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

Annotations cover safety (readOnlyHint=false, destructiveHint=false, openWorldHint=true), and the description adds substantial context beyond them: booking is irreversible (invoice can no longer be changed), the return value (voucher number), failure modes, per-call confirmation behavior, and an API-key/access prerequisite. No contradiction with the annotations.

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 immutability point is stated twice ('A booked invoice can no longer be changed' and 'After booking Fortnox does not allow changes'), which is mildly redundant, but the important scoping and irreversibility facts are front-loaded and the whole is efficient for a mutation tool.

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 one-parameter irreversible mutation with no output schema, the description covers everything an agent needs: effect, irreversibility, the prerequisite get_invoice check, failure conditions, confirmation behavior, and auth requirements. Nothing material is missing.

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% and there is only one parameter, so the schema already documents invoice_number fully. The description adds no format or source detail beyond what the schema states, 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?

States a specific verb ('Book') and resource ('saved invoice'), and distinguishes itself from siblings like fortnox_get_invoice and fortnox_create_invoice. It even clarifies scope ('a saved customer invoice') and analogizes to the UI button ('as the Bokför button does'), so an agent can tell what operation this performs without opening the schema.

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?

Explicitly routes the agent to check totals first with get_invoice, and states the failure conditions (already booked, missing financial year) that gate a successful call. This is clear when-to-use and when-not guidance naming the relevant alternative.

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