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

Substantially exceeds the annotations: it discloses per-factor credit pricing, a refund-on-failure policy, the auth requirement (API key), input format and dimension caps, and a 7-day result-link lifetime. Since annotations only cover readOnlyHint=false and openWorldHint=true, this is exactly the extra context an agent needs before spending credits.

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?

Four short sentences with the capability first, then cost, then constraints, then return value — no filler. Each sentence carries information the agent cannot get from the schema or annotations.

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 credit-charging mutation with no output schema, the description covers cost, failure handling, auth and return value well. The remaining hole is how the two mutually exclusive image source parameters are chosen between and that at least one is required.

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 67%, and the description compensates by adding concrete validation rules for the inputs (PNG/JPEG/WebP, 10 MB, 2048 px at 2x vs 1024 px at 4x) that the schema omits. It also reinforces the factor enum. The one gap: it never clarifies that one of image_url or image_base64 is needed even though neither is marked required.

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?

Opens with a specific verb+resource+capability: 'Upscale an image 2x or 4x with AI.' That reads clearly against siblings like ai_extend_image and ai_remove_background, which do different transformations, so an agent can route without opening the 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?

The prerequisites (CropDen API key), cost (8/12 credits) and input limits imply when the tool is viable, and the credit cost implicitly tells the agent to check get_credit_balance first. But there is no explicit when-to-use/when-not guidance and no direct comparison to ai_extend_image, which is the closest alternative for enlarging an image.

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