Paint: Ellipse, circle or arc

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

Annotations already declare this is a non-readonly, non-idempotent, non-destructive mutation. The description adds useful context beyond that - the canvas must have been created with paint_canvas, a layer index is optional, and undo applies - but it never states what happens on an unknown canvas or that the operation commits to the canvas surface.

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 geometry semantics that matter most, no filler. The second sentence is slightly generic (it explains shared cross-tool behaviour) but still earns its place as a routing hint.

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 18 params, no output schema, and existing annotations, the description covers the key ambiguities: shape mode selection, arc/pie/rotation semantics, and canvas/layer targeting. It is nearly complete, missing only error/precondition behaviour and a note on the preview return image.

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?

With only 44% schema coverage, the description carries real weight and does so for the ambiguous geometry params: it maps radius to circle, rx/ry to ellipse, explains start_angle/end_angle are degrees forming an arc, pie closes to centre, and rotation tilts. This compensates well for the low coverage, though cx/cy and stroke_width remain unexplained.

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 pins down the resource (circle vs ellipse vs arc) and the exact parameters that select each mode (radius, rx/ry, start_angle/end_angle, pie, rotation), so an agent immediately knows what this draws. It stops short of an explicit verb or drawing attention to how it differs from siblings like paint_path or paint_brush, which handle other geometry.

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?

It gives context on how to target the operation: name the canvas (created via paint_canvas) and optionally a layer index, and notes that layers/filters/undo behave like every other drawing tool. There is no explicit guidance on when to choose this over alternative shape tools or what preconditions must hold (e.g., canvas must already exist).

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