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

Annotations already declare read-only, idempotent, non-destructive behavior, and the description adds substantial context beyond that: data provenance ('registry-derived list ... refreshed daily'), latency ('answers in milliseconds'), response structure ('split into available and taken, plus an unknown list'), and a critical interpretation rule ('never report those as available'). No contradiction with readOnlyHint=true, since checking availability is a pure read.

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?

Four sentences front-load purpose ('Check whether names are available') and progress logically through provenance/performance, default behavior, and the 5,000 cap. Every sentence carries operational load — the unknown-list warning, the defaults, and the limit — with zero filler or repetition of schema content.

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 supplies the return structure (available/taken/unknown) and the interpretation rule for unknown entries. Annotations cover the safety profile, and the description covers defaults, limits, and result semantics, so 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 coverage is 100%, so the baseline is 3; the description adds genuine value with the cross-parameter constraint 'Names x extensions must not exceed 5,000 in one call — 25 names ... at most 200 extensions,' which no single parameter schema can express. It also reinforces the tlds default behavior, though the schema already documents that.

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 opening sentence states a specific verb and resource: 'Check whether names are available, across many extensions at once.' This distinguishes it from siblings like check_usernames (username availability) and generate_domains (name generation), and the 'This is the main tool' positioning reinforces its role as the primary domain-availability lookup.

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?

The description provides clear operational guidance: omit tlds to fall back to the caller's favorites or most popular extensions, respect the 5,000 names×extensions cap, and treat the unknown list as non-available. It positions this as 'the main tool' but does not explicitly name alternatives or state when-not-to-use conditions relative to siblings like check_usernames, so it stops just short of full exclusion guidance.

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