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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered; the description's real contribution is the error contract (never raises a protocol error, returns {"error": ...} with a fix hint) and retry safety, which annotations cannot express. The restatement of "read-only and idempotent" is redundant with annotations and doesn't add value, keeping it out of the top band.

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-loaded with the purpose and the two usage modes, then return shapes, then exclusions, then errors – a sensible ordering. It is somewhat longer than needed, and the "FREE." fragment and redundant re-statement of idempotency are filler.

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?

Output schema exists, but the description still sketches both return shapes and fully specifies the error response behavior for a stateless calculator, which is everything an agent needs to call it and recover from bad input. No meaningful gap remains.

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 a 3 is the baseline, but the description adds genuine meaning by explaining the mode switch between add_days and end_date and supplying a concrete worked input/output pair. It stops short of discussing negative add_days or the holidays array in prose, which the schema already handles.

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 and resource ("business-day math that skips weekends and your holiday list") and immediately enumerates its two modes, add vs. count. It also explicitly separates itself from the sibling deadline_planner, 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 an explicit when-to-use ("when the span must exclude weekends and named holidays") and two explicit when-not cases: plain calendar-day subtraction and backward scheduling from a deadline, the latter naming the alternative sibling deadline_planner. Nothing is left to inference.

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