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

Annotations already declare readOnlyHint=true, so the read-only nature is covered. The description adds the distinctive behavioral context that this is a draft/preview invoice rather than a finalized one, and it names the specific Orb API endpoint, going beyond what the annotations provide.

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?

The description is two short sentences with no wasted words. The primary purpose is front-loaded, and the API endpoint is a useful, non-redundant addition.

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 simple one-parameter, read-only tool with no output schema, the description sufficiently covers the purpose, the draft nature, and the API endpoint. There is little ambiguity about what the tool does and how to invoke it.

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% and the single parameter subscription_id is already well documented as 'Orb subscription id.' The description only restates the notion of a subscription without adding further detail about the parameter, so the baseline of 3 applies.

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?

The description states a specific verb ('Preview') and resource ('upcoming (draft) invoice for a subscription'), and also gives the exact API endpoint. This clearly differentiates it from siblings like orb_get_invoice, which concerns finalized invoices, and orb_get_subscription, which gets subscription data rather than invoice data.

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 description implies the tool is for previewing a draft invoice for a subscription, but it does not explicitly state when to use it over alternatives such as orb_get_invoice or orb_get_subscription. No exclusions or alternative routing are mentioned, leaving usage to inference.

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