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

Annotations already cover the safety profile (readOnly, idempotent, closed-world), and the description adds substantive behavior: how out-of-era dates are handled (still counted, within_era=false, outside_era flag) and weekday validation (weekday_matches=false, weekday_mismatch). It does not describe error behavior for unparseable input, which is the main remaining gap.

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 core conversion and output-format facts are front-loaded in the first sentence, and the edge cases are ordered logically. The bilingual duplication restates the same content in Japanese, which is reasonable for a Japanese-date tool but costs some density.

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 carries the burden of explaining returns and does so via within_era, weekday_matches, and the flags object. Minor omissions remain (invalid input handling, era range limits), but an agent has enough to call and interpret 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?

Schema coverage is 100% for the single wareki parameter, so baseline is 3, but the description adds genuinely new semantics: kanji numerals, weekday suffixes, full-width characters, and formats like H31/4/30 and Reiwa 8.10.7 are not enumerated in the schema. This meaningfully expands what the agent knows it can pass.

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 (convert Japanese era date to Gregorian) and gives the output format YYYY-MM-DD. It is clearly the inverse of the sibling to_japanese_era, so an agent can route between them without opening schemas.

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?

Conveys the context well by enumerating many accepted input shapes and the out-of-era conversion behavior, which implicitly tells the agent when this tool applies. It never explicitly names to_japanese_era as the alternative for the reverse direction, so it stops 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.