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

Annotations already indicate read-only, idempotent, and non-destructive behavior. The description adds valuable context: exact billing conditions ('$0.002 ONLY when filtered_count >= 1...'), idempotency (same question free if set unchanged), authentication method, and HTTP route. This goes well beyond the annotations and fully discloses the cost and behavior.

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?

The description contains redundant billing explanations (e.g., '$0.002 ONLY when we filter...' and 'Costs $0.002 USDC per call... ONLY when filtered_count >= 1'). While informative, the repetition wastes space. The core purpose is front-loaded, but the duplication and long billing segment make it less concise than it could be.

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?

There is no output schema, so the description carries the responsibility of explaining return values. It states that the tool returns only the new items and says 'If every item is new we tell you so,' covering the main behavior. It also covers auth, billing, and route. It lacks an exact response shape, but for a simple filtered-list tool, the provided description is sufficiently complete.

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%, and the schema already describes each parameter (set, items, agent_key) with examples. The description mentions agent_key in the authentication context and references the set name implicitly ('this exact set of already-seen items'), but it does not add new semantic meaning beyond what the schema provides. Baseline of 3 is appropriate.

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 clearly states the tool's function: 'Given a batch of items, return only the ones not already in the set — the work you actually still have to do.' This is a specific verb (return) plus resource (items not in set), and it distinguishes the check operation from the sibling tool seen_add, which would add items.

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 provides clear context for when to use the tool ('the work you actually still have to do'), implying it's used to filter out already-seen items before processing. It also explains billing conditions that affect usage, such as when calls are free. However, it does not explicitly name alternatives like seen_add.

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