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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safe-read profile is covered. The description adds genuinely new context: the validation uses the same rules and messages as Edit Profile, which tells the agent that errors surfaced here will match the profile-editing flow. It does not mention rate limiting or response shape, so it is not exhaustive.

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?

A single sentence that front-loads the verb and resource and ends with the routing hint. Every clause earns its place; there is no filler or restatement of the tool name.

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-only validator with no output schema, the description covers what is checked and when to call it. It could state more explicitly what the result distinguishes (invalid vs taken vs available), which matters since there is no output schema to fall back on, but the essentials are present.

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?

One parameter with 0% schema description coverage, so the description carries some burden. Writing '@handle' implies the input format includes the leading @ and that the value is a handle string, but it gives no length, character-set, or normalization guidance beyond the schema's maxLength=64. Useful hint, not full compensation.

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 (check) and resource (@handle) plus the two properties being verified: validity and availability. It also names the sibling it precedes (set_username), so an agent can separate it from set_username and search_users without opening any schema.

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?

Explicitly frames the use case as a pre-flight check 'before set_username', which tells the agent when to reach for it and which alternative it feeds into. It stops short of stating when not to call it (e.g. that it is unnecessary if the handle is already known valid), so it is clear context rather than a full decision rule.

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