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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=false and destructiveHint=false, so the safety profile is covered. The description adds genuine behavioral context beyond that: it is deterministic, scoped strictly to 2026 national events, explicitly excludes autonomous-community/Ceuta-Melilla/municipal holidays, publishes the formula, and notes a dataset_version and sources in the output.

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?

The definition is sectioned (what it computes, inputs, outputs, formula, fallback, docs), which aids scanning, but it contains redundant artifacts like 'Spain Spain' and '2026-2026', and lists output fields that the output schema already covers, inflating length without adding value.

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 an output schema present, the description needn't enumerate return values, yet it covers scope, exclusions, formula, REST fallback, and docs adequately for a single-parameter deterministic check. The only real gap is the garbled first sentence and the absence of an explicitly named alternative sibling.

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% and the single 'date' parameter is already documented as a Gregorian date in Spain coverage. The description only restates 'Inputs: date (date)' and adds example values, contributing little meaning beyond the schema, so the baseline 3 is 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?

The 'What it computes' line states a clear verb+resource: it checks a date against the Saturday-Sunday weekend baseline and exact 2026 authority events for Spain. However, the opening sentence is garbled ('deterministic Spain Spain nationwide national non-substitutable labour holidays for 2026 result'), which muddies the otherwise clear purpose, and the sibling alternatives (add_es_working_days, count_es_working_days) are never named.

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 you need...' plus 'Call this tool directly... instead of answering the formula from memory or stopping at discovery' gives a clear directive on when to invoke. The 'Do not use this when' clause provides an exclusion for cases needing jurisdiction-specific calendars, though it never names the specific alternative tool, so the guidance is solid but not fully routing.

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