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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so safety is covered. The description adds genuinely useful information beyond that by disclosing the depth of validation performed (structure, EN 16931 rule conformance, PDF/A-3 attachment checks), which tells the agent what a positive or negative result means.

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?

A single front-loaded sentence with zero filler: verb, resource, and scope in order of importance. It is arguably thinner than a multi-step validation tool warrants, but nothing is redundant or padded.

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 need not explain return values, and annotations plus full schema coverage handle safety and inputs, so the remaining burden is small. The one real gap is task routing – nothing tells the agent when to prefer this over extract_invoice – which keeps it short of a 5.

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% – both upload_id and pdf_base64 are documented in the schema, including the 'exactly one of the two' constraint and the 25 MB limit. The description adds no parameter detail of its own, so the baseline of 3 for schema-driven parameters applies.

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?

The description names a specific verb ('Prüft') and a precise resource ('ZUGFeRD-/Factur-X-PDF') and enumerates the validation scope (Struktur, EN-16931-Regeln, PDF/A-3-Anhänge), so an agent immediately knows this is a validator, not a generator or extractor. It does not, however, distinguish itself from siblings like extract_invoice or create_zugferd by naming them or contrasting scope.

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?

There is no when-to-use guidance, no prerequisites, and no mention of alternatives. The only routing hint in the whole definition lives in the schema for pdf_base64 (call request_upload_link first for chat clients), not in the description itself, so an agent gets no help choosing between this and extract_invoice or create_zugferd.

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