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

Annotations already establish read-only, idempotent, closed-world behavior, and the description adds genuinely new context: the tool works without an API key and ships a concrete endpoint example. It omits any description of the response shape, which is a gap given there is no output schema, but the auth-access disclosure is real value beyond 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?

The lead sentence front-loads the outputs and the parameter, and the remaining two sentences each add something (key-free access, usage scope). The curl example is useful but slightly casual for a tool definition; otherwise there is little waste.

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-optional-parameter read tool with a 100%-covered schema and no output schema, the description covers what it returns conceptually and when to reach for it. The only real gap is that the actual response structure is never hinted at.

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 100% and the single date parameter is fully documented in the schema, including format and the today-by-default behavior. The description repeats the format and default rather than adding syntax or edge-case meaning, so it sits at the baseline for a fully documented 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?

The description names concrete resources and outputs: planetary positions, Moon phase, and the coming week's sky events for a date. It also distinguishes itself from chart-based siblings with the clause 'when no one's chart is involved', so an agent can separate it from natal_chart/transits without opening schemas.

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 an explicit positive usage list ('where the planets are on a date, the Moon phase, or the coming retrogrades and ingresses') plus a negative condition ('when no one's chart is involved'). It does not name the specific alternatives (transits, daily_transits) that should be used instead, so routing is implied rather than fully explicit.

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