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

With no annotations, the description carries the full burden and does well: it discloses determinism, that there is no network call, that it will report whether check digits were actually verified versus format-only, and the cost ($0.005 per call via x402/USDC). It omits error/edge behavior (e.g., unknown country prefix, non-EU input), which keeps it short of a 5.

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?

Five compact sentences, all earning their place: what it does, what it guarantees, its determinism, its intended use, and its pricing. The core capability is front-loaded and nothing is redundant.

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, offline validator with no output schema and no annotations, the description covers purpose, behavior, determinism, and cost, and even hints at the return content (whether check digits were verified). Only the exact response shape and error cases remain unspecified, which is acceptable given the tool's simplicity.

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% and the single parameter is fully documented in the schema itself, including tolerant handling of prefix and spacing. The description adds only that a country prefix is central to validation, so 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 (validates an EU VAT number) and enumerates exactly what is checked: country prefix, country-specific format, and the check-digit algorithm where it exists. This clearly distinguishes it from adjacent utilities like util_iban or util_phone in the sibling list.

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 a context of use ("cleaning a list of companies one at a time") and notes it is offline vs. a network-backed check, which is useful implicit guidance. However, it never states when to prefer this over an alternative or when it is inappropriate (e.g., a company whose VAT must be verified live against VIES), leaving the boundary implied rather than explicit.

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