design_set_units

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

With no annotations, the description carries the full burden and does well: it discloses the mutation's limited blast radius (display only, tool inputs unchanged) and states the return value references design_get_settings. It does not mention persistence with the document, undo behavior, or whether an active design is required, which keeps it from a 5.

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, followed by the critical caveat and return note in two short sentences. The example line is mildly redundant given the schema enum, and the odd line wrapping costs a little polish, but nothing is wasted.

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 single-parameter tool with no output schema, it covers purpose, the key semantic caveat, and where the return value is documented (design_get_settings). Missing only edge-case handling such as requiring an active design or persistence behavior, which are minor for this operation.

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% and the enum already constrains values to mm/cm/m/in/ft, so the schema does most of the work. The description adds real meaning by explaining what the parameter governs (display units and units for unitless expressions) and gives a concrete call example, going slightly beyond the schema's terse 'Distance display units'.

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 ('Change the distance units Fusion displays and uses') and scopes it to the active design, plus explicitly clarifies what the change does NOT affect (numbers passed to tools stay in mm). This distinguishes it cleanly from the related design_get_settings, which it names as the return-value reference.

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 gives clear context for use (changing display units / unitless expression units in the active design) and pre-empts the most likely misuse by stating that numeric values in tool calls are unaffected. It lacks an explicit 'use X instead when...' exclusion, but the scope is unambiguous.

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