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

Annotations already declare readOnly/idempotent/openWorld/safe, so the bar is lower, yet the description adds real semantics: the three-valued vat.status and specifically that 'unavailable' is not 'invalid', and that peppol.listed:false means not-found-by-VAT in an opt-in index rather than proof of non-membership. Those are exactly the non-obvious behavioral traits an agent needs to interpret output correctly, plus a pointer to `sources` and a non-advice disclaimer.

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 value proposition is front-loaded, then each following sentence handles a distinct interpretation pitfall (status semantics, Peppol false-negative, sources, disclaimer) with no filler. Minor formatting quirks aside, every sentence earns its place.

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-value structure need not be described, and the description nonetheless covers the ambiguous output values and their limits. Combined with full schema coverage and annotations, an agent has everything needed to call and correctly interpret this tool.

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 both parameters already document prefix/format handling and the GR/EL, XI country-code quirks, so the schema carries the parameter burden. The description adds no additional parameter meaning beyond naming that the VAT number drives both lookups, matching the baseline-3 expectation.

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?

The description opens with a concrete verb+resource framing ('does this EU supplier exist and can I send it an e-invoice?') and names the two underlying operations it performs (validate_vat/VIES and Peppol Directory search by VAT number). This distinguishes it clearly from the siblings validate_vat and peppol_lookup, which each do only one of those halves.

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 'One call for ...' framing implicitly routes the agent here when it wants both checks at once rather than invoking validate_vat and peppol_lookup separately, and it explains the Peppol caveat that selects interpretation of the result. It does not, however, explicitly state when NOT to use it or name the alternatives as such.

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