billing_get_last_month_summary

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

With no annotations provided, the description must carry the burden of behavioral disclosure. It only states that a summary is returned; it does not mention that this is a read-only call, how 'last month' is defined, what timezone or currency is used, or whether any authentication or account context is required.

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 short and front-loaded with the core purpose. The 'Returns' line adds the new detail 'with service breakdown', but it also repeats 'last month usage summary', creating slight redundancy rather than fully earning its place.

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 tool with an output schema, the description is close to adequate: it states the resource and the returned breakdown. However, it leaves 'last month' ambiguous and gives no sense of how this tool relates to other billing summary tools, so an agent may still be unsure when to call it.

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 and the schema description coverage is 100%, so there are no undocumented parameters for the description to clarify. This earns the baseline score of 4 for a no-parameter tool.

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 uses a specific verb and resource ('Get last month usage and cost summary') and adds that the result includes a service breakdown. It is clear about what the tool returns, though it doesn't explicitly distinguish itself from billing siblings like billing_get_current_month_summary or billing_get_monthly_usage_summary.

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 about when to use this tool versus nearby alternatives such as billing_get_current_month_summary, billing_get_cost_breakdown_by_service, or billing_list_billing_history. There are no exclusions, prerequisites, or decision rules to help an agent select it confidently.

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