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

With no annotations, the description carries the full burden and does a decent job: it declares the operation read-only, states that it 'sends nothing' (no side effects or data egress), and flags the auth requirement ('needs a free API key'). Missing are finer traits such as caching/freshness of the 7-day window or any rate limits.

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 compact sentences: the first front-loads what is returned, the second covers the safety/auth profile. Every clause earns its place with no filler.

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?

With no output schema, the description compensates by enumerating the returned sections, and it closes the annotation gap with read-only/auth statements. Minor gaps remain: the boundary definition of the 7-day window (rolling vs. calendar, timezone) is unstated.

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 takes zero parameters, so there is nothing to disambiguate and the baseline is 4. The description correctly spends no words on input syntax.

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?

Concrete verb+resource: a plain-English summary of the last 7 days of spend, with the exact contents enumerated (total, top runs, tools, models, busiest day). The fixed 7-day scope implicitly separates it from siblings like cost_report, but no sibling is named, so the differentiation is left to inference.

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?

Usage is implied by the name and the 'last 7 days' framing, but the description never states when to prefer this over cost_report or usage_and_upgrade, nor any when-not condition. It is adequate routing-wise only because the scope is narrow and obvious.

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