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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint=false, so the safety profile is covered. The description adds real behavioral context beyond that: it states the required input ('Needs the first day of their last period'), the medical disclaimer scope, and the `period_may_be_late` output behavior plus its doctor-referral advice. It stops short of describing latency, data sources, or the general return shape.

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-loads the core purpose, then usage, then exclusions, which is a sound order. It is slightly redundant in re-stating the pregnancy/contraception exclusion twice in one sentence, which costs a little efficiency but does not obscure the message.

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 read-only, no-output-schema tool the description covers purpose, triggers, exclusions, the required input, and one notable output behavior (`period_may_be_late`). An agent has enough to call it correctly; only the broader return structure (kit links, food ideas) is left implicit.

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 all 8 parameters (tz, date, diet, country, cycle_length, period_length, last_period_start) are documented in the schema with defaults and ranges. The description only reinforces that `last_period` is required, adding no format, unit, or default detail beyond the schema, 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?

States a specific verb+resource: returns today's cycle phase along with workout and food ideas, and the title restates it consistently. It is clear what the tool produces, but it never references its siblings get_week_ahead or list_phases, so the agent gets no explicit differentiation from them.

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 explicit trigger scenarios ('what phase am I in', 'what should I train today?', 'why their energy is up or down') and explicit exclusions ('Not for getting pregnant, fertile days, ovulation timing, contraception or pregnancy'). It even directs those out-of-scope users to a doctor or pharmacist, leaving nothing to inference about when this tool is and isn't appropriate.

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