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

Beyond the annotations (which already declare a non-read-only, idempotent, open-world, non-destructive write), the description discloses the authorization requirement, the billing/cap constraints that can block submission, and the exact retry contract ('keep inputs and idempotency_key unchanged'). It also preempts a common failure mode by warning that no verification results come back in the response.

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?

Four short sentences, front-loaded with what the tool does and capped off with the return value and retry rule. Every clause carries operational information (volume limits, auth, caps, return shape, idempotency) with no filler.

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?

For a submission tool with no output schema, it covers the essentials: what to send, what comes back, and how to retry safely. The remaining gaps are the purpose of the 'name' field and an explicit pointer to the polling sibling for retrieving results.

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 only 33%, so the description must carry weight: it documents the 1–5,000 email volume and the idempotency_key retry semantics, which are the operationally critical parts. However, the optional 'name' parameter is left unexplained in both schema and description, leaving a real gap.

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 states a specific action (submit a batch of 1 to 5,000 email addresses) and immediately clarifies its output boundary: it returns a job_id for polling, not completed mailbox results. That framing distinguishes it from fairvalid_get_results and fairvalid_get_job without needing the sibling schemas.

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?

It gives clear preconditions for use (submit scope for live keys, workspace balance and key spending caps) and clarifies the tool's role in a two-step submit-then-poll workflow. It stops short of naming the polling sibling explicitly or explaining when to prefer fairvalid_verify_email for single addresses, so routing is implied rather than stated.

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