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

Annotations establish read-only, open-world, non-destructive behavior, and the description adds crucial operational context absent from annotations: paid per call, charged against SingIt limits in USDC, and limited to Base accounts for now. That materially informs an agent before invocation.

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 short sentences, front-loaded with the purpose, then use case, then cost/account caveat. No filler; each sentence contributes selection or invocation context.

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 simple one-parameter read tool with annotations covering safety, the description includes source, cost, and account restriction. It does not describe the profile fields returned, which would help an agent know what to expect since there is no output schema, so it falls just short of fully complete.

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?

The single username parameter is fully described in the schema, including the 'without @' format. The description only restates lookup by username and adds no syntax or semantic detail beyond the schema, so the baseline of 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 exactly what resource is returned (public profile data for an X/Twitter account), the lookup key (username), and the upstream API. It also frames the practical intent (check who is behind a handle), so an agent can distinguish it from unrelated price/news/search siblings.

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 a concrete use case: checking who is behind a handle. It does not spell out when not to use it or name alternatives, but among the listed siblings there is no competing profile-lookup tool, so the missing exclusion is minor.

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