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

Annotations already declare readOnlyHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description adds genuinely useful non-schema context: the $0.006 per-request billing and the shape of the returned profile data. It does not mention rate limits or error behavior for missing accounts, so it falls short of a 5.

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?

Three tight sentences, each earning its place: purpose, return payload, alternative tool, and price. Front-loaded with the core action, no filler or restatement of the title.

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 compensates by listing the returned fields, and annotations cover the safety profile. Cost disclosure plus sibling routing make this complete enough for an agent to select and call it correctly.

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?

Single parameter with 100% schema description coverage ('Twitter username (without @, max 50 characters)'), so the schema already carries the syntax constraints. The description only restates 'by username' and adds no format or edge-case detail beyond the schema — the baseline 3 for full coverage.

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 ('Get a Twitter/X user's profile by username') and enumerates what comes back (display name, bio, follower/following counts, verification, metadata). It is readily distinguishable from search_twitter_users, which it names explicitly.

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?

Gives the exact lookup key (username) and routes the other case to the correct sibling: 'Use search_twitter_users to find an account by keyword.' The when-to-use/when-to-use-something-else pairing is explicit rather than implied.

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