Detect faces

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

Adds real behavior beyond the annotations: nothing is persisted, each call costs 93 credits, and a quality gate can suppress faces from the count. The credit cost and non-storage guarantee are exactly the details an agent needs before deciding to call. The idempotency/409 semantics live in the schema, so nothing material is lost there.

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 sentences: capability, return shape, then use case plus cost. No repetition of the name or title and no filler; every clause carries information the agent would otherwise have to guess.

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, describing detected_face_count, gated_face_count and the per-face fields is genuinely necessary and is done well, and the credit cost is disclosed. The only gap is that the meaning of the quality gate (what min_px or quality_passed thresholds are) is unexplained, which matters for interpreting gated_face_count.

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 each of the five optional image-source parameters is fully documented in the schema, including size limits and source trade-offs. The description adds only the '(ratios)' note about bounding boxes, which is a return-value detail, not parameter guidance, so baseline 3 applies.

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 ('Find the faces in an image') plus the crucial distinction from its closest sibling: it does not store anything, unlike index_photo. The return payload and credit cost are named up front, so an agent knows exactly what this call produces.

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 a clear use case ('check a photo or selfie before indexing'), which implicitly routes the agent away from index_photo and toward this tool for a dry run. It stops short of naming index_photo or compare_faces as explicit alternatives, so the routing is inferred rather than stated.

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