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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description adds genuinely useful non-annotation context: it costs 1 credit per request (including failures) and requires the VAT Validation API key, which matters for how an agent should budget calls.

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?

Front-loads the purpose and return value in the first sentence, then tacks on cost and key requirements. Mostly efficient; the fragment 'VAT Validation API.' is redundant filler that could be dropped.

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 one-parameter read-only lookup with no output schema, the description covers purpose, return fields, cost, and auth requirements. It stops short of noting any error/edge-case behavior (e.g. unrecognized country prefix), but nothing critical is missing for correct invocation.

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 description coverage is 100% for the single parameter, and the schema already documents the country-prefix format and example. The description adds no syntax or format detail beyond it, so the baseline 3 applies.

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 and resource ('Check whether a VAT number is valid') plus the value returned (registered company name and address), which separates it from siblings like abstractapi_calculate_vat and abstractapi_list_vat_rates. An agent can tell what it does without opening the schema.

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?

Usage is implied by the verb 'validate' and the return description, giving clear context for what the tool is for. However, it never explicitly says when to prefer this over siblings such as calculate_vat or list_vat_rates, or what prerequisites/invalid-input behavior look like.

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