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

Annotations already cover read-only, idempotent, and non-destructive behavior, so the safety profile is known. The description adds useful behavioral context beyond annotations: sorting ('worst first'), computed fields ('days overdue and the amount'), and currency grouping/totals. It does not detail as_of edge-case handling, but that is minor.

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 behavior is front-loaded in a single tight sentence, and the sibling routing is clear. The standalone 'Free.' contributes little to an agent's selection or invocation decision, preventing a perfect score.

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 optional-parameter read-only report, the description is nearly complete: it explains what is returned, how results are sorted and grouped, and when to prefer invoice_list. It does not enumerate exact response fields, but the text sufficiently describes the report content given no output schema exists.

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?

There is only one optional parameter and the schema already documents its format ('YYYY-MM-DD') and default ('defaults to today'), giving 100% schema description coverage. The description adds no additional parameter-level meaning, so the baseline of 3 is appropriate.

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 resource ('invoices'), the exact filter ('not paid in full whose due date has passed'), and the output shape ('days overdue and the amount, then a total per currency'). It also distinguishes itself from invoice_list by saying that invoice_list covers everything still open.

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 explicitly routes the agent to the sibling tool when appropriate: 'Use invoice_list for everything still open.' This is a clear when-not/alternative statement, and the first sentence makes the intended use case obvious.

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