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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=false, so the safety profile is covered. The description adds real value beyond that by describing the return shape (similarities, differences, factual observations) and the screen-vs-target alternative mode. It stops short of noting what happens with unregistered screen ids or partial failures.

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?

Front-loaded with the comparison content and scoping, then usage context. The two worked examples (Linear/Stripe, Vercel/Notion) are illustrative but add length; the description could shed one pair without losing signal.

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 an output schema present, return format need not be spelled out, and the description nonetheless previews it. Purpose, both modes, and usage triggers are complete; the only unaddressed element is what focus controls, which the schema enum covers adequately.

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%, so screenA, screenB, and targetDescription are already documented inline, including the 'if omitted, provide targetDescription instead' constraint. The description reinforces the dual-mode behavior but adds no format or constraint detail beyond the schema, and never mentions the focus parameter. Baseline 3 applies.

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?

Specific verb (compare) plus resource (two captured screens) with an explicit enumeration of what is compared: layout structure, sections, actions, dimensions, screen types, design guide differences. It is clearly distinguishable from siblings like get_screen or search_screens, which retrieve rather than diff.

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?

Names concrete when-to-use triggers (understanding how two products solve the same problem) with two example pairs, and explicitly covers a second mode—comparing one screen against a target description of what the user is building. Both selection conditions are stated, not inferred.

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