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

Annotations already communicate read-only, non-destructive behavior. The description adds a small but useful nuance by saying the balance is 'derived' and scoped to 'this caller_id,' and it exposes the GET endpoint. It does not disclose additional behavioral traits like rate limits or error conditions, but the annotation coverage lowers the bar.

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 description is a single, tight sentence with no filler. The most important information ('derived prepaid balance') appears first, and the endpoint reference is included compactly.

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, the description gives the essential purpose and endpoint. However, there is no output schema and no mention of the response format or whether the balance is current/pending, so an agent still has some ambiguity about what exactly will be returned.

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 input schema has zero parameters, so the baseline is 4. The description still adds meaning by clarifying that the balance is implicitly tied to the authenticated caller_id, which helps explain why no explicit parameters are needed.

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 states a specific resource ('credits/balance') and the result ('derived prepaid balance for this caller_id'). It is clear enough to be distinguished from most siblings, though it does not explicitly contrast with similar credit-related tools like credit_events or list_credit_packs.

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?

No guidance is given on when to use this tool versus alternatives. The description does not mention when to prefer credit_balance over credit_events or list_credit_packs, nor does it state any prerequisites or exclusions.

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