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

Annotations only supply readOnlyHint, so the description carries the rest of the behavioral load; it contributes a genuinely useful fact in 'No API key needed' (an auth/access requirement) and describes the shape of the explanation returned. It still omits rate limits or whether the rule corpus can go stale, keeping it short of a 5.

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?

One sentence that front-loads the action and input, then a colon list of the value delivered, plus a short trailing availability note. No filler and nothing to trim.

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 read-only reference lookup with a single required id and no output schema, the description must cover input format, returned content, and access requirements — it does all three. Nothing essential to calling 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?

With 100% schema coverage the baseline is 3, but the description adds real semantic breadth the schema lacks: a national CIUS rule is a valid id form and several naming families (BR-CO, BR-DE, PEPPOL-EN16931) are enumerated. It stops short of saying what happens for an unrecognized id.

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?

States a specific verb and resource ('Explain an e-invoicing business rule by its id') and pins down the input space with concrete ids (BR-CO-15, PEPPOL-EN16931-R001, national CIUS rules), which separates it cleanly from explain_error. It also enumerates the returned content (message, level, rule sets, failure reason, remedy), so an agent knows exactly what it gets.

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 implies usage ('why invoices fail it and how to fix them') but never states when to choose this over explain_error or the other siblings, nor any preconditions beyond the API-key note. Guidance is inferable but not explicit.

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