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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), and the description adds genuinely useful context beyond them: exact integer arithmetic, the returned fields (tax amount, both prices, formula), and the 13-digit yen ceiling. It does not discuss error handling for invalid rates or amounts, which keeps it from a 5.

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?

A single dense sentence that front-loads the core operation and then the parameter options, with no filler. It is slightly overloaded with enumerations, but every clause carries information.

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 names what is returned, and it documents all parameters plus the numeric limits. An agent has enough to call it correctly; only error/failure behavior is left unspecified.

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 all four parameters are already documented in the schema; the description largely restates the rate and rounding choices. It adds little syntax or format detail the schema lacks, so the baseline of 3 applies.

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 and resource ('Calculate Japanese consumption tax'), spells out both conversion directions, and makes clear this is a tax calculator rather than any of the date/unit/conversion siblings. An agent can identify it immediately without opening the schema.

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?

Explains the two operating modes (tax-excluded to tax-included and the reverse) and enumerates the rate and rounding options, which effectively tells the agent when each parameter path applies. It stops short of explicit exclusions or 'use X instead' routing, but the sibling tools are unrelated utilities, so no routing is really needed.

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