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

Annotations cover safety (readOnly, idempotent, non-destructive) but the description adds substantial non-obvious behavior: it is a paid x402 call at $0.005, it returns the x402 request rather than the data itself, the caller must supply an x402 or MPP client, and bad input is refused before any charge. This is exactly the kind of cost/payment/refusal context annotations cannot convey.

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 usage trigger, then contents, then the critical payment/return mechanic, then sibling routing. Dense but every clause carries decision-relevant information; no filler.

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 must explain what comes back, and it does: the exact paid x402 request (GET /domain/report, $0.005 per call) plus response schema and example, to be replayed with an x402/MPP client. An agent has everything needed to invoke it correctly and understand the two-step flow.

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 description coverage is 100%, so the domain parameter, its URL-accepted behavior, and its refusal of IP/private names are already fully documented in the schema. The description adds no additional syntax or format detail beyond noting valid input is required, so the baseline 3 is appropriate.

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 states a specific resource and scope: 'one report on a domain' enumerating DNS records (A, AAAA, MX, NS, TXT, CAA) plus SPF/DMARC parsing and RDAP registration data. It explicitly differentiates from siblings by naming domain_report_batch and check_email_address, so an agent can pick it 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 a direct when-to-use trigger ('when you need one report on a domain') and routes to alternatives by condition: 'For several domains use domain_report_batch; for a single mailbox use check_email_address.' Nothing is left to inference.

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