set_active

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

Annotations state readOnlyHint=false and destructiveHint=false, so the agent knows this mutates state but non-destructively. The description adds that the change is UI-state-oriented and mostly relevant in live mode, but says nothing about reversibility, what happens when no live session exists, or how batch mode behaves.

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 sentences, front-loaded with the core action and scope, then the live-mode caveat. No filler, though the long enumeration makes the first sentence slightly dense.

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 ten-parameter catch-all mutator with full schema coverage and no output schema, the description is adequate but shallow: it does not clarify how simultaneously supplied fields interact, nor the live-vs-batch behavioral difference hinted at by the backend parameter. It covers the surface but omits the operational detail an agent would want.

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 description coverage is 100%, so the schema already documents all ten parameters including brush, range, colors, and backend defaults. The description only re-lists the categories of changeable state without adding format or interaction details beyond the schema, so the baseline of 3 applies.

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 states a specific verb ('Changes') and resource ('what is active in the UI') and enumerates the concrete targets: sprite/layer/frame, fg/bg color, tool, brush, zoom, and timeline range. This is far clearer than a tautology, though it does not explicitly distinguish itself from potential siblings like select or view_settings.

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?

'Mostly useful in live mode' gives a contextual hint about when the tool applies, and the schema notes 'range' is live-only. However, there is no explicit guidance on when to prefer this over alternatives such as select or view_settings, leaving usage largely implied.

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