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

Annotations only declare readOnlyHint=false, openWorldHint=true, destructiveHint=false; the description adds real behavioral context the annotations do not cover — the 0.3-credit cost per address, the free re-check within 7 days, and the fact that results are returned synchronously for the whole batch. It does not explain partial failures or how errors are surfaced, 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?

A single dense sentence with parentheticals; the pricing and the deliverability question are front-loaded. It is efficient, though the nested parentheses make it slightly harder to parse than a two-sentence split would be.

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 carries the burden of describing return values and does so by enumerating the four result states. Combined with cost, caching, and batch-size details, an agent has enough to call this correctly and budget for it.

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 50%: the `language` parameter is documented in the schema but ignored entirely in the description, and the description's 'up to 25 addresses per call' merely repeats the schema's maxItems/maxItems constraint. It does add the useful idea that multiple addresses are answered together, but contributes little else beyond what the schema already encodes.

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?

The description names a specific verb and resource ('verify e-mail address') and enumerates the four possible outcomes (valid, invalid, catch_all, unknown), which is more than a restatement of the title. None of the siblings (check_records, enrich_records, find_person) touch email deliverability, so an agent can route correctly.

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?

Usage is implied rather than stated: it is clear you call this when you need deliverability, and the 7-day free-recheck note hints you should not re-verify recently checked addresses. However, there is no explicit when-not guidance, no naming of an alternative tool, and no note on what to do with 'unknown' or 'catch_all' results.

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