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

Annotations already declare readOnlyHint and openWorldHint, but the description adds genuinely new operational context: deterministic/no-AI behavior, per-call cost ($0.002 USDC via x402 on Base), and a free-tier rate limit of 5 calls/day/client. Cost and rate-limit disclosure is exactly the kind of behavior structured annotations cannot carry.

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 purpose, then the field list, then applicability, then pricing — a sensible priority order with no filler sentences. The field enumeration is dense but each item earns its place since there is no output schema.

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 compensates by enumerating return fields and even the derived status vocabulary (ok/expiring_soon/expired), plus cost and coverage limits. An agent has everything needed to decide to call it and to interpret the result.

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?

Only one parameter and schema description coverage is 100%, so the schema already fully documents `domain` (including an example). The description adds no format, normalization, or edge-case guidance (e.g., IDN, subdomains) beyond what the schema provides — baseline 3.

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?

States a specific resource (domain registration data) and source (registry RDAP server via IANA bootstrap), then enumerates the exact returned fields: registrar, dates, EPP status, nameservers, DNSSEC. This is unmistakably distinct from the email-authentication siblings (dmarc_check, spf_lookup_count, email_auth_report), which operate on DNS TXT records rather than registry data.

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?

"Works for gTLDs and ccTLDs that publish RDAP" gives a clear applicability boundary that implicitly excludes non-RDAP TLDs, so an agent knows when this will fail. It stops short of naming alternatives or stating explicit exclusions (e.g., what to do for a TLD without RDAP coverage).

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