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

Adds substantial context beyond the destructiveHint=true annotation: it emails an external third party, is irreversible ('cannot be recalled'), falls back to the client's address and the account's template, and requires user confirmation. This is exactly the behavioral disclosure an agent needs before invoking an irreversible outbound action.

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 tight sentences, front-loaded with the action and its irreversibility, followed by defaults and the POST path. The endpoint reference mildly duplicates what an agent already infers from the tool call, but nothing is bloated.

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 an 8-parameter irreversible action with no output schema and only a destructiveHint annotation, the description supplies the precondition, defaults, safety warning, and confirmation requirement. It omits only error/rate-limit behavior and result reporting, which are secondary here.

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 baseline is 3. The description restates the fallback semantics for `to` and the template defaults for subject/body, which the schema already documents per-parameter; it adds no new syntax or format detail.

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 ('Send an issued invoice or proforma by email') and constrains scope to already-issued documents. No sibling tool sends documents, so it is clearly distinguishable from the cancel/create/restore family.

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 default behavior when `to`, `subject`, and `body_text` are omitted, warns that the action cannot be recalled, and instructs the agent to confirm the recipient with the user first. It does not name any alternative tool, but no sibling performs a comparable send operation, so the guidance is effectively complete.

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