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

With no annotations, the description carries the behavioral burden and does disclose the return format as a PNG of the phone screen. However, it does not explicitly confirm that the operation is read-only/non-destructive, nor does it mention capture constraints or whether the session state is affected.

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 short sentences and is front-loaded with what the tool returns. The second sentence efficiently adds the sibling-alternative guidance without redundant restatement.

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 output schema, the description supplies the essential output type and the main usage decision. It could explicitly confirm read-only behavior, but otherwise the agent has enough context to call it correctly.

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?

There is one required `session` parameter, and the schema already documents it at 100% coverage as coming from `open_app`. The description adds no parameter-level detail, which is acceptable because the schema baseline carries this dimension.

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 states that the tool produces a PNG of the phone screen and scopes it to visual checks such as layout, colours, and images. It clearly distinguishes this from the accessibility-tree output of `screen`, though it describes the artifact rather than using an explicit action verb.

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?

It gives a concrete use case for this tool (checking layout, colours, images) and explicitly routes the agent to `screen` when the tree is sufficient to act. That is clear when-to-use and when-to-prefer-an-alternative guidance.

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