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

Annotations already declare readOnlyHint=true, so safety is covered structurally. The description adds genuinely useful behavior beyond that: the caching note ('responses can be cached; they rarely change'), which informs an agent that re-fetching is usually unnecessary. It stops short of describing cache invalidation or return shape, but the output schema covers the latter.

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?

Three tight paragraphs: what it returns, when to use it with the alternative named, and the anti-guessing/caching caveat. Every sentence earns its place and the most decision-relevant content (purpose plus usage) is front-loaded.

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 one-parameter discovery tool with a full output schema, annotations covering safety, and 100% parameter coverage, nothing an agent needs in order to call it correctly is missing. Discovery-before-query guidance is exactly the context this tool needs.

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 100% with a single well-described resource_name parameter, so the baseline is 3. The description reinforces the parameter's domain (resource names like 'campaign', 'ad_group') but largely repeats schema content, and the metrics./segments. prefix note describes output fields rather than the input parameter.

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?

States a specific verb (retrieves) and resource (selectable/filterable/sortable fields for a Google Ads resource), plus the scope of what's returned (compatible metrics and segments). An agent can distinguish this discovery/meta tool from the sibling query and mutation tools at a glance.

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?

Explicitly says when to use it ('to find out which fields you can select, filter by, or sort by ... with the `search` tool') and adds a clear directive not to guess fields before constructing a query. The alternative path (the `search` tool) is named, so routing is unambiguous.

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