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

The annotations already establish that this mutates state and is idempotent. The description adds useful behavioral context beyond that: the email contains a server-rendered PDF attachment, and the operation has an account prerequisite with a specific Guest onboarding path. No contradiction with annotations.

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 description is compact and front-loaded with the core action, then quickly covers prerequisites, exclusions, and pre-call steps. Every sentence earns its place without redundant 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 mutating email-sending tool, the description covers the core action, account prerequisite, guest fallback, exclusions, and required pre-call verification. Combined with the rich 100%-covered schema and output schema, nothing essential is missing for an agent to call it 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 description coverage is 100%, so the individual parameter meanings are already fully documented in the schema. The description adds no parameter-level meaning beyond what the schema provides, which matches the baseline for high coverage.

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 opens with a specific verb and resource: 'Send an existing invoice as a server-rendered PDF attachment by email.' It clearly delineates what the tool does and distinguishes itself from sibling tools by explicitly saying it is not for creating, editing, or downloading an invoice.

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?

Usage conditions are explicit: a registered account is required, Guest connections must use create_account_claim_link first, and the user should identify the invoice via get_invoice/list_invoices and confirm the recipient before calling. This provides both when-to-use and when-not-to-use guidance.

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