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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered structurally. The description adds genuine behavioral context beyond that: the result is scoped by the calling key's grants, includes queued sends in the quota counter, and reveals counter-reset and billing state. It stops short of describing refresh timing or caching, so it is not exhaustive.

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 sentences, front-loaded with what is returned and closed with the imperative to call it first. The field enumeration is long but each item is decision-relevant (sending domains verified-or-not, remaining quota). Slightly dense, but no filler sentences.

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?

There is no output schema, so the description carries the full burden of explaining return values — and it does: it lists the workspace, scopes, plan, quota, reset time, billing, limits, domains, and senders. For a zero-argument, read-only tool that is sufficient for an agent to decide to call it and interpret the response.

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 per the baseline rule a score of 4 applies — there is nothing for the schema to document and nothing for the description to compensate for. The description correctly implies no input is required.

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 (returns the workspace/account this key grants access to) and then enumerates the exact fields returned — scopes, plan, remaining emails, reset timing, billing status, limits, sending domains, senders. This clearly distinguishes it from every sibling tool, which all act on campaigns, subscribers, suppression lists, or emails.

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?

'Call this FIRST: it tells you which address to send from before attempting anything' gives explicit ordering guidance and the purpose that ordering serves (resolving the sending address). It does not name alternatives or when-not conditions, but for a no-argument discovery tool there is no competing sibling, so the omission is minor.

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