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

The annotations already establish read-only, idempotent, non-destructive, closed-world behavior. The description adds valuable context beyond that: returned markers, UTC minute precision, ephemeris source, and inclusion of periods that overlap the year.

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 compact sentences, with purpose and scope front-loaded. The source details and the astronomical-not-horoscope boundary each add useful information without filler.

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 simple one-parameter read-only tool with no output schema, the description covers the year range, returned fields, time precision, source, and overlap rule. It stops short of specifying output structure or explicit sibling routing, but it is otherwise complete enough to call correctly.

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?

The schema already fully documents the year parameter and its 2025–2030 range. The description adds a useful boundary rule by saying periods that 'fall in or overlap' the year are included, which clarifies cross-year behavior.

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 gives a specific verb and resource: it lists every Mercury retrograde period for a year. Its annual, Mercury-specific scope clearly separates it from siblings such as next_mercury_retrograde, is_mercury_retrograde, and planet_retrograde_dates, even without naming 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?

Usage is implied by the annual list scope, but there is no explicit when-to-use or when-not-to-use guidance. It also does not route the agent to alternatives like next_mercury_retrograde or is_mercury_retrograde.

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