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

The description goes well beyond the read-only annotations by disclosing the conditional dependency on xmllint and the vendored XSD, enumerating the business-rule checks, stating that errors are returned in Polish, and clarifying no KSeF submission occurs. The no-API-key note also removes an authentication uncertainty the annotations do not cover.

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 definition is front-loaded with the core action, packs the scope, dependencies, error language and boundaries into a tight paragraph, and wastes no sentence on restating the name.

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 an output schema present, the description does not need to explain return values, and it still covers the key operational caveat (xmllint/XSD may be unavailable) plus the error language. Together with the annotations, an agent has everything needed to decide whether and how to call it.

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 documents the single 'xml' parameter at 100% coverage, and the description adds only that it is a FA(3) invoice. That aligns with the schema without adding syntax or format guidance beyond it.

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 (validate) and a precise resource (KSeF FA(3) invoice XML), and scopes it to schema plus named business rules. This clearly distinguishes it from siblings such as generate_fa3_xml or summarize_ksef_invoice.

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?

Makes the boundary explicit with 'Does not send the invoice to KSeF' and notes it is callable without an API key, which is useful context for an agent. It does not name sibling alternatives or state when to call it versus generate_fa3_xml, so it stops short of full routing guidance.

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