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

Annotations carry only the generic safe-read profile, and the description adds substantive behavior beyond them: provider fallback ordering, five-minute evidence expiry, the meaning of a null premium result ('unconfirmed, not standard tier'), and the fact that provider attempts and check times are returned. These are exactly the traits an agent cannot get from readOnlyHint/idempotentHint.

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?

Very dense and front-loaded, with the cardinality constraint and provider chain leading. The second sentence packs three ideas into one semicolon chain, which is efficient but slightly heavy for a definition of this length.

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 and four parameters, the description carries the return-value burden and does so: it explains that availability and premium facts are returned separately, that premium may be null with a specific interpretation, and that provider attempts and timestamps accompany the result. Nothing needed to call it 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 every parameter (tld, name, fresh, registrar) is already documented in the schema, including the fresh-check cooldown and registrar gating rules. The description adds output semantics rather than parameter meaning, 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 first sentence states a specific verb and resource with an explicit cardinality constraint ('Check one exact domain'), which separates it cleanly from the batch sibling. It also names the internal provider chain (Cloudflare, fallbacks, registrar), so the agent understands what is actually being queried.

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 clearly bounds usage by ruling out adjacent tools ('No price lookup, purchase or reservation') and states the flow condition ('a taken name ends the chain'), which is real routing guidance. It never names a sibling tool explicitly, so an agent must infer that check_availability_batch handles multiple names, but the context is unambiguous.

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