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

Annotations already cover the safety profile (readOnly, idempotent, openWorld, non-destructive), so the bar is lower. The description adds real behavioral context beyond them: it requires the last period start, discloses that the 'say' line flags a late period when period_may_be_late is true, and warns against medical interpretation. It does not describe pagination or response shape, but that is minor for a single-result tool.

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 triggers, then inputs, then exclusions — good ordering with every sentence carrying weight except the mild redundancy of restating the pregnancy/contraception exclusion twice ('not for getting pregnant... not for pregnancy planning or contraception').

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?

With no output schema, the description carries return-value burden and does reasonably well: it says the tool yields phase, workout and food ideas, and a 'say' line that can flag a late period. It is complete enough to call correctly, though it doesn't spell out the full response structure.

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 the schema already documents tz, date, diet, country, cycle_length, period_length and the last_period/last_period_start aliasing. The description only reinforces that last_period is required, adding no syntax or edge-case detail, so 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 and resource — today's cycle phase plus workout and food ideas — and the scope ('Today's') cleanly separates it from syncmycycle_get_week_ahead and syncmycycle_list_phases. An agent can route between the three siblings without opening schemas.

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 with two quoted user utterances ('what phase am I in, my last period started September 12th', 'what should I train today?'), names the required input, and states clear exclusions (pregnancy planning, fertile days, ovulation, contraception) with a redirect 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.