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Jinero

by online.jinero
OverviewDiscussionsAdmin

Check Contrast Tool

check_contrast
Read-onlyIdempotent

Check the WCAG contrast ratio between a foreground and background color, with AA/AAA pass/fail for normal and large text.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "fg": "<fg>",
      "bg": "<bg>"
    },
    "name": "check_contrast"
  }
}

Input Schema

NameRequiredDescriptionDefault
bgYesBackground color (hex, rgb(), or hsl()).
fgYesForeground color (hex, rgb(), or hsl()).

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4.2/5.0
Behavior4/5

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

Annotations already declare readOnlyHint=true and idempotentHint=true, so the safety profile is clear. The description adds valuable behavioral detail about AA/AAA pass/fail evaluation for normal and large text, which goes beyond the basic 'check' verb. It doesn't mention return format or error handling, but the annotations lower the bar for this dimension.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single sentence that is front-loaded with the core action and outcome. There is no redundant phrasing, and every part contributes meaning.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the simple two-parameter tool with good annotations and full schema coverage, the description adequately covers the tool's purpose and key outputs (AA/AAA for normal/large text). It does not describe the exact return structure, but no output schema exists, and the description mentions the essential result criteria.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%: both fg and bg are described as color strings with acceptable formats. The description does not add extra parameter semantics beyond what the schema already provides, so the baseline score of 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description uses a specific verb ('Check') and names the exact resource ('WCAG contrast ratio between a foreground and background color'), making the tool's purpose immediately clear. It also distinguishes itself from sibling tools like extract_colors or get_palette by focusing on contrast checking rather than color extraction or palette generation.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description clearly implies when to use the tool: whenever you need to evaluate contrast between two colors against WCAG standards. However, it does not explicitly mention alternatives or situations when not to use it, missing the top tier of guidance.

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

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