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

Annotations already cover read-only/idempotent/non-destructive, so the bar is lower, yet the description adds real behavior: it needs the last period start, and when `period_may_be_late` is true the returned say line flags a possibly late period and recommends a doctor. It does not describe the phase output itself, but that gap is minor.

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-loaded with purpose, then usage, then the safety boundary, then the edge-case note — a sensible priority order. It is on the longer side, and the two quoted user utterances plus the safety paragraph are verbose, though each carries real routing value.

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 an 8-parameter tool with no output schema and rich annotations, the description covers trigger, required input, medical scope limits and one rendering edge case. It does not sketch what the phase/workout/food response contains, which would help, but nothing essential to calling it correctly is missing.

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%, so every parameter (tz, country, cycle_length, diet, last_period_start alias, etc.) is already fully documented. The description only restates the required input ('Needs the first day of their last period'), contributing nothing beyond the schema — the baseline 3.

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 opens with a specific verb+resource ('Today's cycle phase, with workout and food ideas'), making the output self-evident. The word 'Today's' implicitly separates it from get_week_ahead and list_phases, but it never names those siblings or states the contrast 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?

It gives explicit trigger conditions (user asks what phase they are in, what to train, why energy is up or down) with two concrete example utterances. It also names when NOT to use it (fertile days, ovulation timing, contraception, pregnancy) and routes the user to a doctor or pharmacist.

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