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

Goes well beyond the annotations by disclosing that the copy is always a draft, costs no credits regardless of the original's status, has no invoice number, is non-idempotent in a user-visible way ('repeating a call that succeeded leaves a duplicate'), and triggers a customer-record creation side effect without emailing anyone. These are exactly the traits the annotations cannot convey.

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 effect, followed by alternatives and caveats in a logical order. It is dense and sentence-heavy, but essentially every sentence carries non-redundant operational 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 mutation with no output schema, the description covers the outcome (draft, unnumbered), cost, idempotency behavior, side effects, and follow-up tool. Nothing an agent needs to call it correctly 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?

Only one parameter, and the schema already documents it at 100% coverage ('Invoice ID to duplicate'). The description's 'chosen by id' adds no syntax or format detail beyond the schema, so the baseline 3 for high coverage 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 and resource ('Creates a draft copy of one existing invoice') plus the exact scope of what is copied (recipient, line items, discounts, dates, sender details). It explicitly contrasts with the sibling create_invoice_from_previous, so an agent can distinguish the two without opening a 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 explicit routing: use this to reuse a specific invoice, use create_invoice_from_previous to copy a recipient's latest invoice with field overrides. It also names update_invoice as the follow-up for setting a number and finalizing, and warns that repeats create duplicates.

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