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

Annotations already declare readOnly/idempotent/non-destructive, so the bar is lower, yet the description adds substantive traits: no new full-resolution grab is spawned, capture is at physical resolution with no downscaling, and no page-source is bundled. It omits the failure behavior when the shared stream is not already running, which is the one precondition it implies but never states.

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 PREFERRED/ALREADY-RUNNING qualifiers and the core tradeoff; each sentence carries information. Slightly repetitive in restating the resolution invariant that device_screenshot already promises, which costs a point.

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 single-param, no-output-schema read tool, the description covers return format (single JPEG frame), the fast-path rationale, and the sibling tradeoff. The main gap is not stating what happens if the shared stream is not running, a precondition this tool depends on.

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 description coverage is 100%, so the schema fully documents it. The description adds no format or sourcing detail for udid, so the baseline 3 applies.

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 (screenshot of an Android device) and a distinguishing mechanism (single JPEG frame decoded from the already-running MJPEG stream, no fresh grab). It explicitly names and contrasts with the sibling device_screenshot, so an agent can separate the two without opening either schema.

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?

Explicitly says when to use it ('routine per-action screen checks', 'make it your default for iterating on a flow') and when not ('reserve device_screenshot for when you need the bundled labeled-elements list or a full-fidelity image'). Alternatives and selection criteria are both given.

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