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

Annotations already declare readOnly/idempotent/non-destructive/openWorld=false, but the description adds substantial behavior beyond them: the deterministic pinned-holiday model, the rest-day limitation, the exact step algorithm (decrement only when status.is_working_day=true), and the richness of fields returned (source_quality, dataset_version, rest_day_policy). This is above the bar set by 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-loads the when-to-use and when-not-to-use guidance well and is clearly sectioned ('What it computes', 'Formula', 'Inputs'). However, the long enumerated outputs list duplicates the existing output schema and does not earn its place, adding length without new information.

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 explain return values, and it nonetheless covers scope limits, the algorithm, example arguments, and a REST fallback. Nothing an agent needs to invoke this correctly is missing.

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 both parameters are already fully documented in the schema, and the description's 'Inputs:' line and JSON example are essentially restatements. It adds no syntax or edge-case meaning beyond what the schema already provides, so the baseline 3 applies.

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 (add/subtract), resource (Singapore working days), and mechanism (pinned national public-holiday dates + CLDR Saturday/Sunday baseline). It explicitly scopes to 2026-2027 and distinguishes itself from employer-specific, bank/exchange, and out-of-range calendars, so an agent can separate it from the many add_*_working_days siblings.

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?

Provides an explicit 'Use this when...' clause with the determinism and limitation precondition, and a 'Do not use this when...' clause naming the excluded cases (employer roster, bank/exchange calendar, out-of-range dates). It also instructs calling directly rather than answering from memory, leaving nothing to inference.

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