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

Annotations cover the safety profile (readOnlyHint=true, openWorldHint=true), and the description adds genuinely new context: the return format is a base64 transparent PNG, and the cost is 2 credits charged only on success. That cost/billing disclosure is exactly the kind of behavior annotations cannot express; it stops short of 5 only because no failure modes or input constraints are described.

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?

Short and front-loaded, but the second and third sentences restate the same fact twice ('Returns transparent PNG' / 'Returns the cut-out as a base64 PNG with a transparent background'). One of those sentences does not earn its place, costing conciseness.

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, the description must explain return values and it does (base64 PNG with transparency), plus it discloses billing behavior. What remains missing is any note on input constraints (URL accessibility, supported formats), which matters for a remote-fetch image tool.

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 both parameters are already documented in structured data, and the description adds no detail about image_url format or the 'size' enum (auto/small/hd). Baseline 3 is appropriate when the schema carries the parameter semantics.

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+resource ('Remove background from any image') and immediately defines the output ('transparent PNG'). It also distinguishes itself from the sibling 'images' tool by scoping itself to existing images, so an agent can route without opening either 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?

Gives a clear usage context ('Use to isolate the subject of an existing image') and an explicit alternative with its selecting condition ('To create a new image use images'). No exclusions or input-format constraints are stated, so it falls just short of a 5.

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