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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description adds genuinely new behavioral context: the Indie plan requirement (with trial counting), which materially affects whether an agent should call this. It does not add pagination or return-format detail, but the plan gate is the important addition.

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?

Essentially one dense sentence that front-loads the resource and its contents, then the routing instruction and the plan requirement. The nested parentheticals make it slightly heavy to parse, but no sentence is wasted.

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?

With no output schema, the description compensates by describing the returned fields (canvas dimensions, store, creative-asset entries) and by disclosing the plan prerequisite. That is sufficient for a zero-parameter read tool, though it could say more about ordering/format of the returned list.

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 tool takes zero parameters, so the baseline is 4 by rule. The description correctly implies a no-argument listing call and adds no misleading parameter claims.

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?

States a specific verb+resource (list device sizes for app-store screenshot sets) and enumerates what each entry contains: canvas dimensions, owning store, and the landscape App Store creative-asset category. It is clearly distinguishable from siblings like sonar_screenshot_layout_guide or sonar_get_screenshot_set.

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?

"Pick a device here before sonar_create_screenshot_set" explicitly names the downstream tool and the ordering constraint, which is strong routing guidance. It lacks a when-not-to-use clause or an alternative source for device data, so it stops short of a full 5.

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