Set a solid fill

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

Beyond the annotations (destructiveHint=true), the description discloses that the operation destroys all existing fills, that the response includes the read-back color so failed writes are detectable, and that one call maps to one undo step. This is rich behavioral detail well beyond the boolean annotation flags.

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, each earning its place: the core action, the read-back safety behavior, the destructive replacement warning, the stroke routing, and the undo granularity. It is front-loaded with the primary purpose and avoids redundancy with the 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?

Despite having no output schema, the description explains what the tool returns (the read-back color and what was replaced). It also covers the destructive nature, the undo behavior, and the boundary with strokes, making it complete for an agent to safely invoke this tool on a node with any fill type.

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 coverage is 100%, so the schema already fully documents nodeId, color, and opacity. The description adds context about the fill being solid and the replace-all behavior, but it does not add new meaning to the individual parameters beyond what the schema provides. 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 uses a specific verb ('Replaces') and a precise resource ('a node’s fills') with the outcome 'one solid colour'. It also distinguishes itself from the sibling figma_set_stroke by noting that borders are strokes, not fills, which removes ambiguity about what the tool operates on.

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 explicitly states when not to use this tool: 'Borders are strokes, not fills: use figma_set_stroke for those.' It also warns about a key contextual consideration — the tool replaces every existing fill — which is exactly the kind of guidance that helps an agent decide whether this is the right tool for a gradient or photograph.

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