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

The annotation readOnlyHint=true already declares the read-only nature. The description adds value by noting 'It's free,' explaining the consequence of running out ('paid calls are turned away until you top up'), and clarifying that deposit addresses cannot be added via this tool (use the dashboard). These details go beyond the annotation and set correct expectations about side effects and costs.

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 informative but not overly verbose. It front-loads the primary purpose and then lists specific data elements, followed by essential usage notes. Each sentence contributes: the 'free' note, the consequence of zero balance, and the dashboard redirect for deposits. It's slightly long but efficiently organized, with no redundancy.

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 balance check with no output schema, the description is fully sufficient. It enumerates what information will be returned (credit, hold, bonus, expiry, pause status, pricing, deposit addresses), clarifies the cost, and explains the behavioral outcome of insufficient funds. An agent has everything needed to decide when to call and what to expect, without needing additional schema or annotations.

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 has zero parameters, so schema coverage is trivially 100%. The description does not need to explain any parameters, and it doesn't. Per the rubric, a baseline of 4 is appropriate for 0-parameter tools; the description adds no parameter-specific information because none is 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 opens with a specific verb and resource: 'Check your prepaid balance before a paid call.' It clearly enumerates the exact data points returned (credit, hold, bonus credit, expiry, pause status, pricing, deposit addresses), which distinguishes it from siblings like system_get_usage (usage) and system_get_notifications (notifications). An agent can immediately understand the tool's scope.

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?

It explicitly states when to use it: 'before a paid call.' It also explains the consequence of insufficient balance and instructs that adding a deposit address requires the web dashboard, implying this tool is read-only. While it doesn't name alternative tools explicitly, the usage context is clear and provides practical guidance on when to invoke this tool versus external actions.

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