validate_vat_id

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

The description discloses key behavioral traits: automatic failover across two sources, cached fallback behavior, and return characteristics (structured company data with parsed address and source freshness indicator). With no annotations provided, the description carries the full burden and does disclose meaningful behavioral context about how the tool operates and what it returns.

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?

Three sentences, all information-dense and meaningful. The description front-loads the core purpose before behavioral details, and every sentence earns its place — multi-source validation, failover, return characteristics, and a reliability note.

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 tool with 2 params, 100% schema coverage, and no output schema, the description covers the core value proposition well. It explains the multi-source strategy, failover, caching, and return format. It could slightly benefit from noting what happens on total validation failure (both sources down), but given the simplicity of the interface, this is adequately complete.

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% — both parameters are documented in the schema. The description adds some context about the force_live parameter by mentioning 'cached fallback' and the description's 'Works when VIES is down' hints at the failover. However, the description doesn't add substantial new meaning beyond the schema's own param descriptions, 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?

The description clearly states the verb ('Validate') + resource ('EU VAT ID') and describes its specific behavior: checking multiple sources (VIES + BZSt) with automatic failover and cached fallback. This clearly distinguishes it from the sibling tool 'check_api_status' — this tool validates actual VAT IDs while the sibling checks API status.

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 description states when it works ('Works when VIES is down') and implies its multi-source advantage, which signals a use case where single-source tools would fail. However, it doesn't explicitly state when NOT to use it or name alternatives beyond the implied sibling distinction. The 'force_live' param also implies there's a cache-fallback mode that's the default.

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