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

Annotations already cover safety (readOnly, idempotent, non-destructive, openWorld), so the bar is lower. The description adds genuinely useful non-schema behavior: the call is synchronous, blocks up to 80s, and returns a definitive result including catch-all detection. It does not mention timeout/unknown-result handling or rate limits, which keeps it from a 5.

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 purpose and the important latency caveat. The example call largely duplicates the schema's `examples` field, which is mild redundancy rather than bloat.

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 the return-value burden — and it only gestures at it ("definitive result, including catch-all detection") without describing the result fields or the possible verdict values an agent would need to act on. Latency and safety context are adequate; the result contract is not.

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 `email` and `_apiKey` are already documented with their meaning and where to find the key. The description's inline example repeats the schema's own examples block and adds no new parameter semantics, so the baseline 3 applies.

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 ("Verify"), resource ("a single email address"), and the mechanism ("BounceBan's synchronous waterfall endpoint"). No sibling tool overlaps with email-address verification, so the agent can distinguish it at a glance.

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 description implies the use case (verifying one address, with a blocking 80s wait) but names no alternative tool or condition for when-not to use it. No guidance on whether batch verification exists elsewhere or when to prefer a faster/lower-confidence path.

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