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

With no annotations, the description carries the burden. It discloses important behavioral traits: 'Format check only; no VIES call' (no external calls) and 'Pure; price 0.0 (free)' (side-effect-free and no cost). This helps the agent understand constraints, though it doesn't detail return format or error handling.

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?

Two sentences, front-loaded with the core action, and zero waste. Every word adds meaning, from the validation scope to the free-of-charge point.

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 single-parameter tool with no output schema, the description covers all essential aspects: what it validates, how it operates (format only, no VIES), purity, and cost. It is fully sufficient for an agent to use 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?

Schema coverage is 100%, so baseline is 3. The description adds value by explaining the expected format structure 'country prefix + national pattern', which enriches the schema's brief example. This extra semantic context justifies a 4.

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 purpose: 'Validate the structural format of an EU VAT number' with explicit scope details ('country prefix + national pattern'). It also distinguishes itself from siblings by noting 'Format check only; no VIES call', making the action and resource unambiguous.

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 clear context for when to use this tool: for structural format validation only, not for live VIES checks. It explicitly excludes VIES calls, which implies alternatives for VIES lookups. While it doesn't name sibling tools, the exclusion is precise and actionable.

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