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

Annotations already declare readOnly/closed-world/idempotent/non-destructive, so the bar is lower; the description still adds real substance by stating the check is offline and limited to format and checksum, which tells the agent no network call or registry verification occurs. It omits what the failure mode looks like (boolean vs. error vs. reason codes), leaving one behavioral gap.

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 short sentences, zero filler, with the capability stated first and the important limitation immediately after. The text is front-loaded and entirely made of decision-relevant facts.

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 2-parameter read-only validator with full schema coverage, the description covers scope, method, and the key limitation. The only shortfall is that with no output schema present, it does not hint at the response shape (valid/invalid flag, error detail), which an agent would need to plan around.

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%, including the 'with or without country prefix' example and the conditional role of 'country', so the schema already does the work and the baseline of 3 applies. The description adds nothing parameter-specific beyond that, which is acceptable here.

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?

Specific verb ('Validate') plus precise resource ('VAT number') with explicit jurisdictional scope ('EU/EEA plus GB, NO, CH and more') and the exact check performed ('format and checksum'). It also carves itself away from any VIES lookup, so an agent can distinguish it from both siblings and non-sibling tax-check tools at a glance.

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 clause 'Does not check VIES registration status' functions as an explicit when-not-to-use rule, and 'offline' signals that it is a local computation rather than a network lookup. It stops short of naming a concrete alternative for registration-status checks, so the routing advice is implied rather than fully closed.

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