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

Annotations already declare readOnlyHint, openWorldHint and destructiveHint=false, so the safety profile is covered and the description's 'Read-only' is largely redundant. It does add genuinely non-obvious behavior: the result is also rendered as a visual card and the default period is last 30 days, both of which affect how the agent should present output.

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?

Four short sentences, front-loaded with the use case and return shape, with no filler. The 'Read-only' clause is the only near-redundant sentence against the annotations.

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 7-parameter, zero-required read tool with no output schema, the description gives enough: what is returned per campaign, the campaign-type scope, the default window, and the card rendering. Nothing critical for correct invocation is missing, though pagination or row limits are unaddressed.

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 enum semantics for detail, date_range, language, and the account/folder and start/end date split are already documented. The description adds only the default period, which the schema also states, so the baseline 3 applies.

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 states a specific verb and resource — returns unique users reached and average impressions per user, per campaign — and scopes it to Display, Video and Demand Gen campaigns, which separates it from generic tools like google_ads_performance or google_ads_breakdown. It does not name a specific sibling to avoid, so it stops short of a 5.

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?

It clearly frames the triggering context (reach questions on Display/Video/Demand Gen campaigns) and supplies a default time window, so an agent knows when to pick it. It offers no explicit exclusions or named alternatives for overlapping questions such as frequency on Search campaigns.

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