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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds meaningful behavioral context beyond annotations: it is FREE, needs no API key, covers all 321 rule ids, and is cheaper than re-validating a document. No contradiction with annotations.

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 longer than minimal but every sentence adds information: output contents, coverage scope, usage trigger, and cost/access advantages. It is front-loaded with the core purpose and then provides practical guidance. Slight redundancy in the rule-id enumeration could be trimmed, but it is not wasteful.

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 single-parameter, read-only, idempotent tool with a fully documented schema, the description is complete. It explains what the tool returns, which rule ids it covers, when to use it, and its access constraints. No output schema exists, but the description adequately describes the explanation content.

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 coverage is 100% and the schema already documents rule_id with examples and forgiving case/separators. The description adds value by enumerating the accepted rule id families (BR-*, BR-CO-*, BR-<category>-*, BR-DE-*, PEPPOL-EN16931-*, ATW-*) and confirming that every reportable rule id is supported, which helps the agent choose valid parameter values.

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 and resource: 'Explain one EN 16931 / XRechnung / Peppol BIS rule in plain English' and details the output contents (requirement, rationale, business term, fix, example). It also distinguishes itself from sibling validation tools by covering 'every rule id this validator can report' and by noting it is read-only versus re-validation.

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 gives explicit usage guidance: 'Reach for it first whenever a rule id appears in an error, a log or a question, and to re-read an error you already have.' It also names an alternative to avoid ('validating again costs a document, this does not'), though it does not explicitly enumerate when not to use the tool or compare it to diagnose_invoice.

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