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

Annotations already declare readOnly/idempotent/openWorld=false, but the description adds real value beyond them: no network, no stored state, determinism, and concrete limits (64 KB input, 1 s per call, 1 MB result). Those operational bounds are exactly what an agent needs and cannot get from annotations.

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?

Each language block is tight and front-loaded with the return value, but the entire content is duplicated in Spanish and English, roughly doubling length without adding information. One block plus a short locale note would suffice.

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 one-parameter, read-only, deterministic tool with an output schema, annotations and explicit limits, nothing an agent needs is missing. Return values, scope, non-goals and failure-relevant constraints are all 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?

The single 'color' parameter is fully documented in the schema (formats, examples, maxLength), and there is an output schema, so the description adds little parameter meaning. Its only extra is the input-size ceiling, which is a payload limit rather than a semantic for 'color'.

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?

Concrete verb-plus-resource: returns WCAG relative luminance, OKLCH lightness, the light/dark reading, and which text color contrasts better. It also names the specific sibling it is not (color-convert), so an agent can separate it from the rest of the color-* family.

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?

Explicit 'When:' (exact color/contrast/palette computation, e.g. accessibility checks) and 'Not for:' routing to color-convert. It does not address the other near neighbors color-contrast / color-contrast-fix, which an agent may reasonably confuse with this, so it falls short of a full 5.

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