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

Annotations already indicate readOnlyHint=true and openWorldHint=false. The description adds meaningful behavioral context: it recognizes RRNs, applies one of four masking modes, returns the masked image, and discloses a 20-point cost per call plus a legal-use requirement. No contradiction with annotations.

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 main action is front-loaded and the description is reasonably compact. The bilingual repetition is slightly redundant, but the required-input note, type enumeration, legal caveat, and cost disclosure are all meaningful and add value without excessive clutter.

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 two-parameter tool with no output schema, the description covers the essential operational details: what it does, required inputs, masking modes, legal conditions, and cost. It does not specify the output representation beyond "masked image," but that is likely sufficient for invocation.

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?

The input schema already provides 100% parameter coverage, including descriptions for both type and image_url. The description repeats the type mapping and emphasizes that both inputs are required, adding little semantic value beyond what the schema already states.

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 begins with a specific verb and resource: "Mask resident registration numbers found in an image and return the masked image." This clearly distinguishes the tool from sibling image tools like face_blur, draw_watermark_image, and general image_edit.

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 clearly states the required inputs (image file and type) and adds an important condition: use only when there is a lawful basis such as consent. It does not explicitly compare against alternatives like face_blur, but the RRN-specific scope provides clear context for when this tool is appropriate.

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