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

With no annotations provided, the description carries the full burden. It usefully discloses pricing ($0.02 per call), the payment mechanism (clink shop credits), how to obtain a key, and the failure policy (failed or empty calls are free). This is meaningful behavioral context beyond the schema.

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?

Two tight sentences: the first states the purpose and return fields, the second covers cost and failure policy. No filler, and the most important purpose information is front-loaded.

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 lookup tool, the description is largely complete: it lists all returned data fields, explains cost and failure behavior, and the schema covers the required input. It could optionally mention whether to include the '@' prefix, but the example makes this clear.

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 parameter 'username' is already fully documented in the schema with a clear example ('elonmusk'). The description adds little param-specific meaning beyond saying 'one handle', so the baseline 3 applies due to high schema 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?

The description opens with a specific verb and resource: 'X/Twitter profile lookup'. It enumerates the exact fields returned, which clearly distinguishes it from sibling tools like x_search and x_digest that cover posts or market-specific data.

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 when to use the tool — when you need profile metadata for a single handle — but it does not explicitly state when to prefer it over alternatives like x_search. It also provides practical prerequisite information about credits and keys, but no explicit exclusion guidance.

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