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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so safety is covered. The description adds genuinely non-structured behavior: free within a daily allowance, then $0.001 USDC per call via x402, plus that results carry a checked-date. That is real cost/rate-limit context beyond the annotations.

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 sentences, front-loaded with what is returned and the scope, followed by the cost model. No filler or redundancy; every clause carries information.

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?

An output schema exists, so return values need not be described, and annotations cover the safety profile. The description adds scope, freshness and pricing, leaving nothing an agent needs in order to call it correctly.

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 the single parameter's omit-for-all semantics are already spelled out in the schema ('ISO code (omit for all)'). The description's 'one country or all of EU27 + UK' restates that without adding format or syntax detail, so 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 resource (standard, reduced and super-reduced VAT rates) with clear scope (one country or all of EU27 + UK) and even the freshness metadata (date checked). An agent can tell this is a rates-lookup rather than a calculator, though it never names vat__calculate or vat__validate-format explicitly.

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: fetch rates for a country or all of EU27+UK. There is no explicit when-to-use vs when-not, nor any pointer to the sibling tools that handle calculation or format validation. The pricing sentence adds context but not routing guidance.

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