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

Annotations already cover readOnly, idempotent, non-destructive, closed-world. The description adds genuinely non-structured traits: cost (1 credit), determinism, and rounding to cents — all useful for an agent deciding whether to call or cache. It does not elaborate error behavior (e.g. rate > 100), which keeps it just short of a 5.

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?

Front-loaded with the use case and an example, then cost and determinism. There is mild redundancy between "Use this when you need to add VAT..." and the following "Add VAT to a net amount or extract..." sentence, but overall no wasted passages.

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?

A low-complexity, three-parameter calculation tool with an output schema, so return-value explanation is optional and already partly covered. Annotations plus the description cover safety, cost, and output precision; only input-validation behavior is unaddressed.

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% — mode enum, rate bounds, and amount are all documented in the schema itself. The description's prose about add/extract and the "21 for Dutch BTW" example roughly restate the mode and rate semantics rather than adding new meaning, so the baseline 3 applies.

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 (add/extract) and resource (VAT), with explicit scope: net→gross and gross→net at any rate. No sibling tool overlaps this domain, so an agent can select it unambiguously from the list.

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?

The opening sentence plus a concrete example prompt ("What is 21% BTW on 1,250 euros?") make the trigger condition explicit. It does not name exclusions (e.g. other tax types or multi-line invoices), but no sibling offers an alternative, so nothing is left ambiguous in practice.

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