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

With no annotations, the description carries the full burden and does disclose meaningful behavior: negative values subtract, business_days=true skips weekends, and the return content of each mode (days, weeks, months+days, weekdays, weekend days, resulting date and weekday). It omits what happens on conflicting inputs (e.g. both end and add_days supplied) and invalid date handling, keeping it from 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 dense sentences, front-loaded with the core purpose before the mode breakdown, with no filler. The telegraphic parentheticals are compact rather than bloated, though the mode pairing takes a moment to parse.

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 9-parameter, no-output-schema tool, the description covers both operating modes and their return shapes adequately, and defaults for start/timezone/include_end live in the schema. Missing edge-case guidance (mutually exclusive parameters, invalid ranges) leaves a modest gap.

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 only 56% and four numeric params (add_days/add_weeks/add_months/add_years) have no descriptions at all; the description compensates by explaining sign semantics ('negative to subtract') and mode interaction. It leaves include_end and timezone behavior to the schema, so it adds value but does not fully close the coverage gap.

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 states a specific verb+resource ('Exact calendar maths') and then splits it into two concrete modes: interval measurement (with 'end') and date shifting (with add_days/add_weeks/add_months/add_years). It does not name or contrast against siblings like convert_timestamp or get_current_time, which holds it below a 5.

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 guidance is implied through mode-by-mode phrasing ('With end: ... With add_days...:'), which tells the agent how to pick parameters but not when to prefer this tool over convert_timestamp or get_current_time. 'Use this instead of counting days yourself' is a nudge, not explicit when/when-not criteria.

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