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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds useful context beyond that: the standard rate, the ht/ttc interpretation, and the computed outputs (HT, TTC, tax amount). It does not, however, mention that the rate is overridable or what happens at non-default rates.

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 purpose and rate are front-loaded, and the 'Inputs / Returns' shorthand is efficient with no filler. The input list is slightly incomplete (no rate), a small imperfection in an otherwise tight definition.

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 read-only, idempotent calculator with an output schema and full schema coverage, the description supplies enough to call it correctly. The only gap is the unmentioned overridable rate parameter, which the schema covers anyway.

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 the schema already documents amount in XOF, the ht/ttc enum, and the rate parameter with its default and bounds. The description's 'amount, mode (ht/ttc)' merely restates the schema and omits the rate parameter entirely, adding no meaning beyond structured fields.

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?

The description states a specific verb and resource ('Compute Senegalese VAT (TVA)') and pins the jurisdiction and default rate (18% standard). It is clearly distinguishable from generic or other-country VAT calculators because of the explicit national scope, though it never names a sibling such as calculate_french_vat or calculate_vat_generic to reinforce the boundary.

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?

'Use for invoicing in Senegal' gives a concrete usage context, which is more than most sibling calculators offer. It stops short of stating when not to use it or which alternative applies for other countries, so the exclusion logic is left to inference.

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