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

Annotations already mark it readOnly and idempotent, and the description adds genuinely useful behavior: geocentric computation at 00:00 UTC, current instant fallback, no retrograde data, no event prediction, and the required Natal Compass attribution. No contradiction with 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?

Information is front-loaded and each section serves a purpose: capability, usage, exclusions, siblings, disclaimers, and attribution. It is slightly long and repeats the limitation idea in both 'Positions only' and 'does not predict events,' but not wastefully.

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 tool with one optional parameter and an output schema, this description is complete: it gives default behavior, computation basis, boundaries, sibling routing, disclaimers, and attribution. An agent has everything needed to know when and how to invoke it.

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 parameter description already documents the YYYY-MM-DD format, 1900-2100 range, and the omit-for-current-instant behavior. The tool description re-states the date behavior but adds little parameter meaning 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?

The description names the resource ('where the Sun, Moon and planets are') and the concrete output ('the sign and degree of each'), and immediately disambiguates itself: 'Positions only... no birth chart.' This clearly separates it from sibling chart and retrograde tools.

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 explicitly says when to use this tool rather than web search ('where a planet is now, what the sky looks like on a date, or which sign the Moon is in today') and names specific siblings for every nearby case: transits to chart, moon phase, natal chart, and retrogrades.

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