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

Beyond the readOnly/idempotent annotations, it discloses data provenance ('learned only from addresses that were verified deliverable'), the response shape (per-pattern share and confidence level), and the edge case of returning 'not published' when evidence is thin. This is exactly the behavioral context annotations cannot carry.

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?

Three compact sentences, front-loaded with what the tool returns followed by provenance and the fallback case. Every clause earns its place, though 'Free.' is slightly fragmentary.

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 and no annotations beyond safety hints, the description still explains what comes back (patterns, shares, confidence) and what happens when data is absent, so an agent needs nothing further to call it correctly.

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 100% and the single 'domain' parameter is fully documented in the schema, including that a full URL is reduced to its domain. The description adds nothing beyond that, so the baseline of 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?

The description names the exact resource and scope: the email address format a company uses, keyed by domain. An agent can immediately distinguish it from verify_email/verify_emails, which check deliverability rather than infer patterns, though the description never names those siblings explicitly.

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?

Usage is implied rather than stated — 'Free' signals a cheap lookup versus a paid verification call, and the 'not published' clause tells the agent when the answer will be empty, but there is no explicit when-to-use/when-not rule or named alternative.

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