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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false. The description adds meaningful behavioral context beyond this: it queries a live registry rather than a dictionary, normalizes arbitrary input forms, converts Cyrillic to punycode, and defines fallback behavior when no zone is supplied. It does not cover rate limits or exact return shape, but the added context is substantial.

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?

The description is front-loaded with the core purpose and remains compact. Each sentence earns its place by adding operational detail: supported zones, accepted input formats, normalization rules, and no-zone fallback behavior.

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?

For a simple one-parameter, read-only availability check, the description covers purpose, input normalization, zone coverage, and fallback behavior well. The main missing piece is the exact return format or error behavior, which is more important here because no output schema exists.

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 schema already documents the single domain parameter. The description still adds value beyond the schema by explaining accepted input forms (URLs, any casing/format, Cyrillic), automatic stripping of extraneous parts, punycode conversion, and the no-zone behavior that checks .ru and popular zones.

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 description clearly states the specific task: determine whether a domain is free or taken via a real registry query, not a dictionary. It names the resource and scope, but does not explicitly differentiate from the sibling check_domains tool, which likely handles multiple domains.

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 purpose, and the description gives useful input conditions such as any zone being accepted and the no-zone fallback behavior. However, it does not state when to use this tool versus alternatives like check_domains or whois_domain, nor does it give exclusions.

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