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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and a closed world, so the safety profile is fully covered. The description's only added behavioral content is naming outputs (HT, VAT amount), which duplicates the existing output schema; nothing about rounding, default rate behavior, or limits is disclosed.

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?

Short, front-loaded sentences in the order an agent needs them: direction, use case, inputs, outputs, related tools. Minor redundancy between 'extract the VAT and net price' and 'back out tax from a gross invoice', but no wasted bulk.

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 two-parameter deterministic calculator with full schema coverage, an output schema, and safety annotations, the description supplies the direction, required inputs, and a pointer to the related finance bundle. Only omission is any disambiguation from the country-specific reverse-VAT scenarios, which is a minor gap.

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% and both parameters are documented there ('Amount including VAT', 'VAT rate %' with default 20), so the schema does the heavy lifting. The description restates the same two inputs without adding format or default details, which is the baseline 3 case.

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 action (extract/back out) on a specific resource (VAT) with an explicit direction that separates it from the forward-VAT siblings like calculate_vat_generic, calculate_french_vat, or calculate_belgian_vat. 'Reverse-VAT' plus 'TTC amount' makes the direction unambiguous without opening a 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?

'Use to back out tax from a gross invoice' gives a clear triggering context. It does not name the nearest alternative (calculate_vat_generic) or state when NOT to use it, but the reverse-direction framing makes the choice largely self-evident.

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