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

With no annotations, the description carries the full burden and does so: it discloses that lookups are live DNS queries, that the free tier is 5 calls per day across all tools, and the exact prepaid cost (0.005 USDC per call) when passing `key`. Rate limit, cost model, and the no-side-effect read nature are all surfaced.

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 the core question and verdict, then the return inventory, then usage and billing. The enumeration of returned records is dense but every item is relevant signal for a DNS/email-audit tool; no filler sentences.

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?

There is no output schema, so the description must describe returns — and it enumerates them (records plus a spoofable verdict and findings). It also covers cost, rate limits, and the domain parameter semantics, leaving nothing an agent needs to call it correctly.

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 the baseline is 3; however the description adds real meaning about `key` that the schema does not — that it draws on a prepaid balance at 0.005 USDC per call, versus the 5 free daily calls. That is added value beyond the schema's 'Optional API key from create_key'.

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?

It poses a concrete question ('Can mail from this domain be spoofed?') and then names the exact resources returned (MX, SPF strictness, DMARC, DKIM, MTA-STS, TLS-RPT, BIMI, spoofable verdict). An agent can distinguish this from siblings like check_domain or tls_certificate 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?

It gives two clear use cases: judging a sender's domain and auditing your own before sending. It stops short of naming alternatives (e.g., check_domain or tls_certificate) or stating when not to use it, so it is clear guidance without explicit routing.

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