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Capture the current screen as a PNG to verify UI state after each action. Automatically retries black frames and explains FLAG_SECURE or sleeping displays.

Instructions

Capture the current screen as a PNG so you can SEE the UI state. Call it after every action to confirm the screen changed before acting again — driving blind chains taps onto the wrong screen. The image is auto-downscaled (default max 760px) so it is accepted by the image reader; this is for seeing only — derive tap coordinates from describe_ui, not from this image. Auto-retries an all-black frame (an intermittent capture glitch) and, if it stays black, says why (FLAG_SECURE content like a native PIN pad, or a sleeping display) — when black, use describe_ui instead. Works on multi-display foldables (the default display captures correctly); pass display="cover"/"inner"/an index to grab a specific panel.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
serialNoTarget device serial (adb -s). Optional when exactly one device is attached.
displayNoWhich physical display to capture on a multi-display device (a foldable). Omit for the default/built-in screen. Accepts a name alias ('inner'/'primary' or 'cover'/'outer'), an HWC index ('0','1'), or a raw physical display id. Only needed to grab the NON-default panel — the default display already captures correctly without it.
max_dimNoMax width/height of the returned image in pixels. Omit for the default 760; pass 0 (or a negative) to disable downscaling and get the full-resolution image.
Behavior5/5

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

With no annotations, description discloses auto-downscaling, retry logic for black frames, handling of FLAG_SECURE and sleeping displays, and multi-display foldable support. No contradictions—fully transparent.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Dense but efficient; each sentence adds value. Could be slightly more concise by merging some details, but front-loaded with core purpose and well-organized for a complex tool.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Covers edge cases (black frames, multi-display), retries, and limitations (no coordinate derivation). Missing explicit output format (e.g., base64), but otherwise complete for a screenshot tool with optional parameters and no output schema.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% with good parameter descriptions. The description adds context like default values for max_dim, alias usage for display, and when to omit serial, enriching the schema with practical usage hints.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool captures a PNG of the current screen to see UI state, using specific verbs and resource. It distinguishes from siblings like describe_ui by emphasizing it's for visual confirmation, not coordinate derivation.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly tells when to use (after every action to confirm screen change) and when not to (when frame is black, use describe_ui instead). Provides alternative tool and explains consequence of driving blind.

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

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