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

Annotations already declare this a safe, idempotent, non-destructive, closed-world read, so the safety profile is covered. The description does add real value by disclosing the return shape { hostname, available, reason } (there is no output schema) and naming two unavailability causes. It stops short of enumerating possible 'reason' values or any rate-limit/error behavior.

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?

Two sentences, zero filler. The core action leads, and the return shape plus unavailability rules follow in priority order.

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 one-parameter read tool with full annotation coverage, this is nearly complete: it explains what is checked, what comes back, and the two known unavailability causes. The only meaningful gap is that the possible values of 'reason' are not described, which matters since 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 description coverage is 100% and the single hostname parameter already carries an example format, so the schema does the heavy lifting. The description only restates that a hostname is being checked, adding no syntax or normalization guidance beyond the schema.

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: 'Check whether a hostname is available to register as a custom domain.' An agent can immediately tell this is an availability probe rather than a mutation. It does not explicitly name the sibling it pairs with (add_domain/activate_domain), so it falls short of a 5.

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 only implied: the tool is clearly a pre-registration check, but the description never says 'call this before add_domain' or excludes any scenario. The note that reserved awsys.co and already-registered domains are unavailable gives useful context but is a result rule, not a when-to-use rule.

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