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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, so the bar is lower, but the description still adds substantial value: the exact return shape, the partial-result edge case when no A record exists (ip null, domain-only lists, note field), and the fact that no login is required. These are behavior facts the annotations cannot express.

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?

Purpose is front-loaded in sentence one, return shape and the edge case follow, and the usage triggers close. No redundant restatement of the tool name or title; every sentence carries a distinct payload.

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 compensates by enumerating the response fields, and it covers the one meaningful failure mode (missing A record). For a single-parameter read tool, 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?

Schema coverage is 100% with a single required `domain` parameter already documented with an example, so the schema does the heavy lifting. The description clarifies that IP-based lookups derive from the domain's A record rather than being a separate input, which is mild but real added meaning; baseline 3 is appropriate.

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?

Opens with a specific verb+resource ("Check whether a domain or its IP address appears on ... blacklists") and enumerates the actual blocklists consulted, so the agent knows exactly what is and isn't covered. This clearly separates it from siblings like check_email_auth or lookup_dns, which touch adjacent email/DNS territory.

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 four concrete triggering situations (deliverability problems, blacklist status, spam folder complaints, IP reputation), which is strong when-to-use guidance. It stops short of explicitly excluding or naming the nearest sibling (check_email_auth) for overlapping email-deliverability questions, so it falls short of a 5.

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