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

Annotations already cover readOnly/openWorld, and the description adds real context beyond them: the upstream provider (FxTwitter, an open-source relay), the cost model ($0.002 USDC per successful call via x402) and the fact that failed calls are not charged. It stops short of rate limits, auth mechanics, or data freshness.

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 purpose and pricing are front-loaded, which is good, but the middle of the description is a long enumeration of every returned field (followers, following, posts, likes, media count, avatar, banner, verification type...) that duplicates the existing output schema and bloats the definition.

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 an output schema present and annotations covering safety, the description focuses correctly on source, cost, and failure handling. Only minor gaps remain: no rate-limit or freshness guidance, and no explicit displacement of the sibling X tool.

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% and the schema already documents the @-optional, URL-accepted format of usernames. The description only reinforces the count ceiling (1–10) and the notFound outcome, adding modest value over the structured field.

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 — fetching public X/Twitter profiles — with a precise scope of up to 10 usernames and a named data source (FxTwitter). It does not, however, differentiate itself from the sibling social_x_com, so an agent must infer which X tool to pick.

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 implies batch usage (up to 10 usernames) and tells the agent that unknown or suspended accounts land in notFound, which is useful boundary guidance. It never states when to prefer a sibling such as social_x_com or another profile source, and offers no exclusions.

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