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

Annotations already establish read-only, idempotent, non-destructive behavior, so the bar is lower. The description adds real context beyond them: what the result includes (count, weekend days, each holiday in the span) and a hard span limit of 3,660 calendar days, which is not present in the annotations or schema.

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?

Two sentences, zero filler, front-loaded with the core action and scope before the return contents and limit. Every clause carries information an agent needs.

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 no output schema, the description helpfully enumerates the return contents, and the 2021–2100 date coverage lives in the schema parameter descriptions. Minor gap: it does not say what happens for out-of-range dates or spans over 3,660 days (error vs. clamp).

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%, so the baseline is 3; the description earns above baseline by clarifying inclusive boundary behavior ('both dates included') and imposing the 3,660-day span constraint, which the schema does not state. It does not add format detail beyond the schema's YYYY-MM-DD examples.

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 (count) and resource (U.S. business days), plus the exact span semantics ('both dates included, leaving out weekends and observed federal holidays'). This is clearly distinguishable from siblings like check_business_day (single date) and add_business_days (offset from a date).

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?

Usage is implied by the counting semantics: an agent can infer this is for measuring the number of working days across a span. However, no sibling is named and no when-not condition is given (e.g., use check_business_day for a single date, add_business_days for shifting a date).

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