Capture Screen Region

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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the agent knows it's a safe read operation. The description adds valuable behavioral context by noting the Screen Recording permission requirement and the fallback mechanism to 'screencapture -R' if the helper is unavailable. This goes beyond the annotations without contradicting them.

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 two sentences, front-loaded with the core action, and every word contributes useful information (action, coordinate system, permission, fallback). There is no wasted content or unstructured rambling.

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?

The description covers purpose, coordinate system, permission, and fallback, but it does not mention the return format (e.g., file path, base64, image object) despite the lack of an output schema. This is a notable gap for a screenshot tool, as the agent would need to know what to do with the result. Given the schema is well-documented and annotations cover safety, the description is adequate but not fully complete.

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 covers all parameters with descriptions, so the baseline is 3. The description adds a minor clarification about 'top-left origin' for the coordinate system, but this is largely redundant given the schema's 'left edge' and 'top edge' phrasing. It does not add meaningful semantics beyond the schema, so a score of 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 clearly states a specific action ('Take a screenshot of a rectangular region') and specifies the scope ('in global screen points (top-left origin)'). This distinguishes it from sibling tools like 'capture' (presumably full screen) and 'capture_window' (window-specific), making the tool's unique purpose obvious.

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 provides clear context by mentioning the coordinate system ('global screen points') and the requirement for Screen Recording permission. It implicitly differentiates from capture_window by focusing on coordinate-based regions, but it does not explicitly name alternatives or exclusions. Thus, it offers clear context without full when/when-not guidance.

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