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

Annotations already cover readOnly=false/openWorld=true/destructive=false, and the description adds genuinely new behavioral context: the recipient defaults to the client's contact email, the call reaches an external party, per-call confirmation is on by default, and it requires an API key with app access. These are useful beyond the safety hints, though it doesn't describe failure modes or what a successful send returns.

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 opening sentence is duplicated verbatim ('Email a finalized invoice to the client' appears twice), wasting space that could carry real information. The remaining sentences are efficient, but that redundancy lowers an otherwise tight description.

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?

With no output schema and only 2 well-documented parameters, the description supplies the key operational facts an agent needs: recipient resolution, external reach, confirmation default, and auth requirement. It is complete enough to invoke correctly, though it could mention success/failure behavior.

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 the schema already documents both parameters and their defaults. The description restates the to_email default ('unless you pass to_email') without adding format or syntax detail, so the 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+resource ('Email a finalized invoice to the client'), and the 'finalized' qualifier distinguishes it from sibling actions like knowify_create_invoice or knowify_save_proposal. An agent can tell exactly what it does without opening the schema.

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 real usage constraint ('Reaches other people: confirm with the user before each call') and notes confirmations default on but can be switched off. However it never routes to alternatives or states when to prefer this over siblings (e.g., ensuring the invoice is finalized first); usage is implied rather than compared.

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