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

Annotations already declare readOnlyHint, idempotentHint, and openWorldHint=false, so the safety profile is covered. The description adds genuinely useful behavior: the response changes shape when only a date is given (returns every sign for that day), and it names the extra fields returned. It does not discuss caching, precision, or failure modes, so the added value is moderate rather than rich.

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?

Two sentences, front-loaded with the core verb and resource before the qualifiers. Every clause is informative, though the dense single opening sentence stacks many qualifiers and the phrase 'the website's one-line meaning' is slightly ambiguous wording.

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 zero-required-parameter read tool with an output schema, full schema coverage, and explicit annotations, the definition covers scope, timezone, zodiac systems, input combinations, and return contents. Nothing needed to select or invoke the tool correctly is missing.

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 description coverage is 100%, so all three parameters are already fully documented with patterns, ranges, and defaults. The description's contributions ('UK date or UK date and time', the Lahiri-to-'sidereal' enum mapping, and the date-only multi-sign behavior) largely restate or lightly re-frame what the schema already provides, so the baseline of 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 ('Returns the Moon's zodiac sign') and immediately bounds it in time (right now / UK date / UK date and time), zodiac system, and the concrete payload (degree into sign, next sign change, one-line meaning). The narrow astrological-sign framing is enough to separate it from sibling get_moon, get_moon_calendar, and find_moon_events without opening a 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?

Gives clear invocation contexts: omit date/time for the current sign, supply a date alone to get every sign the Moon occupies that day, or date+time for an exact instant. It does not name an alternative sibling or state when a different tool (e.g. get_moon) would be the better choice, so it falls short of full when/when-not routing.

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