fill_selection

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

With no annotations, the description carries the full behavioral burden. It usefully discloses that the tool only affects paint layers and that omitting the colour destroys the selected pixels (an erase), which is valuable context. However, it does not say what happens when no selection exists, whether the operation is undoable, or what the response contains.

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?

A single well-structured sentence with the primary action front-loaded and the erase case attached after a semicolon. No filler or redundant restatement of the tool name.

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 two-parameter destructive-ish paint operation with no annotations and no output schema, the description covers the main action and the erase mode but leaves the layer target and no-selection edge case unexplained. Adequate but with clear gaps.

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 0% – both parameters have only titles and null defaults. The description meaningfully explains that an omitted/null colour means erase, covering the most important ambiguity of 'color'. But the 'layer' parameter is never addressed: whether it takes a name, id, or index, nor what the active layer fallback is.

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: fill the current selection on a paint layer with a colour. An agent can distinguish it from brush_stroke or draw_shapes because it operates on an existing selection rather than drawing new geometry. It does not explicitly name a sibling alternative, so it stops short of a 5.

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 'omit colour to erase' clause is genuine usage guidance for one mode of the tool, but there is no statement of when to prefer this over related tools (set_selection, modify_selection, brush_stroke) or any prerequisite such as an active selection being required.

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