Estimate audience size

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

Annotations already declare readOnlyHint and openWorldHint, so the read-only behavior is covered. The description adds 'Does not create anything' and 'before spending' context, but these largely confirm the annotations rather than revealing new behaviors such as result limits or data freshness.

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 sentences with no filler; the key purpose is front-loaded and the cross-reference to add_targeting is necessary context, not fluff.

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 low complexity, two parameters, and helpful annotations, the description is nearly complete: it names the return concept ('reachable unique users') and references the criteria shape. It lacks a concrete output format, but that is a minor gap for an estimate tool.

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 50 percent: account_id is documented, criteria is not. The description adds value by calling criteria a 'targeting set' and referencing x_ads_add_targeting for shape, but it does not explain the semantic meaning or allowable values beyond the schema.

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?

Description states a precise verb and resource: 'Estimate reachable unique users for a targeting set' and explicitly scopes it 'BEFORE spending.' It also disambiguates from mutation siblings by adding 'Does not create anything.'

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 a clear intended use context ('BEFORE spending') and points to x_ads_add_targeting for criteria shape. It does not explicitly name alternative tools or exclusions, but the context is sufficient for selection.

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