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

Annotations already declare readOnly/idempotent/non-destructive, and the description goes well beyond them: it discloses the exact coverage window (2026-2027), the computation formula (Mon-Fri minus national holidays and observed dates), the distinction between exact and observed dates, the field set returned, and a REST fallback URL with an explicit 'do not guess another /api/* path' warning. That is rich behavioral context an agent cannot get from the 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 routing decision, then organized into labeled sections (when-not, what it computes, example requests, inputs, outputs, formula, fallback, docs). Every block is scannable, though the outputs listing and REST/docs lines add length that is partly redundant given the output schema exists.

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 single-date lookup with rich annotations and an output schema, the description covers coverage window, formula, scope exclusions, an example invocation, and a fallback path. Nothing an agent needs to call it correctly is missing, and the holiday-dataset boundaries (national only) are stated up front.

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?

Only one parameter and schema description coverage is 100%, so the baseline is 3. The description echoes the parameter name and adds a complete JSON argument example, but adds no format constraints or edge-case guidance beyond what the schema already states ('Gregorian date in New Zealand national 2026-2027 coverage').

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 (check a New Zealand date for working-day status) and immediately scopes it to national statutory public holidays versus regional anniversary, school, bank, exchange or employer calendars. An agent can separate it from the many sibling check_*_working_day tools and from add_/count_ variants without opening a 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?

Explicit when-to-use ('Use this when you need a New Zealand national statutory public-holiday result. Call this tool directly ... instead of answering from memory') and explicit when-not-to-use ('Do not use this when you need a regional anniversary, school, bank, exchange or employer calendar'). Alternatives are named by category, leaving little to inference.

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