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

Annotations already declare readOnly/idempotent/non-destructive, so safety is covered. The description adds that the computation works for any date and lists the outputs, but discloses nothing beyond that (no limitations on date range, no caching or precision notes) — modest added value over the 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?

Front-loaded with purpose, then uses, then I/O in a tight block. 'Inputs: date' and the return-value list duplicate what the schema and output schema already carry, so a small amount of text does not earn its place.

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 one-parameter, read-only computed tool with both annotations and an output schema, the description is sufficient: it names the domain, the input, and the returned fields. Only the absence of any edge-case or scoping note keeps it short of full completeness.

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?

With one parameter at 100% schema coverage, the schema already specifies the YYYY-MM-DD pattern and format. 'Inputs: date' merely restates the parameter name and adds no format or boundary semantics beyond the schema.

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 plus scope: 'Compute current moon phase and illumination % for any date.' That is unmistakably distinct from siblings like calculate_sunrise_sunset or calculate_star_magnitude_distance, so an agent can select it without opening 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?

'Use for astronomy, agriculture, fishing' gives domain triggers but no when-not condition and names no alternative tool. The pointer to list_bundles routes to a bundle rather than to a competing calculator, so usage guidance stays implied rather than explicit.

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