Set instance properties

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

Annotations already declare non-readonly, non-destructive, idempotent behavior, yet the description adds real context: the instance stays linked to its component (i.e. this does not detach), names may omit the #id suffix, SLOTs are out of scope, and editor support is qualified ('Design; unverified: FigJam, Slides'). That editor caveat is exactly the kind of extra signal annotations cannot carry.

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 core action, stays dense but readable, and every sentence contributes (scope, naming rule, companion tools, slot exception, editor support). Slightly information-packed for a single paragraph, but nothing is padding.

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?

For a mutation tool with no output schema, the description covers behavior, discovery prerequisites, the slot edge case, and environment caveats. It stops short of describing the return value or failure semantics, which the absence of an output schema leaves unspecified.

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 baseline is 3, but the description adds genuine semantics beyond the schema: the '#id suffix optional' rule for property names in the JSON, and the SLOT parentId convention. It does not elaborate on session or idempotencyKey, both of which the schema already documents well.

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 ('Change an instance's component properties') and enumerates the concrete property kinds (label text, variant, boolean, swapped icon), with the analogy 'the way the right panel does'. An agent can distinguish this from component-definition siblings like figma_edit_component_property without opening the schema.

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?

Explicitly routes the agent to figma_get_design_context to discover an instance's properties and figma_get_component for available options, and states a clear when-not case ('A SLOT is not set here: create layers inside it (parentId = the slot)'). It does not, however, contrast with adjacent mutation siblings such as figma_set_text or figma_swap_component.

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