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

Annotations already declare readOnlyHint=true, so the description does not need to restate safety. It adds useful behavioral context beyond annotations, including the anchor on the fall of Tenochtitlan, verification on the anchor and two modern dates, and alignment with the Maya count under the default constant. No contradictions with the annotations are present.

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 three sentences with each earning its place: the first sentence states the core output, the second provides calibration and verification context, and the third names target use cases. Information is front-loaded, and there is no redundant 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?

With no output schema, the description usefully enumerates the returned items: day sign, coefficient, trecena, and composed reading. It also provides anchoring and verification context that helps an agent trust results. It could be slightly more explicit about the shape of the composed reading or error handling, but for a calculator-style read-only tool it is adequately complete.

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%, and the input schema already provides rich detail on date format, proleptic Gregorian behavior, language defaults, and compact output shape. The description does not add meaningful parameter-level meaning beyond what the schema supplies, so the baseline score of 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 states a specific verb and resource: it 'returns the tonalpohualli day sign, its coefficient 1 to 13 and its trecena for any Gregorian date, with a composed reading.' This clearly distinguishes it from sibling list/table tools and other Mesoamerican calendar endpoints, making the tool's purpose immediately identifiable.

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 clearly implies the use case: 'Built for Aztec calendar apps, Mexica day sign features and AI agents answering what is my Aztec sign.' It gives clear context for when to call this tool, though it does not explicitly name alternatives or say when not to use it, so it stops short of full exclusionary guidance.

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