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

With no annotations, the description carries the full behavioral burden. It usefully discloses the 1,000-item batch cap and the shape of the response (per-email results plus summary counts), but says nothing about rate limits, cost, partial-failure behavior, or what happens when the batch exceeds the cap.

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, zero filler, and the batch scope is front-loaded before the return-value summary. Every clause earns its 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?

There is no output schema, so the description responsibly enumerates the summary categories returned (valid, invalid, disposable, catch_all, unknown) and the batch ceiling. The remaining gap — error handling, rate limits, and auth expectations for an external API — keeps it short of 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?

Schema description coverage is 100%, so both parameters (emails, smtp_check) are already documented in the schema, including the default for smtp_check. The description adds no meaning beyond that — it does not explain the cost or latency tradeoff of enabling the deep SMTP handshake. Baseline 3 is correct.

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 ('Validate a batch of up to 1,000 email addresses') plus the backing service (NoBounce API). The word 'batch' implicitly separates it from the single-address sibling validate_email, but the sibling is never named explicitly, so the differentiation is left to inference.

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?

The 'batch of up to 1,000 ... in a single call' phrasing implies when to prefer this over validate_email, but there is no explicit when-to-use statement, no exclusion for small lists, and no guidance on when the sibling is the better choice.

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