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

Far exceeds what annotations provide: it discloses the critical limitation that no SMTP/RCPT probing occurs so mailbox existence is NOT verified, gives expected latency (0.1-1 s), price (USD 0.002), a rate limit (5/IP/day shared pool), and a prompt-injection warning about untrusted page text. These are exactly the traits an agent needs and none are in the structured fields.

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-loads purpose and input, then the checks, then the return shape, then operational constraints. Dense but every sentence carries information; only the long enumeration of checks and returned fields pushes it toward being list-heavy rather than prose.

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 no output schema, the description still names the return fields (verdict, reasons, normalized, mx_hosts, null_mx, disposable, role_account, free_provider) and the verdict enum values, plus cost, latency and the safety caveat. Nothing needed to call or interpret the tool is missing.

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?

Only one parameter and schema description coverage is 100%, so the schema already documents `email` fully; the description's 'Input: `email`' adds nothing beyond the name. Baseline 3 applies when the schema does the heavy lifting.

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?

Specific verb (verify) plus a narrowly scoped resource (an email address) with an explicit boundary: 'syntax + DNS, no SMTP'. The description enumerates exactly what is checked (MX, null MX, A/AAAA fallback, disposable list, role parts), making it clearly distinguishable from siblings like inspect_domain, rdap_lookup, or ip_lookup.

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?

Gives clear situational context: 'before you send to it or accept it at sign-up'. It also hints at the shared free-tier pool with domain inspections, RDAP and IP lookups, but it never names a sibling tool or states when to prefer inspect_domain or rdap_lookup instead, so the routing guidance is implicit rather than explicit.

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