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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, so the safety profile is covered. The description adds genuinely new behavioral context: the result includes calendar, sources, and a disclaimer; unknown ids error out; early closes stay open; and the not-legal/payroll/financial-advice caveat. That is real added value beyond 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?

The purpose is correctly front-loaded, but the body is a long, repetitive wall of per-calendar exceptions, with near-duplicate phrasing such as 'Friday, July 3, 2026 stays open' repeated across several calendars and much of the enum text duplicated from the schema. Several sentences do not earn their place.

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 need not explain return values, and it still notes the result carries calendar/sources/disclaimer. Combined with date-format and error behavior for unknown ids, an agent has enough to call it correctly; the remaining gap is the absence of sibling routing.

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% and the enum descriptions already document each calendar id, so the baseline is 3. The description largely restates that schema content (us-fed-banking Saturday-holiday rule, us-nyse/us-sifma/us-ca/us-ny/us-nyc scopes) with only marginal additions such as specific example dates, so it does not clearly exceed the schema.

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?

The description opens with a precise predicate: 'Whether a civil date is a business day, and why.' That is a clear verb+resource and is distinguishable from mutation/counting siblings (add_business_days, count_business_days, roll_date) by implication. It does not name any sibling explicitly, so it falls short of a 5.

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?

It gives rich guidance on which calendar id to choose and states 'Omit calendar for us-federal-opm' plus 'Unknown ids are an error,' which is useful selection context. However, it never says when to use this tool versus the siblings (list_holidays, list_calendars, report_discrepancy), so tool-selection guidance is only implied.

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