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

Annotations already declare readOnly, idempotent, openWorld and non-destructive behavior, so the description focuses on value-add context: output contents, a per-successful-call price ($0.005 USDC via x402/Solana), and an equivalent HTTPS endpoint. It does not cover failure/cost handling beyond 'per successful call' or rate limits, keeping it below a 5.

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?

One dense sentence front-loads the purpose and enumerated outputs, followed by two short sentences for pricing and the HTTPS alternative. No filler; every clause carries information.

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 usefully lists returned data (MX records, provider inference, A/AAAA, SPF presence, null-MX) and states cost and an alternate transport. It omits domain input format and error/payment-failure behavior, which are the remaining gaps for a paid, one-parameter lookup.

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 0% for the single required 'domain' parameter, so the description must compensate but does not. It never states that the domain should be a bare registrable domain (no scheme/path), whether IDNs or subdomains are accepted, or any format constraints.

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 gives a specific verb+resource (MX / mail-routing check) and enumerates concrete outputs: sorted MX, inferred provider, A/AAAA, SPF presence, null-MX detection. It signals a relationship to email-auth ('Complements /v1/email-auth') but does not name a sibling tool or sharply delineate scope, 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?

The phrase 'Complements /v1/email-auth' implies this tool handles mail-routing while email auth handles SPF/DKIM/DMARC depth, but there is no explicit when-to-use, when-not-to-use, or named alternative such as email_auth or dns_dump. Usage is inferable but not stated.

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