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

Annotations already declare readOnlyHint=true and destructiveHint=false. The description adds valuable behavioral context beyond this: content is fixed per date, rolls over at midnight UTC, and is cacheable/plannable. This tells the agent the tool is deterministic and timezone-anchored, which is useful for caching and scheduling. It could have mentioned response shape, but annotations lower the bar for safety disclosure.

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?

Three sentences with no filler. The first sentence front-loads the core purpose and result content; the second adds the caching-relevant behavioral detail; the third names target use cases. Every sentence earns its place.

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 read-only tool with zero required parameters and no output schema, the description covers the essential return fields (day sign, coefficient, trecena, composed reading), the date flexibility, and the UTC rollover behavior. Combined with the fully descriptive schema, an agent has everything needed to select and invoke the tool 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?

Schema description coverage is 100%, so the input schema already fully documents date, lang, and compact including defaults, formats, and semantics. The description adds no parameter-specific meaning, but under the high-coverage baseline of 3, that 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 resource and action: it returns the tonalpohualli day sign, its coefficient, its trecena, and a composed reading for a given date. It clearly differentiates from sibling tools by emphasizing the 'daily' / date-specific nature versus listing endpoints like day_signs or trecenas, and by specifying Aztec rather than Mayan.

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 gives concrete use cases: 'daily energy widgets, Mexica day sign feeds and chatbot answers about what today carries.' It also implies the tool is for date-specific queries rather than static reference lists, but it does not explicitly name alternatives or state when not to use this tool.

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