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

With no annotations supplied, the description carries the burden and does well: it discloses the full set of checks, explicitly excludes SMTP probing, and states the $0.002 USDC cost — a material constraint for a metered tool. It stops short of describing auth requirements, rate limits, or the score scale behind 'score+verdict'.

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?

Front-loaded with the core capability and checks, then the scope exclusion, then price, then sibling routing — a sensible priority order. It is telegraphic and dense; the sibling price list is slightly bulky but earns its place as cost-routing signal.

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, annotation-free, output-schema-free tool, the description covers inputs, checks performed, cost, limitations, and alternatives. The main residual gap is the shape/range of the returned score and verdict, which an agent may need in order to interpret 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?

The single 'email' parameter has 0% schema description coverage, but its meaning is self-evident from the tool name and purpose. The description adds no format constraints (single vs. batch, normalization, internationalized addresses), so it doesn't compensate for the coverage gap beyond the obvious.

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 verb (validate) and resource (email) and enumerates the exact checks performed: syntax, MX/A DNS, disposable/free/role flags, provider guess, score+verdict. An agent can distinguish this from dns_lookup or domain_lookup without opening a 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?

The 'Related:' block explicitly routes adjacent needs to /dns, /domain, /check, and /kyb with prices, and 'No SMTP probe' marks a scope boundary so the agent won't expect mailbox verification. It does not state when this tool should be preferred over, say, token_check-style validators, but the sibling routing is clear enough.

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