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

With no annotations provided, the description fully discloses behavior: queries Cloudflare DoH, follows CNAME aliases, 5s timeout, and returns a CheckResult discriminated union with success/error details. This goes beyond what annotations would typically provide and leaves no ambiguity about execution or results.

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?

Two dense sentences pack purpose, usage context, alternative, implementation details, and return format with no wasted words. Information is front-loaded: the core action comes first.

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?

Despite having no output schema or annotations, the description fully specifies the return shape ('on success, {status:"ok", records:[{exchange, priority},...]}...; on failure, {status:"error", reason}') and covers the tool's role among siblings. All necessary context is present.

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?

The input schema covers the 'domain' parameter with a detailed description (public FQDN, rejections of IPs/ports/paths). The tool description adds no further parameter-specific semantics beyond restating 'domain' in the context of MX lookup, so the baseline of 3 for high schema coverage 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?

Description opens with a specific action: 'Look up a domain's MX (mail exchanger) records and return them sorted ascending by priority.' It clearly identifies the resource (domain MX records) and output format, and implicitly distinguishes from siblings by naming dns_lookup and other dossier checks.

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?

Provides explicit use cases: 'Use when verifying inbound-mail routing or as a precursor to SPF or DMARC checks.' Also states a clear alternative: 'prefer dns_lookup with type=MX if you only need the raw DNS answer without the ranked view.'

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