List Screen Elements

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

Annotations already declare readOnlyHint=true and openWorldHint=true, covering the read-only and dynamic nature of the tool. The description adds valuable behavioral context: that refs and coordinates remain valid only until the screen changes, and that a re-list is required after navigation or layout changes. This goes beyond the annotations and helps the agent anticipate when results become stale.

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?

The description is two sentences, front-loaded with the primary purpose and then the usage caveat. There is no redundant information, and every sentence earns its place. The guidance on ref validity and re-listing is concise and directly actionable.

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 simple read-only list tool, the description covers everything an agent needs: what it returns (ref, coordinates, text/label), how to use the ref (with a specific sibling tool), and when to refresh (after screen changes). With annotations covering safety and no output schema required, nothing essential 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?

Schema description coverage is 100% – both device and format parameters are fully described in the input schema. The description adds no extra parameter semantics beyond what the schema provides. Since the schema does the heavy lifting, a baseline score of 3 is appropriate.

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 the tool lists screen elements with ref, coordinates, and display text or accessibility label. It explicitly names the sibling tool mobile_click_on_screen_at_coordinates for using the ref, which distinguishes it from other list tools in the sibling set (e.g., mobile_list_apps, mobile_list_available_devices). The verb 'list' plus resource 'screen elements' is specific and 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 guidance on when to use the tool again: 're-list only after navigation or a layout change' and notes that refs/coordinates stay valid while the screen is unchanged. It also directs the agent to use the ref with a specific sibling tool, providing clear follow-up context. It doesn't explicitly state when not to use it, but there is no direct alternative among siblings, so this is adequate.

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