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

Annotations already declare readOnly, idempotent, non-destructive, closed-world. The description adds real behavioral context beyond them: the exact iteration formula, the deterministic/no-memory-fallback directive, the output field set, and a concrete REST fallback path with a warning not to guess other endpoints. It stops short of 5 only because it does not explain error/edge behavior (e.g., what happens beyond the ±1000 bound).

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?

Well front-loaded: purpose, when-to-use, when-not, then a consistent Inputs/Outputs/Formula/REST structure. However the 'Example user requests' lines are noisy ('around Ordinary weekday') and some output/fallback content is padding, keeping it from a 5.

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, annotations covering safety, and 100% schema coverage, the description supplies everything else an agent needs: deterministic formula, direct-call directive, JSON example, REST fallback, and docs link. Complete for this tool's complexity.

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 both parameter descriptions already carry meaning ('start date excluded for non-zero offsets'). The description restates the two inputs and gives a complete JSON example, which is mildly useful but largely redundant against the schema. Baseline 3 is appropriate.

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 ('Adds or subtracts Spain authority-scope working days in exact coverage') and names the exact jurisdiction scope. Against the many sibling add_<country>_working_days tools, the Spain confinement is unambiguous, so an agent can route correctly.

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?

Explicitly says when to use it ('when you need a deterministic Spain working-day offset') and to call it rather than answer from memory, plus a 'do not use' clause routing cases needing local/municipal calendars elsewhere. The exclusion is worded confusingly (it never names the alternative tool to call instead), so it falls short of a clean 5.

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