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

Annotations already indicate readOnlyHint=true and idempotentHint=true, but the description adds valuable behavioral context beyond those: it discloses authentication/access (no API key), and transparently explains behavior for missing rules ('says so and links the rule reference rather than pretending the rule does not exist'). This is exemplary transparency.

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 concise and front-loaded: the first sentence establishes the core purpose, followed by coverage scope, edge-case behavior, and cost/auth. Every sentence carries essential information without fluff or repetition, earning a perfect score.

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 tool with no output schema, the description is remarkably complete. It not only states what the tool does but also describes the content of the explanation (what it requires, why, business term, fix, example), covers edge cases (unwritten rules), and provides access requirements. This gives the agent full context for invocation and expectation setting.

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?

The schema already provides full coverage of the single parameter rule_id with examples, so the baseline is 3. The description does add some context about the rule families (BR-*, BR-CO-*, etc.) and that some rules are not covered, which helps the agent understand what rule_id values are expected, but it doesn't add significant new semantics 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 uses the specific verb 'Explain' and clearly identifies the resource: 'one EN 16931 / XRechnung / Peppol BIS rule'. It further details what the explanation includes (requires, why, business term, fix, example), making it distinct from sibling tools like validate_invoice or check_vies_status. This leaves no ambiguity about the tool's function.

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 when to use it (to understand a rule) and notes coverage limitations ('most, not all' rules), providing helpful context for expectations. It also mentions 'FREE and needs no API key', which guides usage by distinguishing it as an accessible option. However, it does not explicitly list alternative tools or when-not-to-use conditions, so it stops short of full 5.

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