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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, but the description adds substantial behavior beyond them: per-call pricing ($0.003), the free 'unknown' status path, the api_key/x402 payment fallback, and the explicit limitation that checksums exist only where the authority publishes an algorithm. It even discloses what the response does after a 200.

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-loads the key fact ('PAID ($0.003)') and each sentence carries information, but the single dense paragraph mixes validation semantics with pricing, an upsell for get_changes/register_alert, and fine-grained price annotations, which slightly dilutes scanability.

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 paid, open-world validation tool, the description covers capability, limitation, payment path, and post-call follow-up. An output schema exists, so return-field detail is optional, yet the description still names the key outputs (status, normalised form, id type, check level, failure reason).

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, and the description adds real meaning on top: 'one country of this brand' constrains the country enum, and the api_key parameter's behavior (use it if you have one, otherwise the response explains x402 payment) is described nowhere in the schema.

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 and resource with scope: 'Offline format and check-digit validation of a business tax ID or VAT number for one country'. It also explicitly scopes itself against the alternative ('Does not confirm registration; follow official_lookup for that'), so an agent can distinguish this from a registration-verification tool 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?

Gives explicit when-to-use and when-not-to-use guidance: use this for offline format/checksum validation, use official_lookup for registration confirmation. It also explains the unpaid path (countries without a sourced rule return 'unknown' free) and what to do about credentials, leaving little to inference.

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