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

Annotations cover the safety profile (readOnly, idempotent, openWorld, non-destructive), and the description goes well beyond them: per-call cost ($0.005), a free sample invocation, fallback semantics (available=null when only DNS was checked, then likely_available), a source discriminator (rdap/dns), a confidence field, and an explicit caveat field. That is rich disclosure of cost, degradation behavior, and reliability.

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, then mechanism, cost, sample, and return shape in compact order with no filler. The trailing 'Returns:' run-on is dense and slightly hard to parse, which keeps it short of a 5.

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 carry return-value burden and does: it enumerates available, registered_on, suffix, source, confidence, price sub-fields, and caveat, plus the null fallback path. Cost, degradation, and output shape are all covered for a single-parameter tool.

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?

There is a single parameter and schema coverage is 100%, with the schema already stating 'A bare domain name, e.g. example.com'. The description's free sample repeats that format rather than adding new constraints (no IDN/unicode guidance, no subdomain handling), so the schema does the heavy lifting and 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?

States a specific question answered ('Is this domain name registered?') against a named mechanism (registry over RDAP), and adds the differentiating value of first-year/renewal pricing. This separates it from siblings like domain_whois or domain_age, which the RDAP mechanism and availability focus make distinguishable without opening a 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?

Usage is implied by the framing question and the price/caveat fields, but there is no explicit when-to-use versus when-not, and no sibling is named as an alternative (e.g. domain_whois for registrant data). An agent can infer the use case but must guess at the boundaries.

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