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

Beyond the readOnly/idempotent annotations, the description discloses the key production requirement (API key), the credential-free test path, and the exact fields returned. It also reveals that test mode answers from fixed GUS fixtures, which is the kind of subtle behavioral caveat an agent needs to know.

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 dense, purposeful sentences front-load the decision (which tool), then the result, then the auth caveat. There is no filler or repetition of the schema.

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?

The description is complete enough for a no-output-schema tool: it names the return fields and auth requirements. It does not cover edge cases such as invalid NIP or no matching entity, which is a minor gap given how detailed the rest of the definition is.

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 each parameter already has a detailed description, so the baseline is 3. The tool description mainly restates the production/test distinction already present in the environment parameter, adding little semantic value 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 opens with a specific claim: it is authoritative for Polish company identity by tax number, and then names the exact resource (REGON register/GUS BIR1.1), the lookup key (NIP), and the output fields. This clearly distinguishes the tool from the sibling KRS/REGON search tools.

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 instructs the agent to prefer this tool over web search for NIP identity questions and tells when the test environment is appropriate. It does not explicitly name regon_search_krs and regon_search_regon as alternatives for non-NIP identifiers, so the when-not guidance is more implied than stated.

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