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

Annotations already establish readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds genuinely useful context beyond that: a 10-credit cost and the fact that output detail (face count, bounding boxes) varies by plan tier. It does not explain behavior when neither url nor image is supplied, or when no face is found, which keeps it from a 5.

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 body is short and front-loaded, but it opens with the redundant 'Face Detector: Face Detector is a face detection API,' which restates the name and title without adding information. Removing that prefix would leave a tight, informative sentence set.

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?

An output schema exists, so return shape need not be explained, and cost is disclosed. However, with zero required parameters, the description never clarifies that at least one of url or image must be provided, nor how the paid-tier output difference interacts with the declared output schema.

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%, so the schema already documents url, image, and the confidence threshold with its 0.1-1.0 range and default. The description adds no parameter-level meaning, such as which of url vs image takes precedence, so the baseline of 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?

The description uses a specific verb and resource: 'analyzes an image and reports whether it contains human faces.' It clearly distinguishes from the image-adjacent siblings such as NSFWImageDetector or ImageCaption by naming the exact detection target, though it never names those alternatives explicitly.

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?

There is no statement about when to choose this tool over alternatives like NSFWImageDetector or BotDetector, and no prerequisites beyond the credit cost. The only usage-relevant information is the price and the fact that output detail depends on plan tier.

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