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

Annotations include readOnlyHint=true, which is consistent with a tool that returns a transformed PNG rather than modifying the source. The description adds format support (PNG, JPEG), adjustable parameters, and a per-call point cost, which is useful context. It does not explain potential failure modes, image size limits beyond the schema, or whether the returned PNG is a download URL or base64 payload, so it stays at a moderate level.

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?

Two sentences plus a short cost note. It is compact and front-loads the core action. The cost note is extra but relevant for agent decision-making. Slightly redundant with schema field descriptions, but not bloated.

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 an image transform tool with a 100% documented schema and readOnlyHint, the description covers the essential behavior, supported formats, and adjustable attributes. It does not describe return format details, but with no output schema and a simple visual transformation, the gaps are minor. Sibling differentiation is sufficient via the image/PDF distinction.

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 every parameter is already documented with type, range, and default. The tool description repeats the general concepts of size, color, opacity, density but adds no syntax or behavioral detail beyond the schema. Baseline 3 is appropriate.

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 ('Draw') and a concrete resource ('a visible text watermark across an image'), and it states the output is a PNG image. It is clearly distinct from draw_watermark_pdf, which is a sibling, because it specifies image files. It could name the sibling explicitly, but the resource difference is already clear.

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 implies usage for adding visible text watermarks to images and lists supported image formats and adjustable attributes. It does not explicitly state when to use this tool over draw_watermark_pdf or set_watermark, but the image-vs-PDF distinction is clear from the description and sibling name. No explicit exclusions or alternative routing, but enough context for a typical agent.

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