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

Annotations cover the read-only/open-world profile, and the description adds genuinely non-obvious behavior: the $0.02 charge only on success, free bad input and not-found results, and the x402/USDC-on-Base payment passed via _meta["x402/payment"]. It omits things like latency or freshness of the data, keeping it 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?

Purpose and target come first, followed by the returned fields, then the billing/error-cost rules – every clause carries information. The field list is long but earns its place because there is no output schema.

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 lookup with no output schema, the description supplies the return fields, cost model, charge conditions and the payment transport. Only minor gaps remain, such as failure modes beyond 'not found'.

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 handle parameter already documents accepting an @handle or x.com URL, so the description merely restates it. With the schema doing all the work, the baseline 3 applies.

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+resource ('X User Profile Lookup: one public X account by handle or x.com URL') and enumerates the exact fields returned, which distinguishes it from siblings like x_posts and x_search that operate on tweets rather than a single account.

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?

Makes clear the lookup is keyed on a handle or profile URL and that you get one account, not a search. It does not explicitly route the agent to a sibling (e.g., social_search or x_search) when only a display name is known, so the exclusion is implied rather than stated.

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