Read the colour of one viewport pixel

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

Annotations already declare readOnlyHint=true, so safety is covered. The description adds genuinely useful behavior ('fully deterministic', 'decodes the screenshot in-process', devicePixelRatio applied internally), but omits the return format (hex, rgb(), alpha channel?) which matters because there is no output schema.

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?

Three short sentences, front-loaded with the action and coordinate convention, followed by determinism and the DPR caveat. Nothing is redundant or padded.

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?

With no output schema, the description should specify what 'the colour' looks like (format, alpha handling, behavior when coordinates are out of bounds). Those gaps are significant for a value-returning tool, though the coordinate model and determinism guarantees are well covered.

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%, so baseline is 3. The description adds real meaning beyond the schema by explaining that input coordinates are CSS pixels while the screenshot is physical pixels and DPR is applied internally, which clarifies why x/y are not raw image offsets.

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 ('return the colour at a CSS-pixel coordinate') plus the mechanism ('capture the viewport'). It is clearly distinguishable from browser_eyedropper and browser_screenshot by being programmatic and single-pixel.

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?

'no eyedropper and no human click is needed' implicitly routes the agent away from browser_eyedropper toward this tool. However, it never states an explicit when-to-use condition or a when-not-to-use case (e.g., when a full screenshot is preferable).

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