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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), yet the description adds real behavioral nuance: observed is null on us-fed-banking when the legal date is Saturday, Saturday holidays close Friday, distinct full-day vs. early-close semantics, per-calendar acceptance windows, and the forward-compatibility caveat that post-2030 years use identical rules rather than a later official table.

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 front-loaded and nearly every sentence carries a distinct per-calendar rule. It is a dense wall of prose with mild redundancy, though, repeating the Saturday-closes-Friday rule in both the bank and generic phrasings that the schema already states for us-fed-banking.

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 two-parameter lookup tool with an output schema present, the description covers the caller's real ambiguities: which calendar to pick, what each calendar's rows mean, default behavior, year limits per calendar, and forward-compatibility. Nothing needed to invoke it 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 coverage is 100%, so the baseline is 3, but the description genuinely adds meaning beyond the schema's generic 2021-2100 range by stating per-calendar year windows (us-nyse 2026-2028, us-sifma/ny/nyc 2026-2027, us-ca 2026) and the default-when-omitted behavior. It does not restate the enum values, but that is handled by 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?

Opens with a specific verb+resource+scope: 'Holiday rows for one year.' It immediately distinguishes itself from siblings like list_calendars (which enumerates calendars) and is_business_day (which tests a single date). An agent knows exactly what this tool returns.

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?

Gives rich context for choosing among the seven calendars, including their legal meaning and accepted year windows, which effectively routes the agent. However, it never explicitly names an alternative (e.g., use list_calendars to discover ids) or states when not to use this tool, so it stops short of full when/when-not guidance.

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