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

No annotations are present, so the description carries the burden. It discloses the verification method, the cost ($0.20 per 1,000 domains), and the funding source (prepaid credits). It does not describe the return format or state explicitly that this is a read-only operation, though 'check' implies it.

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 tight sentences front-load the core purpose, then give the use case and cost. No filler or repetition of the schema.

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 one-parameter verification tool with no output schema, the description covers what it does, when to use it, and its cost. The main gap is the lack of an explicit statement about the response shape, but the purpose sentence strongly implies a per-domain deliverability result.

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 100% and the single parameter is already described as 'Domains or full email addresses (max 200).' The description adds the billing unit and campaign context but no additional parameter-level semantics, so baseline 3 applies.

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?

States a specific action ('Check whether domains can receive email at all') and names the mechanism (MX records, catch-all, provider). It clearly separates this verification tool from sibling leads_* tools, which find or pattern-match leads rather than validate deliverability.

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?

Explicitly tells the agent when to run it: 'Run it over a list before a campaign so you stop sending where nothing arrives.' It gives clear workflow context but does not name alternatives or exclusions, so it stops short of a full when-to-use/when-not-to-use guide.

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