accounting.vat_add

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

Annotations already declare readOnly/idempotent/closed-world, so the description's job is to add computation semantics — which it does: rounding to integer places, default HALF_EVEN, and Decimal-string amounts. It does not state the accepted rounding-mode vocabulary or how rate bounds/negative inputs are handled, so it stops just short of full 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?

Four tight sentences, front-loaded with the computation before the disambiguation and format notes; no filler. Density is high but each sentence carries distinct information (formula, rounding default contrast, Decimal format, reverse tool).

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 values need no explanation, and the description covers the formula, defaults, and the reverse-direction alternative. Only the rounding-mode vocabulary and input-format edge cases are left unstated for a tool whose parameters have zero schema descriptions.

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 0%, so the description must carry the load, and it largely does: it identifies net as the supply amount (required), gives rate's default 0.1, and gives rounding's default HALF_EVEN plus the fact that rounding targets integer places. The gap is that the allowed rounding enum values and the expected rate string format (fraction vs percent) are never enumerated.

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 (add VAT to the supply amount to get the gross amount) and names the exact formula (VAT = net × rate). It also names the sibling it is not: vat_extract, which goes the other direction.

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?

Explicitly routes the agent: use vat_add when going net→gross, and use vat_extract when going gross→net. It further disambiguates the two by flagging that their default rounding modes differ (HALF_EVEN vs DOWN), which is exactly the kind of detail that prevents a wrong sibling choice.

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