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

The description discloses that totals are recomputed automatically and that the operation is blocked on paid/void invoices. These are behavioral traits not covered by the annotations (which only indicate non-readonly, non-destructive, non-idempotent). No contradiction with annotations is present.

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 two concise sentences with no redundancy. It front-loads the main purpose and then adds critical constraints and requirements. Every sentence earns its place.

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 description covers the essential usage context (when it works, automatic recalculation, blocking conditions) and notes invoice_id as required. However, it omits details about unit_price and description parameters, which are not explained in the schema either. Given the lack of an output schema, the description provides enough to understand the operation but leaves parameter semantics unclear for an agent.

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 only 25% (only quantity has a description). The description only mentions that invoice_id is required, which is already in the schema. It doesn't explain the meaning or expected format of unit_price or description, leaving the agent without additional semantic guidance for the core parameters. It fails to compensate for the low schema coverage.

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 clearly states the action (add a billable line item), the target resource (invoice), and specific constraints (draft/sent, blocked on paid/void). It distinguishes itself from sibling tools like create_invoice or update_invoice by specifying exactly what it does and its 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?

The description indicates when to use it (adding a line item to a draft or sent invoice) and states a limitation (blocked once paid/void). It doesn't explicitly name alternatives or when-not-to-use scenarios, but the context and constraints make the intended usage clear enough.

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