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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new behavior: results are computed from real astronomy, it's factual data rather than interpretation, and it falls back to client-shared location when no city is given.

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 content is front-loaded with what is returned, followed by example queries and the location/date defaults, in a compact block with no filler sentences. Every clause carries usable information for invocation or selection.

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?

Zero required parameters and no output schema, yet the description enumerates the return content (sign, degree, retrograde status, Moon phase) explicitly. Combined with the location fallback note, an agent has everything needed to call it correctly.

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%, so city, date and timezone semantics and their 'omit for now/user location' defaults are already documented in the schema. The description only restates the default behavior, adding no format or edge-case detail beyond what the structured fields provide, 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 ('where the Sun, Moon and planets are ... which planets are retrograde, and the Moon phase') and enumerates concrete data fields (tropical zodiac sign and degree). The example queries ('is Mercury retrograde', 'today's astrology') make it clearly distinct from siblings like get_four_pillars, get_solar_terms, and convert_lunar_date.

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 usage context ('for now or any date', example question phrasings) and a scope exclusion ('Facts only, no horoscope text'), which tells the agent what this tool will not produce. It stops short of naming alternative siblings for adjacent astrology questions, so no explicit when-not-to-use routing.

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