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

Beyond the annotations, it discloses quota accounting per recipient, the 30-day scheduling ceiling, three named failure modes (422 tenant conflict, 403 sending_paused, 429 daily_quota_exceeded), required API key scopes, and the irreversibility of delivered mail. That is exactly the operational context the annotations cannot express.

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?

Front-loaded with the core action, then organized into constraints, testing, and safety. Dense but nearly every clause earns its place; only the trailing 'API: POST /emails (operationId sendEmail, Emails)' line is filler for an agent.

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 14-parameter mutation tool with no output schema and thin schema descriptions, the description covers auth, quota, scheduling, error handling, and the irreversibility warning. Gaps remain around attachment/header/tag usage and what the call returns (email id? queue status?), but nothing essential to invoking 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?

With only 29% schema coverage across 14 parameters, the description compensates well: it explains the html/text either-or requirement, the tenant_id conflict semantics, the replyTo/scheduledAt aliases, and the natural-language scheduling format. Secondary parameters such as headers, tags, and cc/bcc still get no mention in the description, so it does not fully close the coverage gap.

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 states a specific verb and resource and adds the crucial scope qualifier 'queues one email', which separates it from send_batch_emails and send_broadcast. It also names the underlying operation (POST /emails) so the agent knows exactly which API is wrapped.

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 concrete routing guidance: call list_domains to find a verified `from` domain, use the delivered@/bounced@ test addresses to avoid real mail, and pass `idempotency_key` when retrying. It lacks an explicit 'use send_batch_emails instead when sending many', so the alternative-selection rule is only implied by 'one email'.

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