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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, so the bar is lower; the description still adds real context by asserting the limits are "static" and that the call neither reserves capacity nor proves delivery. That rules out side effects the agent might otherwise assume from a limits endpoint. It stops short of describing freshness or what the limits are keyed to.

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?

Two short sentences, zero waste, with the positive statement of purpose front-loaded and the caveat immediately after. Ideal size for a no-arg read tool.

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?

No output schema exists, yet the description does not indicate what the returned limits contain (per-domain, per-day, account-wide?) or their units. For a zero-param read tool with full annotation coverage the description is nearly sufficient, but that single return-shape gap keeps it off a 5.

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?

The tool takes zero parameters, so the baseline is 4; there is nothing for the description to disambiguate beyond the empty schema.

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 gives a specific verb and resource ("Read static sending limits"), and the qualifier "static" plus the negative claims distinguish it from the mutation/verification siblings (voidmail_send_once, voidmail_send_status). It does not name an alternative explicitly, so an agent must infer the routing from the negations.

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?

"This does not reserve capacity or prove delivery" is explicit when-not guidance: it tells the agent not to treat this as a pre-send reservation or a delivery-verification step, which is exactly the confusion the sibling send tools would create. No positive statement of when to call it (e.g., before composing a campaign), but the exclusion is clear context.

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