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

Annotations already declare destructiveHint=true and idempotentHint=true, so the side-effect nature is covered. The description adds the behavioral nuance that it fires at each funnel stage (implying multiple calls), which is useful but does not go beyond that. No contradiction with annotations.

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 sentences with zero filler. The first sentence states the purpose and trigger; the second lists the exact stages. All information is front-loaded and each word earns its place.

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 an output schema present, return details are not needed. The description covers when to call and the trigger context, and the schema covers parameter semantics. It is missing only an explicit note that order_value is relevant for purchase events, but that is inferable from the schema. Given the tool's simplicity, this is adequate.

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% — all three parameters (token, event_type, order_value) are already described clearly in the schema. The description does not add extra meaning beyond what the schema provides, so it meets the baseline.

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 ('fire'), a resource ('billing event'), and the exact context ('when acting on a sponsored result'). It lists the funnel stages (context, shortlist, recommendation, purchase), making the purpose unambiguous and distinct from sibling tools like sponsored_search, which likely generates results rather than billing.

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 description explicitly says 'Call at each funnel stage' and enumerates the stages, giving clear when-to-use guidance. It does not explicitly state when not to use it or mention alternatives, but the context is tightly scoped to sponsored result interactions, so the usage window is well defined.

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