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 behavior beyond that: it discloses that the response includes the sign's date range and that boundary-day births produce a warning. It does not mention error behavior for an invalid date, which is a minor gap.

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?

One sentence, front-loaded with the core mapping, followed by the two output details that matter (date range, boundary warning). No filler and nothing buried.

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-parameter lookup with no output schema, the description covers the input meaning, the primary return value, and the notable edge case (boundary day). Only the invalid-date/error path is unaddressed, which is minor given the schema's regex constraint.

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?

There is a single required parameter with 100% schema description coverage, including a JJJJ-MM-DD pattern, so the schema already carries the semantics. The description restates the parameter's meaning ('geboortedatum') but adds no format or edge-case detail beyond it. Baseline 3 is appropriate.

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 a specific verb+resource: it maps a birth date to a zodiac sun sign. It also previews the output (the sign's date range plus a boundary-day warning), which separates it from the horoscope-by-period siblings. It stops short of naming any sibling explicitly, so it is clear but not fully differentiated.

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?

The use case is strongly implied by the description ('welk sterrenbeeld hoort bij een geboortedatum'), but there is no explicit when-to-use, when-not-to-use, or pointer to an alternative sibling. An agent can infer the trigger from the name and description alone, but must do that inference itself.

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