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

Annotations cover safety (readOnly, idempotent, non-destructive, closed-world), and the description goes well beyond them: data provenance (pinned OpenHolidays open-data snapshot), explicit non-authority and substitute-day limitations, the CLDR weekend rule, the coverage window, and a REST fallback path. This is exactly the extra behavioral context annotations cannot supply.

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 use-case, then clearly sectioned (Do not use / What it computes / Inputs / Outputs / Formula / Fallback / Docs). It is longer than strictly necessary — the REST fallback and docs URLs plus the output-field list add bulk — but each block is labeled and skimmable.

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 a two-parameter reference checker with an output schema, annotations, and a documented formula, nothing an agent needs to select or call it is missing. Coverage limits, provenance, and the fallback path are all stated, so the definition is self-sufficient.

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% and both parameters are documented in-schema, so the baseline is 3. The description adds real value by restating the 2026-2028 date bound, the enum-driven territory semantics, and a complete JSON example, though it does not add syntax beyond the schema.

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 description states a specific verb (checks a date), resource (working day), and scope (supported OpenHolidays countries, nationwide-only, 2026-2028), and explicitly contrasts itself with the sibling check_china_working_day / check_uk_working_day / check_global_reference_working_day variants. An agent can route between the four working-day checkers without opening schemas.

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 gives both a positive trigger (source-labelled nationwide reference for supported OpenHolidays countries) and an explicit negative list (official authority results, regional/local holidays, substitute-day rules, employer/bank/exchange closures, dates outside 2026-2028), then names the alternative packages to use instead. Nothing is left to inference.

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