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

Goes well beyond the annotations by disclosing the billing model: $0.01/call via x402 USDC or card key, first 25 free, failed calls free, and repeats are charged — which explains the idempotentHint=false. The statement "reads or computes only and changes nothing" is consistent with readOnlyHint=true and destructiveHint=false.

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?

Purpose and result shape are front-loaded, followed by usage and then billing. Every sentence carries information, though the pricing clause is dense enough that it slightly crowds out the core tool description.

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 compute tool with no output schema, the description covers what is computed (ratio plus AA/AAA verdicts at two sizes), input format via example, safety profile, and the cost model. Nothing an agent needs to decide or call correctly is missing.

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 both parameters are documented, but the description adds the value format via the example (hex strings like #767676 and #ffffff), which the schema does not specify. That is a genuine increment over structured data, though it doesn't cover alternate formats like rgb() or named colors.

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: computes the WCAG 2 contrast ratio between a text colour and a background colour, plus AA/AAA pass/fail at both text sizes. This is clearly separable from siblings like color_palette, color, or random_color, which do not evaluate accessibility.

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" plus a concrete example (fg=#767676, bg=#ffffff) gives a clear usage context. It stops short of naming alternatives or when-not-to-use (e.g. use color_palette to generate colors instead), so it does not reach the top band.

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