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

Annotations already declare destructiveHint=true and openWorldHint=true; the description adds real value on top by warning that messages reach actual external recipients and that the sending domain must be verified. It omits that a sent message cannot be recalled (only cancel-email for scheduled sends) and says nothing about rate limits or failure behavior, keeping it out of 5 territory.

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?

Three sentences, front-loaded with the core action and the verified-domain constraint, ending on the safety instruction. No filler and every clause carries information an agent needs.

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 15-parameter, nested-object mutation tool with no output schema, the description covers the essential prerequisites (verified domain, confirmation with user) and the major feature surface. It leaves gaps on body-mode mutual exclusivity, error/return behavior, and scheduling semantics, which an agent may still have to infer.

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?

With 60% schema coverage, several params (cc, bcc, reply_to, headers, tracking) lack schema descriptions; the description compensates by naming CC/BCC, attachments, scheduling, templates with variables, and open/click tracking, which clarifies what the tracking object actually controls. It does not explain the html/text/template exclusivity or what `headers` and `reply_to` are for.

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 and resource ("Send a transactional email") plus the scoping constraint ("from a verified sending domain") and enumerates supported content modes. It implicitly separates itself from send-campaign by using the word "transactional," but no sibling is named explicitly, so it stops short of a 5.

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?

"Delivers to real recipients, so confirm recipients and content with the user first" gives a concrete precondition for invocation. It does not, however, point to alternatives like send-campaign for bulk sends or retry-email for resends, so routing guidance is clear but not exhaustive.

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