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

With no annotations, the description carries the full burden and does much of it: batch cap of 100, VIES dependency, per-item result granularity, transient-failure retry semantics, GB staying unavailable until HMRC access, and a billing note ('not charged'). It stops short of stating rate limits, authentication needs, or error/partial-failure behavior.

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 core capability and the batch limit are front-loaded in the first clause, with exceptions (retry, GB, billing) packed into a compact follow-up. Dense but every sentence carries information; the stacked parenthetical lists are slightly heavy but not wasteful.

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?

An output schema exists, so return values need not be explained, and the behavioral caveats are solid. What remains missing is material for a billed batch tool: the meaning of the 'requester' parameter, VAT string format, and what happens on partial or total failure.

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 0%, so the description must compensate for both parameters. It partially does for 'vats' by stating the 100-item limit, but never explains the format expected per entry (country prefix, spacing) and says nothing at all about 'requester', which carries a default value an agent cannot interpret.

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 ('Validate up to 100 EU VAT numbers in one call via VIES'), including the batch size cap and the backing service. It also explicitly relates itself to the sibling verify_vat by promising the same per-item result, so an agent can tell them apart immediately.

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?

Names concrete scenarios (supplier/customer list cleanup, onboarding, invoice runs) and gives retry guidance for member_state_unavailable items plus the GB/HMRC exception. It does not explicitly say 'use verify_vat instead for a single number', leaving that inference to the reader, so it falls short of full when/when-not coverage.

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