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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so the safety profile is covered and the bar is lower. The description adds essentially nothing beyond that and the schema – it does not describe the return shape (e.g. field names, types, whether selectable/compatible flags are included), which would be the useful behavioral context for a metadata tool.

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?

One sentence, front-loaded with the verb and resource, with zero filler. Example resources are embedded compactly rather than listed separately.

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?

With no output schema, the description carries the burden of explaining what is returned, and it only says 'selectable fields' without describing the response structure an agent would need to consume. For a simple two-parameter read-only lookup this is minimally adequate, but it leaves the return contract underspecified.

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%, so both parameters are already documented, and the schema even provides examples for each. The description merely echoes example resource names (campaign, ad_group, keyword_view, search_term_view) without clarifying value formats, id requirements, or constraints beyond what the schema states.

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 and resource ('Describes selectable fields of a Google Ads resource') and ties it to GAQL query building, which is distinct from the reporting siblings. It does not explicitly name the complementary tool (ads_query) that would run such a query, so sibling differentiation is implied rather than stated.

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 phrase 'to help build GAQL queries' implies the general usage context, but there is no explicit when-to-use statement, no call out of the alternative (ads_query), and no preconditions mentioned. Usage is left to inference.

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