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

Beyond the annotations (readOnly=false, etc.), the description discloses meaningful behaviors: the invoice number is next and never-reused, unit prices are in major units, lines are rounded then summed, and only one currency is allowed per invoice. It also mentions the free-tier limit of 3 per month, which is useful operational context.

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 only four sentences long and every sentence earns its place: the first defines the action and return value, the second clarifies unit and rounding semantics, the third states a key constraint, and the fourth gives the free-tier quota. Information is front-loaded and 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?

The input schema is fully self-describing, and the description adds the key behavioral facts an agent needs: the returned record includes a never-reused number, unit prices are major units, and rounding/currency rules. It doesn't mention sibling alternatives or idempotency, but for a write tool with a rich schema this is a minor gap.

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?

The schema description coverage is 100%, so the baseline is 3, but the tool description adds global semantics not obvious from the schema: unit_price is in MAJOR units, lines are rounded individually before summing, and one currency per invoice is enforced. These invariants clarify how the parameters interact.

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 ('Create') and resource ('invoice') and defines the input as line items, making the core purpose unmistakable. It doesn't explicitly distinguish itself from the sibling 'invoice_from_hours', but 'from line items' implies that alternative path.

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 siblings such as invoice_from_hours or invoice_mark_paid. No when-to-use, when-not-to-use, or alternative tool is mentioned, so an agent must infer usage from the tool name and parameter names.

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