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

Annotations already declare readOnlyHint, idempotentHint, non-destructive, and closed-world, so the safety profile is covered. The description adds genuine context beyond that: the values are authoritative invoice-matching charges, itemized by SKU line item, and the past-month-only scope. It doesn't discuss latency, API-key requirements, or data freshness, which keeps it short of a 5.

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 core purpose, then argument behavior, then return shape. Every sentence is relevant, though the sentence enumerating `totalUsd`, `sku`, `quantity`, `unit`, and `usd` partially duplicates the existing output schema and could be trimmed.

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 single-optional-parameter read tool with full annotation coverage and an output schema, the description supplies everything an agent needs: default behavior, accepted format, and the authoritative nature of the numbers. Repeating return fields is minor redundancy rather than a gap.

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 coverage is 100% and the single `month` parameter already documents the YYYY-MM pattern and the omit-for-current default. The description restates the same format and adds one concrete example (`2026-09`), plus the implicit constraint that only past months are queryable, but that is marginal value over the schema. Baseline 3 is appropriate.

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: 'See the real cost of calls on this account', itemized by provider calls, browser sessions, and IP leases. It explicitly contrasts with an implied alternative (estimated costs) by asserting this is the EXACT amount matching the invoice, so an agent can recognize this is the billing/cost tool and not a usage-metrics or reporting sibling.

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?

Gives concrete invocation guidance: omit arguments for the current month, pass `month: 'YYYY-MM'` for a past month. It does not name a sibling alternative or exclusion, but no sibling in the list offers cost data, so the routing decision is essentially unambiguous.

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