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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds a genuine prerequisite ('Needs the app's key'), but it does not say how that key is supplied, since the schema has no key parameter, nor does it describe aggregation granularity or return format.

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 short, front-loaded sentences with no padding. The second sentence is terse to the point of ambiguity, but no sentence is wasted.

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 simple read-only aggregation with a single fully documented parameter, the description is close to adequate, but with no output schema it should still convey what the breakdown contains (per-model and per-feature costs, units) and where the required app key comes from.

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?

Only one parameter exists and its schema description coverage is 100%, so the schema already explains 'days' including its default and bounds. The description's 'over a period' restates the same idea without adding syntax or edge-case meaning.

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 names a specific resource and split ('AI cost ... split by model and by feature'), which distinguishes it from pricing/estimation siblings like estimate_llm_cost and get_llm_price. It is clear what is returned conceptually, though it never says whether the cost is in currency units or tokens/credits.

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?

There is no explicit when-to-use or when-not-to-use guidance and no named alternative. The phrase 'over a period' weakly implies historical actuals versus the estimation/pricing siblings, but the agent must infer that routing itself.

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