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

The description discloses something no annotation carries: the $0.001 USDC on Base cost paid via x402 per successful call. That payment/auth requirement is high-value behavioral context, and it plausibly explains why readOnlyHint is false. It does not add further traits (determinism, format handling), so not a 5.

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 core function followed by usage and cost. No filler; every clause carries information.

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?

No output schema exists, yet the description covers the return shape (AA/AAA pass/fail at both text sizes) and the payment model. The only real gap is accepted colour input formats for a tool whose whole job is parsing colours.

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 fg/bg are already documented, and the description maps them to 'text colour' and 'background colour'. However, neither the schema nor the description specifies accepted colour formats (hex, rgb, named), so no meaning is added beyond the baseline.

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 precise verb+resource (compute WCAG 2 contrast ratio between text and background colours) and even enumerates the outputs (AA/AAA pass/fail at normal and large sizes). This clearly distinguishes it from the generic 'color' and 'random_color' siblings.

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?

'Use to check that a design is readable' gives an implied use context, but there are no exclusions, prerequisites, or named alternatives (e.g. vs. the generic 'color' tool). Minimum viable routing guidance only.

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