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

The description adds substantial context beyond the destructiveHint annotation: it discloses the exact audience (client's invited contacts), the irreversible nature ('Cannot be unsent'), and the side effect of marking the invoice sent. This is exactly the behavioral detail an agent needs before invoking a destructive, outward-facing operation.

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 critical warning is front-loaded, and the text is dense with no filler. The trailing API endpoint reference ('POST /api/v1/invoices/bulk with action=email') is implementation detail that does not help an agent select or invoke the tool, slightly weakening conciseness.

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 tool with no output schema and a destructiveHint annotation, the description covers purpose, audience, side effects, irreversibility, and a user-confirmation requirement. Nothing needed 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 fully documented in the input schema. The description adds no further meaning to invoice_id or email_type, so the baseline of 3 is appropriate given the schema already does the work.

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 names a specific verb and resource ('sends a real email with the invoice') and clarifies the side effect of marking the invoice sent. It implicitly distinguishes itself from invoiceninja_mark_invoice_sent by emphasizing the actual email transmission, but it never names that sibling explicitly, leaving a small inference gap.

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?

It gives a clear precondition ('confirm with the user first') and signals the tool is outward-facing, which is strong contextual guidance. However, it does not explicitly state when not to use it or name the alternative tool for merely marking an invoice sent without emailing.

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