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

The description adds 'Input untouched,' clarifying non-destructive behavior beyond the annotations, and notes the 4 MP source limitation. It does not spell out overwrite behavior or return details in the main description, but the schema covers overwrite and the output download link.

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, each carrying distinct information: purpose, fit semantics, aspect-ratio behavior, input safety, and size limit. No filler, repetition, or irrelevant detail.

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 6-parameter tool with full schema coverage and no output schema, the description covers the essential behavioral decisions: fit modes, one-dimension handling, input safety, and size cap. It does not say what happens if both width and height are omitted, and it doesn't route to sibling tools, but the remaining gaps are minor.

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 100%, so the baseline is 3, but the description adds real value by explaining what each fit mode does ('cover fills and crops', 'exact stretches') and clarifying that a single width or height alone preserves the aspect ratio. This goes beyond the schema's terse field descriptions.

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 opens with a clear verb+resource: 'write a resized copy' and explains the three fit modes. It does not explicitly contrast siblings like image_batch_resize or image_crop, but the resize semantics are unambiguous and distinct from those tools.

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?

'Call this tool to write a resized copy' directly states when to invoke it, and the free-tier size cap gives practical selection context. It does not mention when to prefer image_thumbnails, image_batch_resize, or other image tools, so it lacks explicit alternative routing.

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