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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds genuinely useful context by stating what the calculation returns (net, VAT and gross amounts), which matters since there is no output schema.

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, tightly packed sentence that front-loads the two modes of operation and ends with the output composition. No filler and nothing redundant.

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 calculator with full schema coverage, the description supplies the one thing structured fields don't: the shape of the result (net, VAT, gross). Nothing critical is missing, though it could clarify whether output is a single figure or a breakdown object.

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, direction and ratePercent, including the add/remove semantics. The description's mention of 20%, 5% and any other rate largely restates the ratePercent default and enum rather than adding new meaning.

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 (add/remove VAT) and resource (a price amount), making the tool's function immediately clear and separable from siblings like check_vat_registration or check_vat_reverse_charge. It does not explicitly name or route against any sibling, which keeps it at a 4 rather than a 5.

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?

Usage is implied: use it when you need to add VAT to a net figure or extract VAT from a gross figure. There is no explicit when-not guidance and no mention of alternative tools, so the agent must infer the context.

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