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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered elsewhere. The description's only added behavioral facts are the cost (10 credits, Tier 1) and the no-write data nature — real value, but thin; it says nothing about rate limits, caching, or what happens if the date range is omitted.

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?

Two tight sentences that lead with the actual data content, followed by a compact metadata block for group and cost. Nothing is redundant, though the bracketed metadata is bolted on rather than integrated into the 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-value shape needn't be explained. But for a date-range-driven tool the description fails to state the default window when rangeStart/rangeEnd are absent, and it does not clarify the required vs optional inputs — gaps an agent cannot resolve from the structured fields alone.

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% and the description adds nothing about any of the five parameters. Crucially it never explains that birthDate is required nor what rangeStart/rangeEnd do or what the default range is when they are omitted, and it is silent on the compact-mode fields/precision options that the schema only partially documents.

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?

Names the exact resource (biorhythm), the three cycles with their period lengths (23d/28d/33d), and the extra output (critical zero-crossing days). It also distinguishes itself from the sibling astroway_render_biorhythm by calling itself the 'data twin of POST /render/biorhythm', so an agent can route between JSON data and rendered image without opening either schema.

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?

The 'data twin' phrasing implies this is the structural-data counterpart to the render tool, which is a useful implicit routing cue. However, there is no explicit when-to-use statement, no mention of the other wellness siblings (astroway_wellness_cycle, astroway_wellness_sleep_cycles), and no stated preconditions such as whether a natal chart must exist first.

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