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

Annotations already declare readOnlyHint/idempotent/non-destructive, and the description reinforces 'Read-only.' It adds real behavioral context by explaining the two return views (account-level by_field_type vs per-asset detail with link text or phone number), though it does not discuss pagination or limits.

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?

Front-loads the core purpose and the view breakdown before the filter details; every sentence carries information. It is on the long side, but the added sibling comparisons and view semantics justify the length.

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 read-only reporting tool with an output schema, the description covers scope, filters, and return structure adequately. The only gap an agent might need is pagination/limit behavior, which is minor given the output schema exists.

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 0%, so the description carries the burden and does well for days (lookback window), campaign_id (narrow to one campaign), and field_type (enumerates all valid kinds). It leaves limit (default 60) and customer_id unexplained, so the coverage is strong but not complete.

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 (reports performance per ad extension type) and enumerates the extension kinds covered. It explicitly distinguishes itself from list_extension_assets (configuration only) and rsa_asset_performance (headlines/descriptions), so an agent can select it without opening a schema.

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 concrete questions it answers ('which sitelink actually gets clicked', 'how often is the call button used') and names the two nearest siblings with the reason they don't cover this. The routing condition is explicit rather than implied.

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