design_set_mask

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

With no annotations, the description carries the full burden. It does disclose the clipping semantics and one hard precondition (cannot mask modifier-path layers), which is real value. It is silent on what happens when the target already has a mask (replaced? stacked?), whether the source layer is consumed or retained, and whether the operation is reversible.

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 short sentences, front-loaded with the action, then the mechanism, then the constraint. Every sentence adds information; none restates the tool name.

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?

Three simple parameters and no output schema, so the description need not explain return values. Still, for an unannotated mutation tool it leaves the existing-mask behavior and the effect on the source layer undocumented, which an agent would want before mutating a document.

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% and maskLayerId even documents the ordering constraint, so the schema does the heavy lifting. The description adds no meaning about the maskMode enum values (alpha/inverted-alpha/luminosity) beyond what the schema states, so 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?

States a specific verb and resource ("Apply a mask to a design layer") and immediately explains the mechanism: the mask source layer clips the target layer. It never names the closest sibling (design_clear_mask) or design_set_blend explicitly, so differentiation is left to inference.

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 line about modifier-path layers (painting:*, adjust:*, filter:*) being unmaskable is a genuine exclusion rule that prevents an agent from making a doomed call. However there is no guidance on when to choose masking versus other compositing options (design_set_blend, design_set_blend_mode) or how it pairs with design_clear_mask.

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