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

Goes well beyond the annotations by disclosing the pricing model (USD 0.002 per call), the payment rail (x402, USDC on Base), and crucially that failed calls are not charged — an economic risk fact an agent cannot get from readOnlyHint/destructiveHint. It also frames the result taxonomy (valid / risky / invalid) and an explicit accuracy boundary.

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, front-loaded with the capability and verdict taxonomy, followed by the two facts an agent most needs before calling: the non-probing limitation and the cost/refund policy. No filler and no repetition of the title.

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 one parameter and no output schema, the description supplies the essentials: what is checked, the verdict categories, the accuracy ceiling, and the billing behavior. It stops short of describing the response shape (per-check breakdown vs single verdict string), which the agent must discover at call time.

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 0%, so the description carries the burden for the single required parameter. "Check an email address: format, ..." makes clear the argument is a single address whose syntax and deliverability are assessed, which is sufficient for an obvious string input. It does not state normalizations (trimming, case, plus-addressing) that could affect results.

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 and resource ("Check an email address") and enumerates exactly what is checked: format, MX records, disposable/role/free providers, with a valid/risky/invalid verdict. The clause "The mailbox itself is not probed" cleanly separates it from a raw DNS lookup, so an agent can distinguish it from siblings like dns_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 scope statement ("The mailbox itself is not probed") tells the agent the precise condition under which this tool is insufficient, which is a practical exclusion. However, no sibling is named and there is no explicit "use this before sending mail / use dns_lookup for raw records" routing statement, so the agent must infer the alternative.

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