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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real behavioural detail beyond that: results are signed (direction matters) and month arithmetic clamps at month end, neither of which appears in 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?

A single front-loaded sentence that pairs each mode with its defining behaviour and wastes no words. Both branches are covered in one compact clause each, which is exactly the right size for this tool.

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?

An output schema exists, so return values need not be described, and annotations carry the safety profile. The description covers both modes plus clamping, leaving only minor gaps such as inclusive-endpoint defaults (which the schema already documents) and explicit sibling disambiguation.

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% and every parameter (mode, start/end, date/amount/unit, inclusive, weekdaysOnly) is documented in the schema itself. The description restates the concepts of weekdays and calendar units without adding format, default, or edge-case semantics beyond what the schema already provides, so the baseline 3 applies.

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 names a specific verb pair (count / add-subtract) and the exact resource (calendar days, weekdays, calendar units), and it distinguishes the two operating modes ('between' vs 'shift') that the schema's oneOf encodes. It is nearly enough to separate it from neighbours like calendar_age_difference or julian_day_convert, though it never names those siblings explicitly.

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?

Usage is implied rather than stated: the reader can infer that date-range counting uses the 'between' branch and arithmetic uses the 'shift' branch. There is no explicit when-to-use/when-not guidance and no routing away from overlapping siblings such as calendar_age_difference, so an agent must infer selection from the schema.

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