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

Annotations already state readOnlyHint=false, destructiveHint=true, and idempotentHint=true, so the description does not need to restate the safety profile. However, it adds a useful behavioral constraint by specifying that only a 'final invoice' should be marked paid. It stops short of explaining what happens to invoice state, whether the action can be reversed, or what side effects to expect beyond the annotation hints.

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 description is short and front-loaded with the action ('WRITE: mark a final invoice as paid'). Every sentence is relevant, though the trailing method name 'invoice.setpaid' is of marginal value to an agent selecting the tool.

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?

The tool is a simple mutation with full schema coverage and rich annotations, so the description need not explain return values or safety hints. It covers the core action and the key precondition that the invoice must be final. It could still say more about prerequisites or expected post-state, which keeps it from 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%, so the schema already defines invoice_id, paid_date, and payment_method clearly. The description mentions payment date and method, consistent with the schema, but adds no extra format details, constraints, or examples beyond what the parameter descriptions already provide.

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: 'mark a final invoice as paid, with the payment date and method.' This is clear and distinguishes the action from generic invoice updates. It does not explicitly differentiate from nearby siblings such as fastbill_complete_invoice, so it falls 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?

The description gives no guidance on when to use this tool versus alternatives or when not to use it. It mentions 'final invoice,' which implies a precondition, but does not explain prerequisites, exclusions, or the relationship to comparable tools like fastbill_complete_invoice.

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