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

Annotations establish the safety profile (not read-only, open-world, not destructive), and the description adds real value beyond them: the side effect that this emails a real client and requests money, that confirmation is required by default, and that a Cotlet API key with app access is needed. It does not, however, note anything about what happens to the invoice state after sending or whether the send is reversible.

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?

It is front-loaded with the core action, but the second sentence ('Emails the invoice to the client so they can pay online') simply restates the first, and the confirmation/key sentences are somewhat scattered. Roughly a third of the text is redundant.

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 annotations covering safety, a complete 2-parameter schema, and no output schema, the remaining need is behavioral context, which the description supplies (external recipient, money request, confirmation, auth requirement). Nothing critical for a correct call 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 coverage is 100% and both parameters (invoice_id, message) are already documented in the schema, including the note that invoice_id comes from list_invoices or create_invoice. The description adds no parameter-level meaning beyond that, 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?

States a specific verb (email/send), the resource (a draft invoice), and the outcome (client gets a link to pay online). This clearly distinguishes it from siblings like pixieset_create_invoice and pixieset_list_invoices, so an agent can select it without opening a 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 context (this asks the client for money; confirm before each call) but never names an alternative or the precondition for use, e.g. that the invoice must already be a draft created via create_invoice. The 'when to use vs alternatives' guidance is only implied.

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