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

Annotations already declare write, idempotent, and destructive, and the description adds the useful fact that billable amounts get reserved and that the draft is unnumbered with validation feedback returned. However, it doesn't reconcile the 'just a draft' framing with destructiveHint=true (there is an invoice_cancel_draft sibling, but no statement about reversibility or side effects on billables), which is context an agent would want before calling a mutating tool.

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?

Four short front-loaded sentences, no filler; the amount/tax/discount/return information comes in the order an agent needs it and the terse 'No number assigned.' earns its place by disambiguating draft from issue.

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?

There is no output schema, and the description compensates by stating that totals and missing-information feedback are returned, which is exactly what an agent needs to know about the draft response. Remaining gaps are the lack of any error/permission guidance and the unstated handoff to invoice_issue.

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 only 25%, so the description must carry the load — and it does on the hardest parts: it explains partial amounts against billable items, per-line tax treatments, and both line-level and invoice-level optional discounts, matching the deeply nested tax object and discount oneOf. It adds no guidance on supply_date/due_date, client_id, business_id, or request_id retry semantics.

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?

Specific verb and resource: 'Reserve explicit amounts from billable items, including partial amounts' plus 'No number assigned' clearly frames this as the draft stage of an invoice. It implicitly distinguishes from invoice_issue, though it never names a sibling directly.

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 implied rather than stated — 'No number assigned' and the draft framing suggest this precedes invoice_issue, but the description never says when to draft versus issue, check, or go straight to billables_list. No explicit exclusions or prerequisites.

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