covers region/municipality resolution. It could go one step further on whether new-build purchases are out of scope and on currency/rounding assumptions, but nothing essential 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 the schema already documents all six parameters, establishing a 3 baseline. The description only loosely reinforces that the mortgage deed cost is tied to the optional loan parameter and mentions the sole-home reduced rate, adding no syntax or format detail beyond the schema — and flemish_core_city is never surfaced.

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 and resource — computes the cost of buying an existing home — and enumerates the components (registration duty, notary fees + VAT, other costs, optional mortgage deed) and the three covered regions. The 'BELGIUM' scoping plus region names make it instantly distinguishable from purchase_costs_es, purchase_costs_br and taxe_mutation.

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 only implied: the word 'existing' quietly excludes new-builds (which follow VAT rules), and 'optionally the mortgage deed' signals when loan_amount matters. There is no explicit statement of when to prefer this over siblings like frais_notaire_fr or compare_regions_be, and no prerequisites or exclusions are spelled out.

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