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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false, so the description's "side effects: none / no network / no stored state" largely restates them. It does add genuinely new operational facts the annotations cannot express: deterministic output, 64 KB input cap, 1 s per-call budget and 1 MB result cap — real value for an agent sizing inputs.

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 version is tightly front-loaded — purpose, then when, then not-for, then behavior, then limits — with no filler. The full ES/EN duplication roughly doubles the token cost, which is a deliberate i18n choice but does dilute conciseness slightly.

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?

An output schema exists, so return values need no explanation; safety and idempotency are covered by annotations and reinforced in prose; and the operational limits plus the sibling routing cover what an agent needs to call this correctly. Nothing material is missing for a 4-parameter, single-purpose tool.

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%, including the enum for `space`, the 0-1 bounds for `weight` and examples for `a`/`b`, so baseline is 3. The description only restates that mixing is weighted and that OKLab is the default; it adds no color-syntax guidance (e.g. which string formats are accepted for `a`/`b`) beyond what the schema already carries.

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+resource (mix two colors by weight) plus the exact algorithm analogy (CSS color-mix) and the three supported interpolation spaces, with the default named. It also explicitly distinguishes itself from the nearest sibling, color-convert, in the same breath.

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?

Contains an explicit "When:" clause (exact color/contrast/palette computation, e.g. accessibility checks) and an explicit "Not for:" clause routing CSS parsing to color-convert. Both the positive and negative selection conditions are spelled out, leaving nothing to inference.

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