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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered. The description adds genuinely useful behavior: the image is the exact frame VNC viewers receive, sourced from a shared capture (cheap, non-exclusive), and no VNC session must be open first. It does not mention cost limits or staleness, but adds substantive context beyond 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?

Front-loaded with the core action and the return shape, then the coordinate-scaling caveat and the 'no VNC open needed' note. Every sentence carries information, though the em-dash/semicolon packing makes it slightly dense versus a cleaner multi-sentence form.

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?

With no output schema, the description compensates by naming the returned JSON keys (width, height, imageWidth, imageHeight) and the critical scaling rule (multiply image positions by width/imageWidth). Combined with the coordinate linkage to device_tap, an agent has everything needed to call and interpret the result.

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?

Only one parameter (udid), and schema coverage is 100% with its own description, so the schema carries the burden. The description does not elaborate on udid, instead spending its detail on the return payload's coordinate semantics — useful but not parameter semantics.

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 verb+resource ('returns the current screen as an image') plus the exact JSON payload fields. It also distinguishes itself from VNC-viewer infrastructure by noting it reads the device server's shared screen capture, so an agent can tell it apart from android_mjpeg_screenshot or device_screenshot.

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?

Clearly says it is 'cheap enough to call after every action' and that it 'Does not need android_vnc_open first' — an explicit prerequisite removal. It stops short of naming alternative screenshot siblings (device_screenshot, android_mjpeg_screenshot, android_video_stream) and when those would be preferred.

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