set_variable_value

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

Goes well beyond the minimal annotations (readOnlyHint=false, destructiveHint=false) by disclosing that EASING/TIMING variables are rejected by Figma, that the value must match the resolvedType, and the exact return shape { ok, variableId, name }. No contradiction with annotations.

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. Front-loads the purpose, then covers type constraints, read-only exceptions, prerequisite, and return value. No redundancy or filler.

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 only three parameters and no output schema, the description is complete: it covers required value types, the read-only edge case, a prerequisite, the origin of modeId, and the response shape. This is sufficient for an agent to invoke the tool correctly in most scenarios.

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. The description adds context beyond the schema by explaining that the value must align with the variable's resolvedType and by detailing the allowed value forms (color, alias, etc.). It also clarifies the source of modeId. The schema includes an easing type, but the description warns against using it, adding practical guidance.

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 clearly states the action ('Set a variable's value') and the scope ('for one mode'), with the modeId explicitly tied to the variable's collection. It distinguishes this from sibling tools like rename_variable and create_variable, and even differentiates from get_variable_defs for reading.

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 description provides explicit when-not-to-use guidance for EASING and TIMING variables, directing the agent to get_variable_defs and the Figma UI instead. It also states the prerequisite of creating the variable first with create_variable, and clarifies that modeId comes from the collection.

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