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

Goes well beyond the readOnlyHint/openWorldHint annotations: it discloses that invowerk self-validates and never returns an invalid document, that amounts are written as given and never computed or corrected, the 4 MB input cap, the error behavior, and the cost (5 credits per call). This is exactly the operational context an agent needs before invoking.

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 the purpose, then input contract, then return/behavior, then cost. Well organized and free of filler, though the enumerated field list is slightly long for a schema that already documents the shape.

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?

An output schema exists so return values need not be restated, yet the description still names the returned keys and, more importantly, covers limits, validation guarantees, error semantics and cost. Nothing an agent needs to call this correctly is missing.

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 there is only one parameter, so the schema already carries most semantics. The description still adds real value by naming defaults ('ubl', 'en16931'), the conditional requirement that flavor 'xrechnung' needs buyer_reference (Leitweg-ID BT-10), the date format, and the main BT keys.

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+resource+output format: 'Create a valid EN 16931 e-invoice (UBL or CII XML) from JSON.' This distinguishes it cleanly from siblings render_invoice, validate_invoice and parse_invoice, whose names imply different operations.

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 usage through its input contract (JSON keyed on EN 16931 Business Terms, 4 MB cap) but never states when to choose this tool over convert_invoice, validate_invoice or render_invoice. No explicit when-not or routing guidance is offered.

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