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

The annotations already declare readOnlyHint=true and openWorldHint=true, so safety and external-data properties are covered. The description adds useful behavioral context: the shadow-casting body is derived from the object's orbit unless overridden, and the search window defaults to two orbits capped at 12 hours. It stops short of describing the returned states/times in any detail, which matters since there is no output schema.

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 core capability and followed by defaults and the intuition-building fade-out explanation. Dense but every clause carries information; the parenthetical body list is slightly long but earns its place by clarifying which shadow applies for Earth, lunar and Martian orbiters.

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 read-only, six-parameter query tool with no output schema, the description covers what is computed, the default window, and the unit-of-interest (which body casts the shadow). The main remaining gap is the shape of the result — states and entry/exit timestamps — which an agent must infer.

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 coverage is 83%, so the schema already documents date, lang, hours, centre and time_zone. The description only restates the hours default and 12-hour cap, which the schema already encodes as default and maximum; it adds no syntax or semantic detail for id, date or time_zone. Baseline 3 is appropriate.

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 states a precise verb and resource: it determines whether an object is in sunlight, penumbra or umbra, and when it next enters and leaves the shadow, plus which body casts the shadow. This is specific enough that no sibling (perigee_passes, perigee_sky, perigee_events) could be confused with it, although siblings are not named explicitly.

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?

It gives a concrete use case ('Explains why a satellite crossing the night sky suddenly fades out') and states defaults and a hard bound ('over two orbits, at most 12 hours'), which tells the agent what it gets without extra parameters. It does not say when NOT to use it or name an alternative tool for related questions.

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