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

The description discloses the return shape, the expected format, and the exact validation formula for numeric keys, which goes beyond the annotations' read-only/idempotent hints. This gives the agent insight into the tool's behavior and limitations.

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 three sentences, with no fluff. The first sentence states the action, second gives format and return, third gives formula and usage. Every sentence adds value and it is efficiently structured.

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 simple read-only validation tool with one parameter, the description covers purpose, input format, output structure, validation logic, and use cases. There is no output schema, so the explicit return object description fills that gap completely.

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?

The schema already covers the 'tva' parameter with a description and example, so baseline is 3. The tool description adds the structural rule 'FR' + 2 alphanumeric key + 9-digit SIREN, which enriches understanding beyond the schema's simple 'with or without spaces' note. This additional format detail raises the score.

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 opens with 'Validates a French TVA intracom (VAT) number', which is a specific verb and resource. It clearly distinguishes from sibling validation tools by naming the French TVA and explaining it's for EU VAT identification, so the agent knows exactly its scope.

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 provides explicit usage context: 'Use when validating French supplier VAT numbers, processing cross-border EU invoices...'. This tells the agent when to select this tool, though it doesn't mention alternatives or exclusions, just the positive case.

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