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

Annotations already declare this a safe, idempotent, closed-world read, so the safety profile is covered. The description adds genuinely useful behavior beyond that: rates are the *general* regional rates, official texts were read, and the taxable base is the higher of price and Catastro reference value — a non-obvious calculation rule. It omits the return shape, which keeps it below 5.

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 dense sentence, front-loaded with 'SPAIN.' and leading with the verb 'Computes'. Every clause carries information (tax types, scope, rate basis), though the parenthetical '(official texts read)' and '(ITP)/(AJD)' asides make it slightly heavy for one sentence.

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?

For an annotation-covered, four-parameter calculator with full schema coverage, the description supplies the geographic scope, the resale-vs-new tax logic, and the reference-value base rule. No output schema exists, so a brief note on what the result contains (e.g. an itemized tax/fee breakdown) would complete it, but nothing needed to invoke it correctly 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 coverage is 100%, so the baseline is 3, but the description adds meaning beyond the schema: the key rule that the base is the higher of price and reference_value, and the new_home flag's consequence (VAT+AJD vs ITP) tying it to the tax path. It does not add value on region name matching beyond what the schema already says.

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 pair (computes transfer tax/VAT/registry/notary fees on a Spanish home purchase) plus the geographic scope (17 regions, Ceuta and Melilla). It is easily distinguished from siblings like purchase_costs_br, purchase_tax_uk or frais_notaire_fr, and it names the exact tax instruments (ITP, VAT/IGIC, AJD) it handles.

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?

It implicitly explains the branching condition that selects the calculation path (resale home → ITP; new home from developer → VAT + AJD), which is real usage context. However, it never explicitly states when to prefer this tool over siblings such as frais_notaire_fr or transfer_tax_us, nor any preconditions (e.g. region must be covered).

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