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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, and the description adds real substance beyond that: the exact computation formula (Monday-Friday and not a holiday), the dataset coverage window (2026-2027), the holiday model including observed Sundays, the output field list, dataset_version/sources, and a fallback endpoint. No contradiction with annotations.

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 trigger and exclusion, and the labeled sections are easy to scan. It is longer than strictly necessary: the 'complete JSON argument examples' restates the schema example and the output field list overlaps the output schema, but no sentence misleads.

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?

An output schema exists so return values need not be explained, yet the description still supplies the interpretation-critical pieces: coverage years, holiday model, formula, and a documented fallback path. An agent has everything needed to call and interpret this tool correctly.

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?

With one parameter and 100% schema description coverage, the baseline is 3; the schema already documents the date format and coverage window. The description adds a concrete JSON argument example and reinforces the coverage constraint, which is a modest but real increment rather than new semantics.

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 (check) and resource (South Africa working day) and pins the exact scope: Saturday-Sunday baseline plus 2026-2027 national public holidays, explicitly including the 4 Nov 2026 election holiday and observed Sundays. That scope statement is enough to separate it from the sibling check_* and add_*/count_* calendar tools without opening any schema.

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?

Contains an explicit 'use this when' trigger, a 'do not use this when' exclusion list (employer, bargaining-council, school, bank, exchange, provincial, locally exchanged calendars), and a directive to call the tool rather than answer from memory. It also names the alternative path (REST fallback) and the condition under which it applies.

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