Let the user select a region

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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds genuinely useful behavior beyond that: the call blocks until the user finishes, and the Space/Esc escape hatches. It does not say what is returned when the selection completes.

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?

Three tight sentences with zero waste. The core action is front-loaded, with the modifier keys and blocking behavior following in priority order.

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 zero-parameter, non-destructive interaction tool with no output schema, the blocking behavior, cancel path, and user instruction cover nearly everything an agent needs. The one gap is what the call yields on success (coordinates, image, or confirmation).

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 tool takes zero parameters, so the baseline is 4. There is nothing for the description to clarify, and it does not need to compensate for any undocumented inputs.

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?

States a specific action (show native crosshair) and the exact outcome (user drags a rectangle around the part of the screen they mean). It also distinguishes itself from the window-selection path by noting Space picks a window instead, which separates it from siblings like device_screenshot and list_windows.

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?

Gives clear in-context guidance: use this to let the user define a region, press Space for window selection, Esc to cancel, and instruct the user to make the selection. It does not explicitly contrast when to prefer this over device_screenshot or look, so it falls short of full when/when-not routing.

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