Email/domain deliverability and trust check

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

Annotations only set readOnlyHint=false and openWorldHint=true, so the description carries the burden. It adds value by disclosing the paid nature ('Paid (price from the live 402 quote)') and the safety guarantee ('Does not send any email'). This is meaningful extra context beyond annotations, though it omits details like response format or error behavior.

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, zero filler. The first sentence front-loads the paid nature and the list of checks; the second gives a use case and a safety note. Every sentence earns its place, and the structure is clean.

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 only 2 parameters, no enums, and a 100% description coverage, the tool is simple. The description covers the why, the what, and the payment nuance, and even addresses a common safety concern. It doesn't detail the output shape, but given the lack of an output schema, this is a minor gap, not a blocker.

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 already covers both parameters fully (100% coverage), describing email_or_domain and payment_signature precisely. The description adds only context ('Paid', 'Use to vet...'), not new parameter-level semantics. Baseline 3 is appropriate given the schema's completeness.

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 verb ('check') and a precise resource ('email/domain deliverability and trust'), listing concrete checks (MX, SPF, DMARC, disposable-domain flag, trust score). This clearly distinguishes it from generic tools like whois or web_extract and makes the tool's function immediately apparent.

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 a concrete use case ('vet a signup email or an inbound sender's domain') and clarifies scope, but does not explicitly name alternatives or exclusion criteria. While the scenario is clear, it leaves the agent to infer when not to use it rather than spelling out the boundary with siblings.

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