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

With no annotations supplied, the description carries the full burden and does so: plan gating, exact error code on rejection, per-plan daily quota numbers, the shared 120/min rate limit, and the explicit note that there is no send-specific limit. It also discloses the two external prerequisites (verified domain, valid recipient) that will cause failures an agent cannot infer from the schema.

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?

Four short sentences, each carrying distinct operational information — purpose first, then plan/quota gating, then rate limits, then input preconditions. No restatement of the tool name and no filler; the 'no separate send-specific limit' clause preempts a likely wrong assumption rather than padding.

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 5-parameter mutation tool with no annotations and no output schema, the description covers the failure modes and limits an agent most needs. The remaining gap is the return value — nothing says whether a message id or status is returned, which matters since no output schema exists to document it.

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 60%, so the schema already documents to/html/text; the description adds meaning only for the recipient constraint ('must be a valid address') and the inbox-scoping concept. inboxId and subject (maxLength 200) get no additional semantic explanation, so the description does not fully compensate for the uncovered parameters.

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?

Opens with a precise verb+resource: 'Send an outbound email from one of your inboxes.' The 'outbound' and 'from one of your inboxes' qualifiers scope it unambiguously, and among the siblings (all read/manage/verification tools) this is clearly the only send operation, so no disambiguation text is needed.

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 concrete preconditions for use: paid plan only, free tier rejected with code=quota_exceeded, per-plan daily quota, verified sending domain, valid recipient. It does not name alternatives or explicitly state when-not to use it, but there is no sibling that could substitute, so the practical routing gap is small.

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