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

Beyond the readOnly/openWorld annotations, it discloses real behavioral traits: duplicates are removed, unknown codes cause a hard failure that names the offending codes, and the call is free/unmetered. These are operationally relevant facts an agent cannot infer from annotations or schema.

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?

Front-loaded with purpose, then scope, then input/output contract, then cost. Every line earns its place with no filler or repetition.

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?

Covers the input contract, the failure mode for unknown codes, the return shape, and the credit/no-auth situation. Even though an output schema exists, the extra context on error behavior and cost completes the picture.

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 already 100%, so the baseline is 3; the description nevertheless adds the dedup behavior for `codes` and restates the `lang` default and accepted values in prose. The added semantics are real but modest.

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 (explain) and resource (rule codes) and scopes it to the exact code families emitted by validate_invoice. An agent can distinguish it from the sibling validate_invoice, which produces the codes this tool consumes.

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?

Establishes clear context: it explains codes found by validate_invoice and hints (EN 16931, XRechnung, ZUGFeRD, IW-*), and notes it works without an API key. It does not state explicit exclusions or name alternative tools, so it falls short of a 5.

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