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

Annotations already cover read-only/idempotent/non-destructive/open-world, and the description adds substantial behavior beyond that: it is a paid call ($0.003), validation is offline and checksum-only where the authority publishes an algorithm, unsupported countries return 'unknown' for free, and the response explains x402 payment when no api_key is supplied. That is exactly the kind of cost, degradation, and auth context annotations cannot express.

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?

Price and purpose are front-loaded, and each sentence carries a distinct fact (scope, non-registration, unknown fallback, payment, follow-up). It is dense and relies on long compound sentences, and the register_alert pricing tail reads partly promotional, but little is truly 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?

An output schema exists, so return-field explanation is unnecessary; the description instead covers the non-obvious essentials: cost, payment path, degraded 'unknown' behavior, and the follow-up series tools. Nothing an agent needs to invoke and interpret this correctly is missing.

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 id/country are already documented in the schema, so baseline would be 3. The description still adds meaning: the concept that a call validates exactly one country and that country choice determines whether a sourced rule exists (and therefore whether the call is free), plus the api_key-vs-x402 payment path for the third parameter.

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?

Opens with a specific verb+resource: 'Offline format and check-digit validation of a business tax ID or VAT number for one country'. It even enumerates the outputs (valid/invalid, normalised form, id type, check level, failure reason) and explicitly distinguishes itself from the sibling official_lookup, so an agent can route correctly 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?

States a clear boundary ('Does not confirm registration; follow official_lookup for that'), a per-country constraint, and a fallback for unsupported countries ('answer free with status unknown'). It also tells the agent how to proceed on payment (pass api_key, otherwise x402) and what to bind afterwards, so both when-to-use and when-not-to-use are covered.

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