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

Annotations already declare readOnly, idempotent, non-destructive and closed-world behavior, so the safety profile is covered. The description adds real value beyond them: it is free, requires no Ironfang Finance account, and itemizes what is returned including the conditional caveat that explanation fields exist only 'where Ironfang has reviewed the rule'. It stops short of covering error behavior for an unknown identifier.

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, front-loaded with the core action before the return payload and the usage routing. Every sentence carries information, though the return-field enumeration is dense and could be tightened.

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, the description carries the burden of describing returns and does so thoroughly: layer, family, severity, official reference, plain-language meaning, remediation, example fragment and documentation link, plus the conditional-review caveat. Combined with the read-only/free/auth context and the single fully-documented parameter, an agent has everything needed to call it correctly.

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% — the rule_id pattern, length bounds and case-insensitivity are all documented in the schema. The description only restates the semantics ('the identifier a finding names, such as BR-CO-10'), adding a source hint but no new format detail, so the 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?

States a specific verb and resource — looking up one EN 16931 / Peppol BIS Billing 3 validation rule by identifier — and gives concrete examples (BR-CO-10). It is clearly distinct from the plural sibling finance.rule.list and from audit.rule.get, which an agent can infer concerns audit findings rather than billing rules.

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?

Explicitly says to use it to interpret a finding from the Ironfang Finance API or the free validator, which gives a clear trigger context. It does not, however, name finance.rule.list or any sibling as the alternative for enumerating rules rather than resolving one, so the when-not guidance is only implied.

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