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

Adds substantial behavior beyond the annotations: Cloudflare-first with configured fallback ordering, a short-circuit on taken names, nullable premium semantics ('null premium is unconfirmed, not standard tier'), and the evidence TTL of 5 minutes / 30 days for taken names. None of this is derivable from the schema or the hint flags.

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?

Purpose is front-loaded in the first sentence and every following clause carries real information (fallback order, null semantics, TTL, exclusions). It is dense with semicolons and slightly harder to scan than it needs to be, but there is 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?

There is no output schema, yet the description covers the return surface itself: separate availability facts, nullable premium, provider attempts, and check times. Combined with the recheck/expiry guidance and the explicit scope exclusions, nothing an agent needs to call this correctly is missing.

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 schema already documents all four parameters in detail, including the fresh/pending-claims/cooldown behavior. The description corroborates 'optional selected registrar' and recheck timing but adds no new syntax or format detail, 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?

States a specific verb and resource with scope: 'Check one exact domain.' The word 'one' and the paired sibling check_availability_batch make the single-vs-bulk distinction immediately legible, and 'exact' signals no fuzzy matching.

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?

Gives concrete conditions: fallbacks run 'only for missing facts', 'a taken name ends the chain', recheck after expiresAt, and fresh only 'when the user asks'. The closing 'No prices, purchase or reservation' implicitly routes price/purchase intent elsewhere, but no sibling is named explicitly, so it stops short of a 5.

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