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

Annotations already declare readOnlyHint and openWorldHint, but the description adds substantial behavior: the default "common" view's exact coverage, that unqueried types surface under not_queried and must be requested explicitly, that results are untruncated by default with ASN/geolocation/TTLs, and that data comes live from public resolvers. It also tells the agent to relay records verbatim rather than paraphrase — a presentation rule not encoded anywhere else.

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-loaded with purpose, then default behavior, then usage boundaries, then an example — a sensible ordering. It is dense but every clause carries information; the parenthetical record list and repeated "COMPLETE/untruncated" emphasis are slightly redundant but justified as guardrails against summarization.

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?

For a 4-parameter, fully-documented read-only lookup with an output schema, the description covers scope, defaults, exclusions, presentation expectations, and a bulk escape hatch. Nothing an agent needs in order to call it correctly or interpret the default output 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?

Schema coverage is 100%, so baseline is 3; the description goes beyond by explaining the semantic difference between type="common" and an explicit single type (full untruncated set), and between detail="full" vs "concise" (curated view, summarized TXT sets). The resolver parameter is left to the schema, and a worked example reinforces the type parameter, keeping it just short of a 5.

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 ("Look up live DNS records for ONE domain") and immediately scopes it to a single domain. It explicitly contrasts itself with the sibling email_security_check and names the bulk-API alternative, so an agent can distinguish it from every sibling 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?

Gives an explicit trigger ("whenever the user asks about a domain's DNS records, name servers, IP addresses, or dig/nslookup-style questions"), an explicit exclusion ("Do NOT use this to grade email security... call email_security_check"), and a fallback for bulk lists. Both when and when-not are covered with named alternatives.

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