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

Adds substantial behavior beyond annotations: currency cannot change once something is booked, chart is first-call-only, start_date defaults to fiscal-year start, opening balances post to account 3900, post_invoices is on by default, payment_accounts has defaults, rates override ECB rates, and the call costs $0.001 unless it fails. Annotations already mark it as a non-readonly, non-idempotent mutation, and the description supplies the operational consequences.

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-loads the purpose and first-call requirement, and nearly every clause carries useful setup information. It is dense with semicolon-separated accounting details and could be more scannable, but there is no filler.

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 seven-parameter setup mutation with no output schema, the description covers initialization, defaults, accounting consequences, cost, and failure behavior. Return values need not be explained because no output schema exists.

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?

Although schema coverage is already 86%, the description adds real meaning beyond the schema: chart contents, currency as immutable base, start_date default, post_invoices default and effect, payment_accounts defaults, and the purpose of rates. fiscal_year_start is left to the schema, which documents it adequately.

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?

Explicitly states that the tool starts double-entry books or changes their settings, separating it from sibling books operations such as books_get, books_list_entries, and books_save_accounts. The verb, resource, and initialization scope are specific.

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?

Explains the first-call behavior: currency is required and the chart is created only then. It also gives default behavior for start_date, post_invoices, and payment_accounts, which tells an agent when this setup path applies. It does not explicitly name alternatives or when not to use it, so it falls short of a 5.

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