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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new scope limits: KSeF is XSD-only, Factur-X validates XML only, and there is no authority or network acceptance check. It stops short of describing the result shape (errors vs warnings), which max_findings hints at but never explains.

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 short sentences, front-loaded with the core action and supported standards, with zero filler. The telegraphic phrasing ('RO or HR package') is dense but readable; no sentence is wasted.

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 explaining what comes back, yet it never says that findings, errors and warnings are returned or how max_findings and include_guides shape the response. For a validator whose whole value is its output, that is a meaningful gap despite otherwise good scope disclosure.

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%, so the baseline is 3; the schema already documents every parameter including the standard enum values and the 10 MiB XML limit. The description's 'KSeF is XSD-only; Factur-X checks XML only' largely restates the standard field's own schema text rather than adding new meaning.

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 ('Check') and resource ('invoice XML') and enumerates the six fixed rule packages it covers. The phrase 'fixed ... package' implicitly separates it from the sibling schema-driven tools validate_xml and validate_xml_against_xsd, though it never names them.

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 closing sentence 'No authority or network acceptance check' establishes a clear exclusion, and 'KSeF is XSD-only' scopes one standard. However, there is no explicit guidance on when to pick this over validate_xml or validate_xml_against_xsd, so the agent must infer the choice from the word 'fixed'.

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