set_variable

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

No annotations are supplied, so the description carries the full behavioral burden, yet it only says the tool writes a value. It does not state whether the variable must already exist, whether the change is limited to the given mode, whether it persists/is undoable, what permissions or plugin state are required, or what is returned on success or failure.

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?

Two short sentences, both earning their place: the first defines the action, the second supplies the prerequisite lookup. The action is front-loaded with zero 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?

This is a mutation tool with no annotations and no output schema, and one third of parameters are undocumented. The description should disclose at minimum mutation side effects, failure modes, and permission/plugin prerequisites; none of that is present, so an agent cannot predict consequences of calling it.

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 33% — only `value` is documented in the schema, and its type-dependency note is already there. The description does add real meaning by telling the agent where variableId and modeId come from (get_variables), partially compensating for the two undocumented string parameters, but it says nothing about modeId scope effects or ID format.

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+scope: sets a Figma variable's value for a particular mode, which distinguishes it from the read-only sibling get_variables. It does not explicitly contrast with the closely related set_variable_binding, so sibling differentiation is only partial.

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?

Gives an explicit prerequisite workflow: call get_variables first to obtain variableId and modeId. That is clear usage context, but there is no guidance on when not to use it (e.g., versus set_variable_binding) or what errors to expect on invalid IDs.

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