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

Annotations already declare readOnly/idempotent/non-destructive, but the description goes further, disclosing the actual algorithm ('move one day at a time; decrement only on Ireland status.is_working_day=true'), the exact coverage window (2026-2027), and a REST fallback URL. This is substantive behavioral context beyond the 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?

Front-loaded and clearly labeled (Use this when / Do not use / What it computes / Inputs / Outputs / Formula / REST fallback / Docs). Despite its length, each section serves a distinct purpose with no wasted text.

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, yet the description supplements it with coverage window, formula, worked example, fallback endpoint, and docs link, leaving no gap an agent would need before invoking the tool.

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%, so the schema already documents start_date and working_days. The description restates the inputs and gives a complete JSON example, but adds little semantic detail 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?

States a specific verb+resource+scope: 'Adds or subtracts Ireland national working days in exact 2026-2027 coverage.' It distinguishes the operation from the check_ireland_working_day and count_ireland_working_days siblings and from other territories, so an agent can route correctly 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 ('when you need a deterministic Ireland national public-holiday offset'), explicit instruction to return the calculation directly rather than answering from memory, and an explicit when-not-to-use exclusion ('employer-selected benefit day or non-national schedule'). 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.