APIbase

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

No annotations are provided, so description carries full burden. It successfully discloses return values (validity status, company name, registered address), data source (VIES), and geographic coverage. Missing only rate limits or error condition details, but adequately covers core behavioral traits for a lookup tool.

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?

Single dense sentence front-loaded with critical information (action, returns, scope) followed by format examples and attribution. Every clause earns its place with zero redundancy or waste.

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 single-parameter validation tool without output schema, the description is complete. It compensates for missing output schema by explicitly documenting the three return fields (validity, name, address) and covers all necessary invocation context.

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?

Despite 100% schema description coverage, the description adds valuable format reinforcement ('Format: country prefix + number') and multiple format examples that clarify expected input syntax beyond the schema's basic description.

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 provides a specific verb (validate), clear resource (European VAT number via VIES), and explicitly distinguishes from sibling tools tax.vatcomply.currencies and tax.vatcomply.rates by function (validation vs. currency data).

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 clear format guidance ('country prefix + number') with concrete examples (DE123456789) and scope (27 EU states + UK). While it doesn't explicitly state 'when not to use,' the functional distinction from sibling currency/rate tools is self-evident from the description.

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