Validate a company identifier (no network call)

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

Beyond the annotations (readOnly, idempotent, non-destructive), the description discloses meaningful behavior: no network call, checksum validation for Norway vs shape-only validation for the UK, normalisation rules, the guarantee that it never raises for malformed identifiers, and the unsupported_country error contract. No contradiction with annotations exists.

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 description is structured logically: purpose first, then per-country behavior, then usage recommendation, then return/error behavior. It is a bit verbose in places — such as repeating the 'cheap' idea — but every major sentence contributes useful operational detail.

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 validation-only tool with an output schema, the description covers all necessary operational aspects: what it validates, how each supported country behaves, the error contract, and when to use it versus lookup_company. Nothing an agent needs to invoke it correctly is missing.

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?

The schema already describes both parameters well (100% coverage), so the baseline is 3. The description adds extra semantic value by explaining the validation algorithms, the zero-padding and upper-casing behavior, and the meaning of a GB valid:true. This goes beyond what the schema provides, though not a huge margin.

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 specific verb and resource: 'Check whether a national company identifier is well-formed — no network call.' It clearly separates this from lookup_company by framing it as a cheap pre-check, and from search_company by focusing on validation rather than search. The scope is unambiguous.

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?

It explicitly tells the agent when to use it: 'Use it on user input or a spreadsheet column before spending a real lookup_company call, since it is instant and free.' It also routes unsupported countries to list_countries. This gives clear contextual guidance for choosing the tool.

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