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

Annotations only declare readOnlyHint=true; the description adds real behavioral context by disclosing that line items and payments are included by default and that this maps to GET /api/invoices/{id}?include=invoiceitems,invoicepayments. It does not describe the response shape, but for a read-only single-fetch tool that is a minor omission.

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 compact sentences: the first front-loads the operation and its return scope, the second supplies the exact upstream endpoint. Nothing is wasted or redundant.

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 only two fully documented params, a read-only annotation, and no output schema, the description usefully compensates by naming what comes back (totals, line items, payments) and the default inclusion behavior. Only the non-naming of the list sibling and lack of any error/permission note leave a small 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?

Schema description coverage is 100%, so the schema already documents invoice_id and include_items (including its default true). The description restates the default-inclusion behavior but adds no syntax or edge-case detail beyond the schema, which is the baseline-3 case.

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 (Fetch) and resource (one invoice) and even enumerates what is returned: totals, line items, payments. The word 'one' implicitly contrasts with paymo_list_invoices, but the sibling is never named, which is the only thing keeping this from a 5.

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 by 'Fetch one invoice' — an agent can infer this is for retrieving a single invoice by id, but the description never states when to prefer this over paymo_list_invoices or how to obtain the required invoice_id. No exclusions or prerequisites are given.

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