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

With annotations present, the description still adds real value: it discloses the caching/reuse rule and that preparation produces no variant and opens no picker. There is mild tension with idempotentHint=false since cached candidates are reused, but that reflects reuse rather than true idempotency, so it reads as added nuance, not a contradiction. It does not mention auth or cost implications.

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 tight sentences, front-loaded with what the tool does, then the return payload, then the behavioral caveat. Nothing is wasted, though the catalogKey/templateIds/galleryUrl sentence partially duplicates the existing output schema.

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 full schema coverage, annotations, and an output schema, the remaining burden is small, and the description covers side effects and caching behavior. The one gap is that it never points to the sibling tool that consumes the prepared catalogKey/galleryUrl, leaving the next step implicit.

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 all four parameters (including the 3-7 path ordering and deviceType's preview-only effect) are already documented in structured fields. The description only adds that source paths drive cache matching, which is marginal beyond the schema. Baseline 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 opens with a specific verb+resource ("Generates or reuses personalized screenshot-layout candidates") scoped to all three device templates, which cleanly separates it from apply_screenshot_style, preview_screenshots and render_screenshots. The closing sentence explicitly states what it does NOT do ("does not create a variant or open a picker"), removing ambiguity about downstream effects.

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 clear conditions for cache reuse ("matching app context and source paths") and clarifies the tool is preparatory, which implicitly routes the agent to a later apply step. However, it never names the actual follow-up sibling (e.g. apply_screenshot_style) and offers no explicit exclusion beyond the negative-side-effect note.

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