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

Annotations already declare readOnly, idempotent, non-destructive, closed-world, so the safety profile is covered. The description adds real value beyond that: the auth/connection requirement for creators and an enumeration of what comes back (Lines, amount, status, due date, period, receipt, pay link). It doesn't say what happens when the number doesn't exist, which keeps it from 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 with no filler, audience constraint front-loaded before the operation. The second sentence is slightly fragmentary but every clause carries information.

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 single-record read with no output schema, the description covers scope, precondition, and the shape of the response. Only the error path (unknown invoice number) is unaddressed, which is a minor gap given the richness elsewhere.

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% and the single parameter is already documented with the same INV-0042 example, so the description adds no new meaning beyond the schema. Baseline 3 applies for a one-param, fully documented tool.

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 resource and scope — a single invoice fetched by its number, with the identifier format (INV-0042) spelled out. The singular 'One invoice' implicitly separates it from list_invoices, but no sibling is named explicitly, so it stops short of 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?

'Creators only, on their own Piku account (asks them to connect)' is a concrete eligibility and precondition statement, which is more than most read tools offer. There is no explicit when-not or named alternative, but the single-vs-list distinction is clear from the phrasing.

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