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

Beyond the annotations (read-only, idempotent, open-world), the description discloses key behavioral traits: it uses live DNS, performs no SMTP probing, and therefore does not claim mailbox existence. It also reveals important operational details—pricing per call, the x-credit-token header for unlimited calls, a free daily tier, and that no wallet or API key is needed. This goes well beyond what annotations provide.

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?

The description is longer than average but every sentence carries useful information: purpose, detection capabilities, limitation, required input, and pricing/auth. It is front-loaded with the purpose question, then moves to technical detail, then operational specifics. The pricing sentence is somewhat verbose but adds genuinely useful cost and access context, so it 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?

For a single-parameter verification tool with an existing output schema, the description covers all essential context: what it verifies, what it explicitly does not verify, required input, cost, authentication, and availability. It even notes the one-free-call-per-day option. Return values are not described, but the presence of an output schema makes that unnecessary.

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?

The schema has 100% coverage for the single email parameter, so the baseline is 3. The description only restates 'Required input: email' and does not add format details or constraints beyond the schema. It does, however, enrich the meaning by explaining what the tool does with that email (checking syntax, DNS, disposable status), which gives the parameter context but not new structural information.

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 opens with a clear question ('Is this address worth sending to?') and then specifies the exact verifications performed: syntax, live DNS (MX/A), disposable/role/consumer-mailbox detection, and a GO/HOLD/STOP verdict with reasons. It also names concrete examples (throwaway inboxes, support@-style mailboxes, gmial.com typos), making the tool's function unambiguous and distinct from siblings like domain_check or phone_check.

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 strongly implies when to use the tool: before sending to an address, to decide if it is worth sending to. It also explicitly states a limitation—no SMTP probe, so mailbox existence is not claimed—which tells users not to use this when they need guaranteed mailbox existence. It does not name an alternative tool, but none of the siblings directly compete with email verification, so this is a minor gap.

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