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

With no annotations provided, the description carries the full burden of behavioral disclosure. It reveals key traits: it decodes a frame from an already-running stream, spawns no fresh full-resolution grab, captures at physical resolution without downscaling, and matches device_tap coordinates 1:1. It also notes the image-only nature and speed trade-off, but it does not mention what happens if the shared stream isn't running, a minor gap.

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?

Every sentence in the description earns its place, covering purpose, performance, usage recommendation, resolution invariant, alternative guidance, and output format. The key facts are front-loaded ('PREFERRED fast screenshot'), and the section is dense with useful information but contains zero fluff or repetition.

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 one-parameter tool with no annotations and no output schema, the description is remarkably complete. It states the return type ('single JPEG frame'), the resolution behavior, coordinate alignment with device_tap, and explicitly explains when to use the sibling tool instead. Nothing an agent would need to call it correctly is missing.

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 only parameter, 'udid', is fully described in the schema as 'Device serial number (UDID)' with 100% schema coverage. The description does not add any additional meaning to the parameter beyond the schema, so the baseline of 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?

The description clearly states a specific verb+resource: 'PREFERRED fast screenshot of an Android device via the ALREADY-RUNNING shared video stream.' It also explicitly distinguishes itself from sibling device_screenshot and ios_mjpeg_screenshot by naming the alternative and the use case, making the purpose unambiguous.

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 gives explicit when-to-use guidance: 'Use this for routine per-action screen checks' and states it should be the 'default for iterating on a flow.' It also provides when-not-to-use guidance, directing the agent to 'Reserve device_screenshot for when you need the bundled labeled-elements list or a full-fidelity image,' which is a clear alternative and exclusion.

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