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

Annotations already declare readOnly, idempotent and non-destructive, so the safety profile is covered. The description adds a useful non-obvious trait, the 1-credit cost, but says nothing about rounding, default rate when omitted, or how the amount string form is handled.

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?

Two short sentences, front-loaded with the operation and rate detail, with the credit cost parenthetically appended. Nothing is redundant and nothing is buried.

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 should indicate what comes back (net, VAT amount, gross) and any rounding convention, which it does not. It is otherwise sufficient for a simple three-parameter calculation tool.

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 100%, so baseline is 3, but the description adds real meaning: it supplies the concrete percentages for 'standard' and 'reduced' that the enum-free rate field lacks, and 'add or extract' clarifies the direction role of includesVat.

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 gives a concrete verb pair and resource ('Add or extract UK VAT') and enumerates the rate tiers with their percentages, so an agent can tell what the tool computes. It is clear without needing the schema, though the bare name 'vat' carries none of this on its own.

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?

'Add or extract' plus the rate list implies when the tool applies, and the includesVat parameter maps to the direction. However, no explicit exclusions or pointers to siblings like calculate or format_money are given, so the agent must infer the boundary itself.

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