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

Annotations declare readOnlyHint=true, and the description adds non-obvious behavior: the 31-day maximum range and default window, plus the shape of each returned row and the volume exception. It doesn't mention pagination or rate limits, which keeps it short of a 5 given annotations carry the safety profile.

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, front-loading the purpose before the range constraints. The trailing 'Civo: GET /v2/charges' is a minor extraneous API-pointer that an agent doesn't need, but it costs little.

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 two-param read tool with no output schema, the description supplies the returned-field inventory, the default window and the range cap, which is enough to call it correctly. Only return-size/pagination behavior is left unspecified.

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?

Schema description coverage is 100%, so both parameters (from/to, RFC 3339 with pattern) are already fully documented. The description restates the defaults in prose but adds no format or syntax detail beyond the schema, matching the baseline 3.

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+resource ('List billable usage for a period') and enumerates the returned fields (product code, label, region, time span, hours, size_gb for volumes). No sibling tool covers billing/charges, so the agent can immediately distinguish it from the instance/firewall/DNS list 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?

Describes default behavior (from = start of current month, to = now) and the 31-day cap, which tells the agent how to call it in the common case. However, it gives no explicit when-to-use or when-not-to-use guidance against alternatives, and the usage is only implied by the defaults.

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