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

Annotations already declare read-only, idempotent, non-open-world behavior, so the bar is lower; the description still adds non-obvious behavior — that output times depend on language (UTC for English, Moscow time for Russian) and that a page link is included.

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 the verb and scope in the first clause, and the two-sentence structure wastes little; the long content enumeration is dense but each item is genuinely informative.

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?

No output schema exists, and the description compensates by fully enumerating the returned sections (moon sign/phase/illumination, lunations, eclipses, stations, aspects, retrograde list, planet positions) plus the supplementary link.

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% with a date pattern and a lang enum already documented, so baseline is 3; the description adds the cross-parameter coupling that lang also determines the timezone of all timestamps, which is not visible in 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 ("Returns the sky of one calendar day from the Swiss Ephemeris") and enumerates the payload contents, which makes it distinguishable from natal_chart, transits_today and the other chart siblings.

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?

"No birth data needed" gives a clear selection signal that separates it from the birth-data tools (natal_chart, vedic_chart, vimshottari_dasha), but no sibling is named as an alternative nor is a when-not condition given.

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