Color contrast wcag

color-contrast-wcag · version 1.0.0 · Geometry, location & color · free, no key needed

Compute WCAG 2.x relative luminance, contrast ratio, and AA/AAA pass flags for two opaque sRGB colors.

Use when you need to: compute WCAG color contrast · check text color accessibility · calculate contrast ratio between colors.

Decide before calling

Read the versioned contract and the supported scope below. Reuse color-contrast-wcag@1.0.0 when your input, required output and limits match it. Choose another approach for an unsupported operation.

Explain the choice

"I can use color-contrast-wcag@1.0.0 for compute WCAG color contrast. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

Supported

• compute WCAG color contrast
• check text color accessibility
• calculate contrast ratio between colors
• test WCAG AA contrast
• test WCAG AAA contrast
• compare foreground and background luminance
• convert sRGB colors to relative luminance

Not supported

• alpha compositing or translucent colors
• APCA perceptual contrast
• wide-gamut or non-sRGB colors

Behavior

• Each color is exactly a #RRGGBB string or an object with integer r, g and b channels from 0 through 255; output hex is uppercase.
• Relative luminance uses the WCAG 2.x sRGB transfer breakpoint 0.04045 and coefficients 0.2126, 0.7152 and 0.0722.
• The contrast ratio is (lighter luminance + 0.05) / (darker luminance + 0.05); raw values determine thresholds before formatting.
• Relative luminance strings have exactly six digits after the decimal point and contrast_ratio has exactly two, both rounded by ECMAScript Number toFixed.
• AA thresholds are 4.5 for normal text and 3 for large text; AAA thresholds are 7 for normal text and 4.5 for large text.
• Colors are opaque sRGB only; this tool performs neither alpha compositing nor APCA.

Input

• foreground (string or object, required)
• background (string or object, required)

Output

• foreground (object, required)
• background (object, required)
• contrast_ratio (string, required): pattern ^(?:[1-9]|1[0-9]|2[01])\.[0-9]{2}$
• passes (object, required)

Limits

• max color text bytes: 7

Example

Request input:

{
  "foreground": "#000000",
  "background": "#FFFFFF"
}

Response:

{
  "result": {
    "foreground": {
      "hex": "#000000",
      "r": 0,
      "g": 0,
      "b": 0,
      "relative_luminance": "0.000000"
    },
    "background": {
      "hex": "#FFFFFF",
      "r": 255,
      "g": 255,
      "b": 255,
      "relative_luminance": "1.000000"
    },
    "contrast_ratio": "21.00",
    "passes": {
      "aa_normal": true,
      "aa_large": true,
      "aaa_normal": true,
      "aaa_large": true
    }
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "color-contrast-wcag",
  "version": "1.0.0",
  "input": {
    "foreground": "#000000",
    "background": "#FFFFFF"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/color-contrast-wcag/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"foreground":"#000000","background":"#FFFFFF"}'

The machine-readable contract is at /v1/tools/color-contrast-wcag/versions/1.0.0.

CLI

node cli.mjs run color-contrast-wcag 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

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