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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), and the description adds pricing ($0.005 per call), a no-charge-on-bad-input guarantee, and the two execution modes (return the signed x402/MPP request URL vs. auto-pay with a P0 bearer key and deduct the price). That is substantial behavior beyond annotations, though the mechanics of the key path and error responses are only gestured at.

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 trigger and payload, then payment and alternatives; every sentence carries information. The final sentence is dense, with a long inline purchase URL and parenthetical key format that interrupts reading flow, costing some clarity.

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 single-parameter, paid, open-world lookup with no output schema, the description supplies coverage list, TLD limits, price, payment mechanics, pre-charge validation, and sibling routing — everything an agent needs to call it and interpret the result. The remaining return-shape detail is covered by the referenced schema link.

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% and the schema already documents that a URL is accepted (only the host used) and that IPs and private/local names are refused. The description adds no additional per-parameter meaning beyond restating 'for your input', so the baseline 3 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?

Names a specific resource (one domain report) and enumerates its contents: DNS record types A/AAAA/MX/NS/TXT/CAA with SPF and DMARC parsing, plus RDAP registration data with the exact TLD coverage (.com, .net, .org) and fields. It explicitly distinguishes itself from domain_report_batch and check_email_address, so an agent can pick the right tool without opening the 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?

Opens with an explicit trigger ('Use when you need one report on a domain') and names two alternatives with the conditions that select them: domain_report_batch for several domains, check_email_address for a single mailbox. It also states the fallback path for callers without a paid key, which is effectively a when-to-use branch.

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