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

Annotations already declare readOnlyHint and idempotentHint, so the bar is lower, yet the description still discloses the return payload (filing window, direct debit last day, open-today flag, late-filing cost, sources) and a regulatory caveat (Orden HAC/623/2026 changed windows from tax year 2026). It does not state what happens without sale_date, but the schema does.

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 with the trigger condition first and the return contents after; every clause carries information. The nested parenthetical and situation list make it dense but not wasteful.

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?

There is no output schema, and the description compensates by enumerating what is returned, including the late-filing penalty and source citation. Combined with full schema coverage and safety annotations, an agent has what it needs; only edge-case behavior (e.g., missing sale_date output shape) is left implicit.

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 situation, sale_date and language including enum meanings and the fallback when sale_date is omitted. The description adds no parameter meaning beyond that, so 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?

States a specific verb and resource ('when a Spanish Modelo 210 ... is due') and scopes it to a home in Spain across four enumerated situations. That is clearly distinct from the sibling estimate_modelo_210, which computes an amount rather than a filing window.

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?

'Use this when someone asks when ... is due' plus the four enumerated situations gives a concrete trigger condition. It does not explicitly rule out alternatives or name the sibling tools, so it stops short of a 5.

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