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

Annotations already signal readOnlyHint=true and destructiveHint=false, so the description's value is in additional behavioral context: it explains the ratioVsMean semantics, the direction of 'better/worse' for cost metrics versus CTR, and the possible 404 pilot-enrollment condition. This goes beyond the structured data, though it stops short of fully describing the response 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?

The description is dense but purposeful: it front-loads the core comparison purpose, then explains metric interpretation, follows with a dependency note, and closes with an error-handling hint. Every sentence earns its place, and the structure guides an agent from selection to invocation to troubleshooting.

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?

Given there is no output schema, the description compensates well by naming the returned metrics (costPerResult, CPM, frequency, CTR), defining ratioVsMean, and explaining how to interpret ratios. It also covers the key prerequisite and a common failure mode, making it largely sufficient for correct invocation and result interpretation.

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 the baseline is 3. The description repeats the adGroupId sourcing guidance already present in the schema but does not add new parameter-level detail about from/to defaults or provider values, which the schema already documents.

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 opens with a specific verb and resource: 'Compare an ad group's performance metrics against the mean of its campaign sibling ad groups.' It clearly distinguishes this tool from related siblings like nevent_get_paid_ad_group_insights by framing it as a comparative sibling analysis rather than a plain metrics fetch.

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 description gives explicit use context ('Use to detect underperforming ad sets'), a dependency guideline ('Use after nevent_list_paid_ad_groups to get a valid adGroupId'), and a troubleshooting path for 404s ('call nevent_paid_ads_health to confirm'). It does not explicitly state when not to use it or name an alternative tool, so it falls just short of a 5.

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