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

readOnlyHint=true already declares the safety profile, so the description need not restate read-only semantics. It does add genuinely useful behavior context beyond annotations: the account permission required to read billing, and the exact response shape (per-resource fields plus totals).

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 with the resource and its contents in a single dense sentence; the prerequisite follows. The trailing raw HTTP endpoint ('GET /1.3/account/billing/summary/{yearmonth}') earns little for an agent choosing a tool, slightly diluting an otherwise tight definition.

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-parameter read-only tool with no output schema, the description carries the needed return-value burden by enumerating the per-resource fields (amount, hours, plan, zone, labels) and totals, and covers the auth prerequisite. Nothing essential to call it correctly is missing.

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 yearmonth (with its YYYY-MM pattern) and the optional resource_id filter are fully documented in the schema. The description only reinforces the one-month scope and hints at per-resource filtering without adding format or edge-case detail, so baseline 3 applies.

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 ('Billing summary for one month') plus the exact content it returns (per-resource details and totals). No sibling tool covers billing, so there is no ambiguity an agent needs resolved.

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?

The one-line prerequisite ('Needs an account allowed to read billing') tells the agent when the call is valid, but there is no explicit when-to-use/when-not guidance or comparison to alternatives. Usage is only implied by the scope of the returned data.

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