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

Annotations already declare this a safe, closed-world read, so the safety profile is covered. The description adds real behavioral value by disclosing the exact Gregorian rule applied (div by 4, not 100 unless 400) and by noting a second return dimension (days in year), letting the agent predict results for edge cases like 1900 vs 2000.

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 dense sentence with the primary function front-loaded and the rule parenthetical placed after it. No filler, no restatement of the title or annotations.

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 an output schema present, return-value explanation is not required, and annotations cover the safety profile; the description still tells the agent the rule used and that days-in-year is also produced. Only the absence of sibling routing guidance keeps it from being fully 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?

There is a single parameter with 100% schema description coverage including a range (1–9999) and examples, so the schema fully carries parameter meaning. The description adds nothing about the year argument, which is the expected baseline-3 outcome when coverage is complete.

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+resource ('Gregorian leap-year check') plus a secondary output ('days in year'), which is far more than the tautological name restates. It does not explicitly distinguish itself from adjacent siblings such as calculator_days or calculator_weekday, so it stops short of a 5.

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?

Usage is only implied: the phrase 'leap-year check and days in year' signals the intended question, but there is no explicit when-to-use/when-not guidance and no pointer to alternatives for date-based questions. Adequate but leaves routing to inference.

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