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

Annotations cover the safety profile (readOnly, idempotent, non-destructive), so the description's added value is the validation behavior: it rejects dates that do not exist in an era (Heisei 31-05-01) and enforces a 1873-01-01 to 2200-12-31 range. Those are concrete failure conditions an agent needs, beyond what annotations state. Return format is still undisclosed.

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, each carrying a distinct payload: the conversion contract, the accepted input formats, then validation/range limits. Purpose is front-loaded and there is no filler, though the format-example sentence is dense enough that a small amount of scanning is needed.

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 converter with no output schema, the description explains inputs thoroughly but never says what comes back (both representations? era name? gannen rendering?). The lack of required params also means the agent must infer that exactly one input mode should be supplied, which is only implied.

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?

With 67% schema coverage, the schema documents era, date, year and wareki, but the description adds the accepted wareki syntax variants and the 'gannen' (元年) semantics for year 1. It also clarifies that era/year/month/day is a combined alternative to the single date or wareki inputs, which the schema does not express.

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 bidirectional verb+resource: 'Convert between Western (Gregorian) dates and Japanese era dates'. The parenthetical naming of the five eras (Meiji, Taisho, Showa, Heisei, Reiwa) further pins down the domain, and no sibling (add_days, calculate_age, days_between) overlaps with this conversion task.

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?

It explicitly tells the agent the three acceptable invocation shapes: a YYYY-MM-DD date, a wareki string like '令和元年5月1日' / 'R1.5.1', or era + year + month + day. That is real routing guidance for a tool with six optional parameters and no required ones. It does not name exclusions or alternatives, which keeps it short of a 5.

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