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

Beyond the annotations (openWorldHint=true, destructiveHint=false), the description discloses that the tool reaches other people, requires user confirmation before each call, needs an API key with app access, and depends on a customer email being on file. These are exactly the side-effect and authorization facts an agent needs before invoking an outbound-send tool.

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 first two sentences repeat the same idea ('Email an invoice to the customer with a link...' / 'Emails an invoice to the customer, with your optional note'), which is redundant. The remaining sentences are front-loaded and useful, but the opening duplication costs it.

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 no-output-schema outbound email tool, the description covers the prereq (email on file), the side effect (customer sends out), the confirmation policy, and the auth requirement. It is essentially complete; only the absence of any mention of delivery result or failure modes keeps it from a 5.

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% for both parameters, so the schema already documents invoice_id and message. The description only echoes the optional note ('your optional note, for example a reminder on an overdue one'), adding a usage example but no new syntax or semantics. 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?

The description names a specific verb+resource ('Email an invoice to the customer with a link to view it'), which an agent can distinguish from create_invoice or get_invoice siblings. It is fully clear about what happens, though it never explicitly contrasts itself with the neighbouring yardbook_send_estimate or yardbook_create_invoice tools.

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 concrete precondition ('The customer needs an email address on file') and states the confirmation policy ('By default you confirm each call; you can switch that off'), so the agent knows the operating context. It stops short of naming alternatives or explicit when-not-to-use conditions.

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