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

Annotations already declare readOnly, idempotent, non-destructive. The description adds behavioral output context by stating it returns compliance for AA/AAA normal and large text, which is useful beyond the annotations. It does not mention edge cases like non-hex input, but this is a low-risk calculation tool.

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 action, no fluff. Every word adds value, and it clearly states inputs and outputs.

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 simple calculation tool with complete schema, annotations, and an output schema, the description covers purpose and return semantics. No additional context is necessary; it is fully adequate for agent use.

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% with both params (foreground/background) documented as hex strings. The description only says 'two colors', adding little beyond the schema. Baseline 3 applies since schema fully covers parameters.

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?

The description uses a specific verb 'Calculate' and names the resource 'WCAG 2.1 contrast ratio between two colors', clearly distinguishing it from sibling tools like color_convert. It also states the return value (ratio and compliance), fully clarifying intent.

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 description implies when to use the tool (for checking WCAG accessibility contrast) and context of compliance levels. It does not explicitly exclude alternatives or name siblings, but the context is clear enough for an agent to select it appropriately.

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