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

Annotations already declare this a safe read-only operation. The description adds useful behavioral context by explaining that it returns current metrics, previous metrics, and percentage deltas. However, it does not disclose how periods are bounded, what happens with missing dates, or which metrics are included, so transparency is decent but not rich.

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 with no filler. The core action is front-loaded, and the return shape is summarized in the second sentence. Every word earns its place.

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 tool with four parameters and no output schema, the description is too thin. It does not state what metrics are compared, which parameters are required for a valid call, or what the response looks like beyond three broad categories. An agent would struggle to invoke this tool confidently with only this description.

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 only 50%, with start_date and end_date completely undocumented. The description does not compensate: it never mentions these parameters, their formats, how they interact with period_days, or whether they are required. The phrase 'previous period of equal length' hints at the logic but does not clarify parameter usage.

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 uses a specific verb ('compare') and names a distinct resource ('current period metrics' vs 'previous period of equal length'), which clearly separates it from siblings like ppc_daily_trend or ppc_account_summary. It also states the output categories, so an agent knows exactly what this tool is for.

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 guidance about when to choose this tool over alternatives, nor any mention of when not to use it. While the description implies period-over-period analysis, it does not reference sibling tools such as ppc_daily_trend or pnl_period_cards, leaving the selection decision to inference.

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