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

Annotations already declare readOnlyHint and openWorldHint=false, so the description focuses on what the annotations cannot say: which legal sources are used, that the e-invoicing dates are provisional while the ministerial order is a draft, and that a null date is a possible outcome. That provisionality/uncertainty disclosure is genuinely useful context a caller could not derive from structured fields.

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 compact paragraph that front-loads the subject (deadlines from published Spanish rules), then sources, then the return shape. It is dense with legal citations but every clause (regulation IDs, provisional status, null return, obligation keys) carries information an agent needs.

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 correctly compensates by describing the return: a date or null plus explanatory text, keyed by obligation name (verifactu, factura_electronica, estados_facturas). The one gap is that it never says which inputs trigger estados_facturas ('where it applies' is left ambiguous), though the schema hints at it via mas_8_millones.

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%, including an explanation of the forma enum and the mas_8_millones turnover threshold, so the schema carries parameter meaning fully. The description adds nothing about how forma / mas_8_millones / factura_a_empresas change which obligations are returned — baseline 3 is the right level.

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 (returns deadlines) and a specific, narrowly-scoped resource (Spanish Veri*factu and B2B e-invoicing obligations), citing the governing regulations. No sibling tool overlaps with this lookup, and the description makes that uniqueness obvious.

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: an agent can infer this answers 'when do I have to comply with Veri*factu / e-invoicing'. There is no explicit statement of when to call it versus e.g. get_modelo or calculate_modelo_303, and no prerequisites mentioned beyond the schema. The scope caveat 'Information about the rules, not advice' is the only boundary drawn.

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