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

Annotations already declare readOnlyHint and openWorldHint, but the description adds substantive behaviour: the three-stage verification pipeline, the returned company fields (nom, adresse, active ou cessée), the reduced quota without a key, and the free optional key as an authentication path. That is well beyond what the annotations convey.

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?

A single dense sentence, front-loaded with the operation and format, then the scope exclusion, then the quota caveat. No filler and nothing buried.

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?

With no output schema, the description carries the return-value burden and does so by naming the three verification stages and the company fields returned. For a one-parameter read-only tool, an agent has everything needed to invoke it and interpret the result.

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?

Only one parameter, and the schema already documents it at 100% coverage with an example and space tolerance. The description still adds a structural breakdown of the expected value ('FR + 2 caractères + 9 chiffres'), which helps an agent construct or pre-validate input, so it edges above the baseline 3.

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 (Vérifie) and resource (numéro de TVA intracommunautaire français) and enumerates the three checks performed: control key, VIES validity, and associated company in the official registry. The scope restriction 'Numéros français uniquement' cleanly separates it from a generic VIES lookup.

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 an explicit exclusion and alternative: 'Numéros français uniquement ; pour un autre pays, utiliser VIES directement.' It also flags the authentication condition ('Sans clé : quota réduit ; clé gratuite facultative'), so an agent knows both when to call it and how quotas shape the call.

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