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

Annotations already establish readOnly, idempotent, non-destructive and open-world traits, so the description correctly focuses on what they don't cover: it discloses that each request spends one credit (including failed requests) and that the Email Reputation API key is required. It stops short of rate-limit or failure-mode detail, keeping it below 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 well-packed sentences, front-loaded with capability and followed by billing/auth constraints. No filler, though the API-product name ('Email Reputation API') is slightly redundant with the cost sentence that follows.

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 usefully lists the categories of data returned (deliverability, score, flags, sender, breaches), and it covers cost and auth. For a single-parameter read-only tool with annotations handling the safety profile, this is essentially complete.

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%, and the single 'email' parameter is fully documented in the schema itself. The description adds only the single-address constraint ('one email address'), which is marginal beyond what the schema already conveys.

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 (Check/Validate) and resource (one email address) and then enumerates the exact checks performed: format, MX, SMTP deliverability, quality score, and reputation flags. An agent can distinguish it from the sibling validators (validate_iban, validate_vat) without opening the schema.

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?

'Check one email address' establishes the single-address scope, and the cost/key sentences frame the operating context (Email Reputation plan). There are no true alternative siblings for email validation to route against, but no explicit when-to-use or when-not guidance is offered either.

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