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

Annotations already declare readOnlyHint=true, so safety is covered. The description goes beyond that by disclosing a non-obvious data trait: a draft invoice appears with an empty number. It also names the underlying endpoint. It still says nothing about pagination or result size limits, keeping it short of a 5.

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 short sentences plus an endpoint reference, with the primary behavior (whether it was invoiced) front-loaded. Every sentence carries information and nothing is padded.

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 simple two-parameter read tool with full schema coverage and annotations, the description is nearly sufficient, and it partially covers the return shape (invoices with series/number, empty number for drafts) despite the absence of an output schema. It is only missing guidance on empty results or multiple invoices returned.

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 both parameters (series_name, number) are already documented as the proforma's identifiers. The description's mention of 'series, number' refers to the returned invoices rather than adding input semantics, so it neither clarifies nor contradicts the schema. Baseline 3 applies.

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 names a specific resource (invoices generated from a proforma) and its scope (whether invoiced plus the series/number pairs), which clearly separates it from creation/cancellation siblings like smartbill_create_invoice_from_estimate. It is not an explicit verb+resource statement, but the intent is unambiguous.

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?

Usage is implied: an agent reads this as the way to check whether a proforma has already been invoiced. However, no alternative is named (e.g., a general invoice lookup) and no conditions or exclusions are stated, so the agent must infer when this beats other lookups.

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