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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds genuine value beyond that: it discloses the included payload (items, VAT rates, payments, supplier) and an important negative trait, 'Attachments are not returned', which an agent could not derive from annotations or schema.

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 the return contents front-loaded, followed by the exclusion and the raw endpoint. No filler; 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 single-parameter read whose safety profile is covered by annotations, the definition says what comes back and what does not. It stops short of covering failure behavior (e.g., an unknown expense_id) but is otherwise self-sufficient.

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 a single well-documented parameter, so the schema already carries the semantics (integer id, exclusiveMinimum 0, sourced from list_expenses). The description adds no format or constraint detail beyond it, making the baseline 3 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?

States a clear operation (fetch a single expense) and enumerates the returned payload components: items, VAT rates, payments, supplier. The singular framing distinguishes it from superfaktura_list_expenses, though the description never names that sibling the way a 5 would.

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 the per-id detail view versus list_expenses, but there is no explicit when-to-use statement or exclusion. The reference to superfaktura_list_expenses appears only in the schema's parameter description, not in the tool description itself.

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