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

Goes well beyond the annotations (readOnlyHint=false, openWorldHint=true, idempotentHint=false): it discloses the paid workflow, the 2,49–4,99 € price, that sending occurs only after payment, that confirmation/delivery report/invoice arrive by email, and that upload_id is single-use. It also tells the agent how to interpret the outcome (sent, scheduled, or still unpaid), which the annotations cannot convey.

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 what the tool does and the payment gate, then moves to the document-selection rules. Dense and mostly waste-free, though the parenthetical caveats and repeated payment/email details make it longer than strictly necessary.

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 9-parameter, nested, no-output-schema mutation tool this covers the essential behavior: cost, payment gating, required confirmation, document sourcing, and result semantics. The one gap is that it never points to get_fax_status for post-payment follow-up, leaving the 'unpaid/scheduled' path slightly open-ended.

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, but the description adds a constraint the schema cannot express: exactly one document input must be chosen ('DOKUMENT (genau eines)'), plus scenario-based selection guidance and the 700 KB cap for base64 with a recommended alternative. This is genuinely useful meaning beyond 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?

States a precise verb and resource (prepares/sends a real fax via SofortFax) plus the key scope detail that it returns a Stripe payment link and that the fax is only sent after payment. It also names create_upload_link as the source for one of the document inputs, so an agent can place it in the workflow without opening the schema.

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 concrete procedural guidance ('Vorher dem Nutzer Faxnummer, Dokument und ggf. Sendezeitpunkt bestätigen lassen') and routes the agent to the correct document source per scenario (upload_id, file, document_url, base64) with an explicit 'Besser create_upload_link' preference. It does not, however, state when NOT to use this tool or how it relates to get_fax_status for later lookups.

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