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

Annotations already cover read-only, idempotent, open-world behavior, yet the description still adds real value: no email is sent, format/provider rules plus one MX lookup per domain, a daily per-account quota, and that the response exposes remaining quota. It doesn't say what happens at the 1,000-address boundary or on partial failure.

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?

A single tight paragraph that front-loads the batch size and input formats before the output shape and limits; nearly every clause carries information. A few fragments ('Format, provider rules and one MX lookup per domain') read telegraphically but still earn their place.

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?

With no output schema, the description still defines the return payload well: one status plus reason per address and counts per status, along with remaining quota. What's missing is edge-case behavior at the batch cap or with malformed CSV, which an agent would need before invoking at scale.

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%, so the baseline is 3, but the description goes beyond it by specifying that CSV input must carry a column headed 'email' and that raw addresses are accepted as emails, which the schema does not state. It stops short of explaining mutual exclusivity or precedence between the two optional 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?

States a specific verb and resource: bulk-checking a mailing list of up to 1,000 addresses supplied either as an array or as CSV text. The batch scope and the six possible statuses distinguish it in practice from check_email/validate_email, but no sibling is named, so the agent must infer the single-vs-list split.

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?

Implicit guidance comes from the 1,000-address cap and the daily per-account limit, which imply when this tool is the right choice. There is no explicit when-to-use/when-not statement and no pointer to the single-address siblings for one-off checks, leaving the routing decision to inference.

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