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

Annotations already declare readOnlyHint=true and openWorldHint=true. The description adds genuinely useful behavioral context beyond them: this call is free and never charged, which matters for an agent managing a credit budget. It does not discuss rate limits or return format.

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?

Front-loaded and short, with the core purpose in the first sentence. The second line re-states the same 'free' fact already given as 'Always free', which is mild redundancy rather than added information.

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 zero-parameter, read-only tool with annotations covering the safety profile, the description is essentially complete. Without an output schema it could note the return shape (a credit count/number), but 'credits remaining' is sufficiently self-explanatory.

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 tool takes zero parameters, so there is nothing to document beyond the schema. Baseline 4 applies; the description coherently frames what the parameterless call returns.

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 ('Return the credits remaining') and pins the scope precisely to the account behind the API key making the request. This unambiguously distinguishes it from every sibling, all of which are Twitter/X content tools.

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?

Implies the use case clearly (checking your balance) and adds a cost condition ('Always free') that tells the agent it is safe to call without spending credits. It does not name alternatives or exclusions, but no sibling overlaps, so there is little to route against.

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