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

Annotations already declare readOnlyHint=true and idempotentHint=true, so safety is covered. The description adds valuable behavioral detail beyond the annotations: it notes that 'only the month and day matter' (ignoring the year) and specifies the exact output contents (sign, symbol, date range, element). This gives the agent a clear picture of behavior without contradicting the 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?

The description is two sentences plus a source URL, with zero fluff. The purpose is front-loaded, the format note is concise, and the output details are listed compactly. Every sentence earns its place, making it an exemplary model of concise, structured documentation.

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 simple one-parameter tool with an output schema, the description is complete. It covers the purpose, the date format and its nuance, and the expected return values. The output schema handles the detailed return structure, so nothing essential is missing for an agent 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?

The schema already documents the 'date' parameter with format YYYY-MM-DD, achieving 100% coverage. The description reinforces the format and adds the nuance that only the month and day matter, which is not in the schema. This extra semantic detail helps the agent understand the parameter's behavior, going beyond the schema's baseline.

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 clearly states the tool's purpose: 'Find the western zodiac star sign for a date of birth.' This is a specific verb-resource pair that distinguishes it from sibling tools like hopi_age_calculator or hopi_day_of_week_calculator. The mention of 'western zodiac' and the returned fields (sign, symbol, date range, element) leaves no ambiguity about what the tool does.

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 description provides clear context: it is for finding a star sign from a date of birth. It does not explicitly list alternatives or when not to use it, but the purpose is so specific that an agent would naturally select it for star-sign queries. The context is unambiguous and there are no exclusions, meeting the 'clear context, no exclusions' criterion.

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