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

Annotations already declare read-only, idempotent, non-destructive behavior. The description adds valuable context: it describes the monitoring cadence (polls every 5 minutes), the specific metrics exposed (status, latency, 24h/7d uptime), and the free/no-API-key requirement, which goes beyond what annotations provide.

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 concise at three sentences, each contributing meaning: what the service is, what data it provides, and when to use it. It is front-loaded with the core purpose and contains no filler or redundancy.

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 one-parameter read-only tool with no output schema, the description is sufficiently complete. It states the data returned (status, latency, uptime), the polling interval, the use case, and the lack of authentication, leaving no significant gaps for an agent to invoke the tool correctly.

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 input schema already thoroughly documents the optional 'country' parameter with pattern and semantics. The description adds no additional parameter-specific details (e.g., default behavior is already in the schema). Schema coverage is 100%, so a baseline score of 3 is appropriate.

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?

Description clearly states the tool reports current availability of VIES, the EU VAT validation service, with per-member-state status, latency, and uptime. It explicitly ties to its name and distinguishes from sibling validation tools by focusing on service availability rather than VAT number validity.

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?

Provides explicit usage guidance: 'Use this when EU VAT number validation is failing, to tell 'their VAT number is wrong' apart from 'that member state's VIES endpoint is down'. This directly addresses when to invoke this tool versus alternatives, though it does not name siblings explicitly.

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