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

Adds real behavioral context beyond the annotations: credit consumption is conditional per domain (only domains returning data cost a credit), and results are unverified public-website findings. This is exactly the cost/trust information an agent needs before spending credits. Minor gap: no mention of response shape or failure behavior, though annotations already cover the mutation/open-world profile.

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?

Three sentences, each front-loaded with a distinct fact: what is returned, the input cardinality, and the credit/verification caveat. No filler or repetition of the schema.

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 one-parameter lookup with no output schema, the description covers purpose, input limit, cost model, and data trustworthiness — enough to call it correctly. It would be a 5 with explicit guidance on choosing between this and the vat-types/x402 variants.

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% and the single 'lookup' parameter is fully documented in the schema. The description only restates the 1–16 comma-separated domain format, adding no new semantics such as domain normalization, wildcards, or subdomain handling. 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 and resource ('VAT API JSON lookup for VAT, GST, CNPJ, ABN, and other company registration numbers associated with websites') and names the input unit (domains). An agent can distinguish it from sibling 'vat-types-api' (which presumably lists identifier types) without opening either 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?

The description implies usage (look up registration numbers for a set of domains) and gives the 1–16 domain limit, but never states when to prefer it over vat-types-api, x402-vat-api, or domain-api. The only negative constraint is the credit behavior, not a use-case exclusion.

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