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

Annotations already declare readOnlyHint=true, so the read-only nature is covered. The description adds valuable behavioral context: default capture uses the user's current device, and passing viewport temporarily switches the preview only for the capture. This prevents an agent from assuming the device change is persistent. It doesn't mention response format, but that is not central to behavioral safety.

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?

Three sentences deliver purpose, usage timing, default behavior, device-switching semantics, and a guardrail against an anti-pattern. Everything is front-loaded and substantive; there is no filler or redundant restatement of the title or annotations.

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?

The description provides enough context for an agent to decide when to call the tool, what projectId to supply, and how to use viewport correctly. The only minor gap is not describing what the tool returns (e.g., an image path or data), but for a screenshot capture this is largely inferable.

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 viewport parameter is richly documented with concrete presets, pixel dimensions, a custom WIDTHxHEIGHT format, and a recommended default for phone checks. The description body reinforces the default behavior when viewport is omitted. projectId is straightforward as the required project identifier, so the 50% schema coverage is adequately compensated.

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 a specific verb and resource: 'Capture a screenshot of the user app.' It also clarifies why to call it — to see layout, styling, or rendered state, or to debug — which distinguishes it from sibling tools that navigate or return URLs. The anti-pattern warning about building a separate mobile page further sharpens its unique purpose.

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 explicitly says to call it whenever the agent wants to see the current app state, and it gives concrete guidance for the viewport parameter, including that the preview switches only for capture and then back. It does not explicitly compare against sibling read-only tools like get_preview_url or navigate_preview, but the context is strong enough to guide selection.

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