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

Annotations already declare readOnly, idempotent and non-destructive, so the safety profile is covered. The description adds genuine non-schema context: a cost of 10 credits (Tier 1) and the backing endpoint, which lets the agent weigh budget before calling. It does not describe latency, pagination, or how critical days are flagged, but billing disclosure is a real addition beyond annotations.

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?

The first sentence front-loads the useful substance (cycle lengths and outputs) and the bracketed metadata is compact. Nothing is padded, though the metadata block is a flat dump rather than integrated prose.

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?

An output schema exists, so return values need not be explained, and annotations cover the safety profile. What is missing is date-range semantics (default range, whether rangeStart/rangeEnd are required for a meaningful 'per day' series) and a clearer rule for choosing this over render_biorhythm. Adequate but with clear gaps for a 5-parameter tool.

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 only 40%: fields and precision are documented in the schema, but rangeStart, rangeEnd and birthDate have no descriptions anywhere. The description adds little for them — 'per day' hints at a date range and '[Required: birthDate]' merely repeats the schema's required array. With half the parameters undocumented, the description fails to compensate for 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 names the exact output (physical 23d, emotional 28d, intellectual 33d cycle values per day plus zero-crossing critical days), which is far more specific than the title 'Biorhythm (JSON)'. It also differentiates itself from the sibling render_biorhythm by calling itself the 'Data twin of POST /render/biorhythm'. It lacks an explicit action verb (no 'compute'/'returns'), so it reads as an output spec rather than a task statement.

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 only implied: 'Data twin of POST /render/biorhythm' suggests picking this over the render variant when raw JSON is wanted, and the metadata block names the Wellness group and required birthDate. There is no explicit when/when-not statement and no named alternative within the tool set, so the agent must infer the selection rule.

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