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

With annotations only covering the safety profile (readOnlyHint=false, openWorldHint=true), the description carries rich extra context: asynchronous PDF generation, conflict failures, absence of CC/BCC, template language tied to billing language, per-call confirmation default, and a confirm-before-sending guardrail for external recipients.

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 mostly dense with useful operational detail. Some minor redundancy around who receives the email ('The email reaches your customer' / 'Reaches other people'), but every clause largely earns its place.

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 two-parameter action tool with no output schema and annotations that only cover safety, the description supplies failure modes, timing constraints, recipient resolution, confirmation behavior, and auth requirements. 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?

Schema description coverage is 100%, so both parameters are already documented. The description restates the recipients default, which the schema also states, and adds no format or syntax detail beyond it, so baseline 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?

States a specific verb and resource ('Email a customer invoice') and immediately clarifies scope: to the customer or to supplied addresses, sending the invoice PDF. This distinguishes it cleanly from create_customer_invoice and finalize_customer_invoice siblings.

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 recipient default and the failure path when the customer has no email, plus the timing caveat that a call right after creation can conflict and should be retried later. It does not explicitly name a sibling alternative, but a concrete when-it-works condition is given.

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