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

With no annotations at all, the description carries the full burden and does so well: it discloses that the HTML is sanitized (no scripts, no external resource loading), that attachments come back separately, that it does not consume the validation quota but has its own daily limit, and that nothing is persisted. These are exactly the behavioral facts an agent cannot infer from the 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-loads the core purpose in the first clause and then adds one sentence per distinct fact (return shape, sanitization, check flag, quota, no storage), so every sentence earns its place. Slightly dense with stacked parentheticals, which slows parsing marginally but adds no filler.

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 four-parameter tool with no output schema and no annotations, the description covers what the tool returns (HTML plus separated attachments), how output is constrained (sanitized rendering), side effects (nothing stored) and cost (own daily limit, no quota consumption). Nothing an agent needs before calling it 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 description coverage is 100%, so the baseline is 3, but the description adds real meaning on top: it explains what check=true actually yields (a short validation summary) and ties file_type values to concrete formats (pdf for ZUGFeRD/Factur-X, xml for XRechnung/CII). Only the language parameter's effect on the rendered output is left to 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?

Names a specific verb ('zeigt ... an') and resource (E-Rechnung) and enumerates the accepted formats (XRechnung/CII-XML, ZUGFeRD/Factur-X-PDF) plus the return artifacts (HTML rendering and separate attachments). This is clearly a render/view operation, which distinguishes it from validate_invoice, check_consistency and extract_xml in the sibling list.

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?

Gives a conditional usage hint for one parameter ('Mit check=true kommt die Kurzfassung der Prüfung dazu'), which tells the agent when to opt into validation output. However, it never states when to choose this tool over validate_invoice or extract_xml, nor which input types are out of scope, so the routing guidance is implied rather than explicit.

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