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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint false, indicating a safe, read-only operation. The description adds valuable context: the real-time nature, reliance on VIES, occasional unavailability for some member states, and that it returns company name and address when valid. This goes beyond annotations and helps set expectations about availability and output.

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?

The description is two sentences, with no filler. The core action and output are front-loaded, and the note about VIES availability is a concise, useful addition. Every sentence earns its place.

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?

Given there is no output schema, the description correctly explains the success return (company name and address). It also notes potential service unavailability. However, it does not mention what happens on invalid VAT numbers or when VIES is unavailable (e.g., error messages, fallback behavior). These are minor gaps, but for a real-time external service, a bit more detail would be helpful.

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%, so the schema already explains both 'vat' and 'country' parameters. The description does not add extra semantic detail beyond the schema; it only repeats that the VAT number can include a prefix and country is optional. According to the rubric, when schema coverage is high, the description need not duplicate parameter info, so the baseline of 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?

The description clearly states the tool's function: checking an EU VAT number against the VIES service and returning company details. It uses a specific verb ('Checks') and identifies the resource (EU VAT number) and service (VIES). This distinguishes it from validation siblings like iban_validate or email_validate, which operate on different data types.

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 clearly indicates when to use this tool: for EU VAT number validation against VIES. It does not explicitly state when not to use it or mention alternative tools, but the purpose is unambiguous. The context signals list many validation tools, but none are direct competitors, so the absence of explicit exclusions is acceptable.

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