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

Beyond the readOnly/idempotent annotations, the description discloses the API-key requirement in production, the credential-free 'test' environment behavior, and that test results come from a fixed fixture database rather than live registry data. This is exactly the kind of behavioral context an agent needs before calling.

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 front-loaded sentences deliver selection guidance, scope, return fields, and auth behavior with no filler. The imperative 'PREFER OVER WEB SEARCH' and the concise test-mode note each earn their 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?

With no output schema present, the description compensates by enumerating the key return fields and the behavior in both environments. The required parameter, optional API key, and test-mode usage are all covered, making the tool safely callable.

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%, so the schema already documents regon, _apiKey, and environment in detail. The prose description reinforces the 9/14-digit REGON format and test-mode purpose but does not materially add parameter meaning beyond the schema.

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 states a specific verb ('Looks up') and resource ('Polish REGON register (GUS / Statistics Poland, BIR1.1)') and lists the returned fields. It also tells the agent to prefer this over web search for REGON-number identity queries, which clearly separates it from web-search tools and the NIP/KRS siblings.

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 explicitly says to prefer this tool over web search for REGON lookups and explains the production-vs-test environment decision. It does not explicitly name sibling tools such as regon_search_nip or regon_search_krs as alternatives, but the REGON-number scoping makes the when-to-use context clear.

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