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

Annotations only declare readOnlyHint and openWorldHint. The description goes well beyond them: probing is real-time and non-sending, one credit is charged per address actually probed, duplicates and malformed entries are free, and a hard cap of 20 addresses per call applies. That is the billing and rate-limit context an agent needs before invoking.

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?

The core behavior leads, then result interpretation, then the billing/limit constraints. Every clause carries information — no restatement of the tool name or title.

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 must carry return semantics, and it does: one result per address in input order, the four `status` values with their meanings, plus `score`, `reason` and `did_you_mean`. An agent has everything required 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?

Schema coverage is 100% and the single `emails` param is self-explanatory, so the baseline is 3. The description adds genuine meaning the schema lacks: duplicates and malformed entries are not billed, and the 1-20 limit is a per-call constraint requiring batching rather than a hard rejection of longer lists.

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 ('Checks whether email addresses exist and can receive mail') and immediately bounds the mechanism ('in real time and without sending anything'). The only sibling, get_credits, is clearly unrelated, so an agent can route this without opening a schema.

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 strong operational guidance: act on `status`, split lists over 20 into several calls, and the cost implication of duplicates/malformed addresses. It does not name an alternative tool or state an explicit when-not-to-use condition, which keeps it short of a 5.

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