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

With zero annotations the description carries the full burden and does substantial work: it defines which events count (not declined, not marked Free), that tasks and habits also block, the time zone used, the return format, and the important edge case that a Busy all-day event leaves no free time. It never explicitly states the tool is read-only and has no side effects, which is the remaining gap.

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 operation and followed by return format and the key edge case; nothing is padding. The parenthetical '(default today; from now on, if today)' is syntactically awkward and costs a re-read, which keeps it off 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?

There is no output schema, and the description compensates by describing the return as HH:MM-HH:MM slots plus what's busy. For a three-param, all-optional read tool with no annotations, the remaining gap is minor: no explicit statement of side-effect freedom or how overlapping tasks/habits are resolved when they conflict.

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 baseline of 3 applies, but the description genuinely adds meaning: the date defaults to today and, when it is today, the window starts from now; and the 'minutes' threshold is tied to slot length ('at least minutes long'). The 'between' parameter is only implicitly referenced via the schema's 09:00-18:00 default.

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 computation — free slots on a day, derived from calendar events, planned tasks and habits — which is far more than a restatement of the name. It does not, however, distinguish itself from plausible siblings like plan_day or list_events, so an agent must infer where the boundary lies.

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 by the framing ('free time on a day', default today, from-now-on semantics for today) and the eligibility rule for events is spelled out. But there is no explicit when-to-use/when-not or pointer to an alternative tool for scheduling, listing events, or checking a specific meeting.

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