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

Beyond the readOnlyHint, the description discloses the response shape (totals, line items, payments) and that line items and payments are expanded by default, plus the underlying Paymo endpoint. It does not cover auth requirements or rate limits, but the added behavioral context is genuine.

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 tight sentences with no waste; the core action and scope are front-loaded, and the endpoint detail is appended as reference rather than padding the lead.

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?

With no output schema, the description usefully states what is returned (totals, line items, payments) and the default expansion behavior, which is enough to call it correctly. Only permissions and error behavior are unaddressed, which is minor for a read-only tool.

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 coverage is 100% with only two parameters, so the schema already defines both. The description reinforces the include_items default and clarifies that the include flag bundles line items and payments together, which is a small semantic addition rather than new information.

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 uses a specific verb+resource, 'Fetch one invoice', which implicitly separates it from the sibling paymo_list_invoices because it emphasizes a single record by id. It never names an alternative sibling, so it falls just short of full explicit differentiation.

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 only implied: an agent can infer this is for retrieving a single known invoice rather than enumerating invoices, but there is no explicit when-to-use or when-not-to-use statement, and no mention of the list counterpart.

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