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

Beyond the readOnlyHint=false/openWorldHint=true annotations, it discloses high-value traits: the invoice is a legal document that cannot be edited or deleted through the API, send_email causes an external email, each call is confirmed by default (switchable off), and the call reaches other people so user confirmation is needed. That is exactly the behavioral context annotations cannot carry.

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?

The core action and optional email are front-loaded in the first sentence, then requirements and behaviors follow. It is dense and runs several sentences, but each one carries distinct information (numbering, VAT, discount, email, immutability, confirmation), so little is wasted.

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 10-parameter write tool with no output schema, the description covers requirements, key field behaviors, immutability, auth needs and human-confirmation policy, leaving little an agent must infer. It could say more about what a successful call returns, but without an output schema that gap is minor.

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 genuine meaning beyond the schema: the 'fixed discount or a percentage, not both' exclusivity rule and the invoice_number 'leave empty to get the next number' behavior are constraints the schema states only implicitly. It also names the required set, reinforcing 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 specific verb and resource ('Create an invoice for a relation from one or more lines') plus the scope ('Creates one invoice in the books'), so an agent immediately knows the operation. It references sibling-ID sources (list_relations, list_invoice_templates) but does not explicitly differentiate this from other sibling write tools, so it stops just short of a 5.

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 useful conditional guidance ('Pass send_email to mail it... otherwise it only exists in e-Boekhouden'; 'Leave invoice_number empty...'), which implies usage. However it never states when to choose this over an alternative sibling such as create_mutation or get_invoice, and there are no explicit exclusions, so guidance remains implied rather than prescriptive.

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