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

Beyond the readOnlyHint, the description discloses critical behaviors: clipping when reaching off-screen, the meaning of returned dimensions (actual captured size), and the error condition for fully off-screen surfaces. This is valuable transparency not covered by 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 description is concise—two sentences—and efficiently packs key information: the primary function, optional region, clipping behavior, error condition, and parameter semantics. No fluff or redundancy.

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?

Given the tool's complexity (optional region, off-screen handling, window targeting), the description covers all essential aspects. It explains return semantics (clipped dimensions) and edge cases (fully off-screen errors), making it complete for an agent to use without further clarification.

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?

Both parameters (region and window_id) are explicitly described in the tool description with practical semantics (e.g., 'omit for the whole window', 'without changing which window subsequent ops target'). The schema also provides detailed descriptions, but the added context ensures full understanding.

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 clearly states the action (capture), the resource (app window or optional region), and the output format (lossless WebP). It differentiates from sibling tools by focusing on screenshots, and the mention of clipping/off-screen errors adds specificity.

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 implicitly signals when to use this tool (need a screenshot) but does not explicitly contrast with alternatives. It could be improved by stating 'use this when you need a visual representation' or naming sibling tools. However, it provides clear behavioral constraints that guide usage.

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