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

Annotations already establish the safety profile (readOnly, idempotent, openWorld, non-destructive), so the bar is lower; the description nonetheless adds real behavioral context: SPF consumes the 10-lookup limit, MTA-STS fetches a remote policy file and compares it against MX hosts, and BIMI validates logo/certificate. This discloses network-fetch side effects and per-type semantics beyond the annotations.

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 purpose sentence followed by a compact, information-dense mapping of each record type to its check. No filler, though the second sentence is long and comma-heavy.

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 read-only lookup tool with no output schema, the description supplies enough per-type semantics for an agent to understand and interpret the call. The only gap is the absence of explicit routing guidance relative to the sibling tools.

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 75%, and the description meaningfully enriches the enum values by explaining what each record type actually validates (policy, reporting, key size, logo, etc.). It does not add format detail for the optional policy or selector params, but the schema already documents those.

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 pair (look up and validate) plus a precisely scoped resource (a single email DNS record for a domain), and then names all seven record types. Combined with the 'single' scope, an agent can distinguish this from the sibling check_domain without opening any 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?

Usage is implied by the enumeration of record types, but there is no explicit guidance on when to pick this over check_domain, check_blacklists, or analyze_headers. The agent must infer that this is the per-record deep-dive tool.

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