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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so the bar is lower. The description adds genuine domain behavior the annotations cannot: exactly which days 'business' skips, the N range of 0–100,000, and that output includes weekday and wareki. No auth or rate-limit context, so not a 5.

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?

Three sentences, front-loaded with the core verb+resource before the unit nuances. Slight redundancy against the schema (the 0–100,000 range and unit semantics are repeated), but nothing is wasted.

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 5 parameters, no output schema, and full schema coverage, the description gives enough to invoke correctly: direction, units including holiday handling, and lang. It does not detail the full return shape, but the key returned fields (weekday, wareki) are flagged, which is sufficient.

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 every parameter is already documented in the schema, and the description's unit and N-range text largely restates it. Baseline 3 is appropriate; the description adds only marginal meaning beyond the structured fields.

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 ('Find the date N days after or before a date') and even names the returned fields (weekday and Japanese era date). That cleanly separates it from siblings like days_between (a difference, not an offset) and convert_japanese_era (era conversion only), so an agent can route 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?

Gives real conditional guidance: 'business' for Japanese holidays, 'weekdays' for weekends only, plus the 'Good for deadlines in Japan' framing that tells the agent when this tool is the right pick. It stops short of explicitly naming an alternative tool to use instead, so 4 rather than 5.

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