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

Annotations already declare readOnlyHint=false and destructiveHint=true, so the safety profile is covered. The description adds real value beyond that by disclosing a server-side validation rule (status must be DRAFT) and clarifying it does not finalize/send the document. It stops short of covering idempotency behavior, which is notable given idempotentHint=false — repeated calls presumably append duplicate items, and that is not stated.

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?

Two sentences, front-loaded with an uppercase WRITE marker and the core constraint. Every clause carries routing or behavioral information; there is 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 12-parameter mutation tool the description plus a fully documented schema is nearly sufficient, and the draft-only constraint is the key missing piece it supplies. With no output schema, however, it never says what comes back (e.g. the created item id), which would help an agent chain subsequent calls.

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 all 12 parameters — including the article_id fallback semantics for title/quantity/unit_price — are already documented in the schema. The description adds no parameter-level meaning beyond it, which is the expected baseline when the schema carries the load.

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 states a specific verb and resource ('add one line item to an invoice') and immediately scopes it to draft invoices only. An agent can distinguish this from billomat_list_invoice_items or billomat_create_draft_invoice without opening any 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?

It gives an explicit precondition ('an invoice that is still a DRAFT') and an explicit when-not ('Billomat refuses it for any other status'), plus a negative scope ('Does not complete or send the invoice'). Nothing about when to call it is left to inference.

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