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

Annotations already cover read-only, idempotent, non-destructive, closed-world semantics, so the description is not burdened with the safety profile. It adds genuinely non-obvious behavior: UTC to-the-minute times derived from JPL DE421, and a stated 2025–2030 ephemeris window, which tells the agent the tool's validity horizon. It does not say what happens for out-of-range dates, but the added context is substantial.

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 sentences, front-loaded with the action and scope before the result content and provenance. Dense but nearly every clause carries information; the parenthetical about in-progress retrogrades is well placed. Slightly crowded noun lists ('sign, shadows, cazimi or opposition, and stations') reduce readability a touch.

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 the return-value burden and does so by naming the fields returned and their time basis. It also discloses the ephemeris source and date range, which is exactly the kind of limitation an agent needs for a timing tool. The only gap is undefined behavior outside the 2025–2030 window.

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% and both the 'from' default/time interpretation and the planet enum are already documented in the schema. The description adds detail about what comes back in the result (sign, shadows, cazimi/opposition, stations) rather than new meaning for the two input parameters, so the 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 ('Returns the next retrograde of [eight named planets]') plus scope ('after a date, or the one in progress'). The planet-general framing distinguishes it from the Mercury-specific siblings like next_mercury_retrograde and is_mercury_retrograde without needing the 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?

Gives clear context: use it to get the next retrograde after a date or the currently in-progress one, and explicitly frames the result as astronomical timing rather than a horoscope. It stops short of naming when to prefer the sibling tools (e.g., planet_retrograde_dates for full date lists, or the Mercury-only variants).

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