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

Annotations already declare readOnly/idempotent/non-destructive, yet the description still adds real value: it discloses the score range (0-99), the exact verdict vocabulary, and importantly that Gmail/Outlook bypass DNS, which materially affects result interpretation. Missing only operational details such as latency or behavior on malformed input.

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?

The core capability and the no-sign-in/no-email-sent reassurances are front-loaded, and the returns are summarized compactly. The DNS record enumeration is a long list but each item is informative, so it earns its place; minor trimming possible.

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 present, the description compensates by defining both return fields (score 0-99, verdict enum values) and the major behavioral caveat about big providers skipping DNS. An agent has everything needed to call it and interpret the 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?

A single parameter with 100% schema description coverage, so the schema carries the semantics. The description adds no format or normalization guidance (e.g. case handling, unicode domains) beyond what the schema states. 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 verb+resource ('deep check of an email address') and enumerates exactly what is examined: format, provider username rules, disposable/role detection, and MX/SPF/DMARC/DKIM/MTA-STS/TLS-RPT/BIMI records. That is far beyond a restated title. It does not distinguish itself from close siblings like validate_email, verify_check or verify_start, so it falls short of a 5.

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?

Useful context is implied — 'free', 'no sign-in needed', 'No email is sent' — which signals a safe pre-flight check. However, it never says when to choose this over validate_email or verify_start, nor what kinds of inputs are unsuitable. Usage is inferable but not directed.

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