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

Annotations already declare readOnlyHint=true and openWorldHint=false, yet the description adds genuinely new behavioural facts: cost (1 credit = 1 CAD cent per successful call) and the fact that schema-invalid input is rejected before billing. It also lists the exact response keys, which matters since there is no output schema. It stops short of describing failure modes or rounding conventions, 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?

The core behaviour leads, followed by the return shape and pricing in a compact three-sentence block with no filler. The inline enumeration of seven return keys is slightly dense but earns its place given the absence of an output schema.

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, deterministic, read-only tool this is nearly complete: inputs are covered by the schema, outputs are enumerated in prose, and billing behaviour is stated. The only remaining omission is what happens on edge cases such as identical colours or invalid-but-matching-pattern values.

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 already documented as hex strings with an explicit pattern. The description adds nothing about accepted hex forms (3- vs 6-digit, optional '#') or which colour plays which role beyond 'two colours', so the baseline 3 applies.

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 2.2 contrast ratio of two colours") and enumerates the outputs it produces (ratio, AA/AAA pass-fail, nearest passing foreground). No sibling tool covers accessibility or colour maths, and the definition makes that scope unambiguous.

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?

The use case is implied clearly by the purpose, but there is no explicit 'use when' framing, no statement of alternatives, and no guidance on truncating/suggesting versus just checking. With a sibling list that contains no near-equivalent, the omission is low-risk but still a gap.

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