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

Annotations already declare readOnlyHint=false and openWorldHint=true; the description goes further by disclosing the per-call confirmation default (and that it can be disabled), the API key scope required, the external side effect ('Reaches other people'), and the returned series/voucher number. It stops short of describing failure modes or idempotency, so it is strong but not exhaustive.

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?

Front-loaded with the core action and constraints, and every sentence carries rules the caller needs. There is mild redundancy between the opening sentence and the second ('Books one voucher with a transaction date, a description, a voucher series and rows') which repeats fields already enumerated.

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 four-required-parameter mutation with no output schema, the description covers validation rules, the return value, the confirmation/authorization model, and the external effect. Remaining gaps (error shape on rejection, retry/idempotency) are minor.

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 all four parameters. The description restates the row structure (account plus either debit or credit) and the balanced-debits constraint, adding conceptual meaning but no syntax or format detail beyond the schema, which matches the baseline-3 expectation.

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 (voucher) plus the composition of the payload: series, date, description, and rows with account/debit/credit. Distinguishable from fortnox_list_vouchers (read) and fortnox_bookkeep_invoice (invoice booking) without opening either 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?

Gives preconditions for success (debits must equal credits, date inside an existing financial year, accounts must exist and be active) and a confirmation policy, which is useful context. However it never names when to use this instead of sibling booking tools such as fortnox_bookkeep_invoice, so alternative selection is left to inference.

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