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

Annotations only declare destructiveHint=true; the description reinforces this by stressing that it sends a REAL email immediately (an external, irreversible side effect) and discloses the return payload { status: "accepted", message_id }. It does not cover rate limits or delivery-failure behavior, but it adds genuine context beyond the annotation.

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 dense sentences with the high-consequence fact ("SENDS A REAL EMAIL") front-loaded, followed by required inputs, the branch condition, and the return shape. Nothing is redundant with the schema's parameter descriptions.

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?

Despite 11 parameters and a nested object, the description covers the required inputs, the subject/content vs template branch, and the response shape in lieu of an output schema. Nothing an agent needs to call it correctly 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%, so the baseline is 3, but the description adds real conditional logic the schema only implies: the mutually exclusive subject/content vs template_id branch, the "d-" prefix convention for dynamic templates, and the role of dynamic_template_data as handlebars input.

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 ("SENDS A REAL EMAIL immediately via SendGrid") plus the underlying endpoint POST /v3/mail/send. It is unmistakably distinct from every sibling, which are all read/list/contact-lookup 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?

Gives explicit invocation context: supply `from_email` and at least one `to`, plus EITHER subject+content OR a `template_id` (with dynamic ids prefixed "d-" and values passed via dynamic_template_data). It does not name when-not-to-use or an alternative tool, but no sibling overlaps in function, so the routing risk is low.

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