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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds genuine context by marking the invoice as a draft/preview that is not yet final, which tells the agent the values may change. It does not cover permissions or whether a missing subscription errors.

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?

Two tight sentences with zero filler, and the core purpose leads. The trailing API endpoint note is short and additive rather than padding.

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, read-only lookup with no output schema, the description supplies enough to call it correctly and correctly signals the result is a provisional draft. Minor gap: nothing about error behavior for an invalid or non-existent subscription id.

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% with a single well-documented parameter (subscription_id), so the schema does the heavy lifting. The description adds no format or constraint detail beyond it, making the baseline 3 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?

Specific verb ('Preview') plus a precisely scoped resource ('upcoming (draft) invoice for a subscription'), which cleanly separates it from the sibling orb_get_invoice (an issued invoice) and orb_list_invoices. It stops short of naming any sibling explicitly, so it earns a 4 rather than a 5.

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?

The '(draft)' qualifier implies this is for seeing charges before they are finalized, but there is no explicit statement of when to reach for it over orb_get_invoice or orb_list_invoices, and no prerequisites or exclusions. Usage is inferable but not stated.

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