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

Adds substantial context beyond the annotations: a real email leaves the system (consistent with openWorldHint), the call is not repeatable (404 after send, consistent with idempotentHint=false), and it documents the credential requirement (billing write scope) and the 403 refusal for self-registered sandbox keys. It doesn't say what the response body looks like or whether partial failures leave the invoice marked sent, which keeps it short of a 5.

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?

Two dense sentences, front-loaded with the action and side effect, then the parameter note, then the error and auth preconditions. No filler.

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 2-parameter, fully documented tool with no output schema, it covers the external side effect, the non-idempotent failure mode, the required credential scope, and the workspace-claim gate. An agent has everything needed to decide and call correctly.

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%, so both parameters are already documented, and the description largely restates them ('pass invoice_id, optionally to'). The one addition is the cross-reference that invoice_id is the id 'as list_invoices shows it', which is mildly useful.

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 plus the side effect ('Email an invoice ... and mark it sent'), which cleanly separates it from get_invoice, list_invoices, create_invoice and update_invoice in the sibling set.

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?

Explicitly says to check with get_invoice first because the call answers 404 once the invoice is already sent, which is real when-to-use/when-not-to-use guidance. It also names the alternative path (pass `to` vs. default to the customer's own address).

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