@oyerinde/caliper

Essential tooling for detail-obsessed design engineers.

Caliper is a high-precision, framework-agnostic measurement tool that lives in your browser during development. It helps you catch "pixel-drift" and alignment issues before they reach production.

AI Agents & MCP 🤖

Caliper is "AI-Native". It can be connected to AI agents (like Cursor, Claude Code, or Antigravity) via the Model Context Protocol (MCP), allowing agents to perform pixel-perfect audits of your UI.

1. Install Bridge: Available as @oyerinde/caliper/bridge for custom integrations.
2. Run Server: npx @oyerinde/caliper
3. Connect: Add the MCP server to your editor on the default port 9876.

The AI agent gains "layout eyes" and can perform high-precision audits, measurements, and alignment checks directly in your browser.

Features 🚀

• Core Measurement: High-precision boundary detection and distance calculation between DOM elements.
• Interactive Overlay: Real-time measurement lines and labels with smooth, high-fidelity animations.
• Selection System: Lock elements for side-by-side comparison (Cmd/Ctrl + Click).
• Edge Projections: Check alignment across the entire viewport using relative projections (W/A/S/D).
• Viewport Rulers: Draggable guidelines with magnetic snapping and chained distance measurements (Shift + R).
• Integrated Calculator: Precise spatial math for complex component spacing (T/R/B/L/G).
• Agent Bridge: Built-in support for AI-driven audits and programmatic UI inspection.
• Agent State Sync: Real-time "passive observation" synchronization with AI agents, providing stable JSON fingerprints for seamless tool-based hand-off.
• Full Customization: Fully configurable shortcuts and theme colors.

Installation 📦

Caliper is designed to be side-effect-free in production and easy to integrate into any modern stack.

1. Next.js (App Router)

// app/layout.tsx
import Script from "next/script";

export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <body>
        {process.env.NODE_ENV === "development" && (
          <Script
            src="https://unpkg.com/@oyerinde/caliper/dist/index.global.js"
            data-config={JSON.stringify({ theme: { primary: "#AC2323" } })}
            strategy="afterInteractive"
          />
        )}
        {children}
      </body>
    </html>
  );
}

2. Vite

<!-- index.html -->
<script type="module">
  if (import.meta.env.DEV) {
    // Run npm i @oyerinde/caliper then
    import("@oyerinde/caliper").then(({ init }) => {
      init({ theme: { primary: "#AC2323" } });
    });
  }
</script>

3. HTML (Plain)

<!-- index.html -->
<script type="module">
  const isDev = location.hostname === "localhost" || location.hostname === "127.0.0.1";

  if (isDev) {
    import("https://unpkg.com/@oyerinde/caliper/dist/index.js").then(({ init }) => {
      init({ theme: { primary: "#AC2323" } });
    });
  }
</script>

4. Astro

<!-- src/components/Caliper.astro -->
<script type="module" is:inline>
  if (import.meta.env.DEV) {
    // Run npm i @oyerinde/caliper then
    import("@oyerinde/caliper").then(({ init }) => {
      init();
    });
  }
</script>

5. Nuxt

// nuxt.config.ts
export default defineNuxtConfig({
  app: {
    head: {
      script: [
        {
          src: "https://unpkg.com/@oyerinde/caliper/dist/index.global.js",
          "data-config": JSON.stringify({ theme: { primary: "#AC2323" } }),
          defer: true,
        },
      ],
    },
  },
});

6. Vue

<!-- index.html -->
<script type="module">
  if (import.meta.env.DEV) {
    // Run npm i @oyerinde/caliper then
    import("@oyerinde/caliper").then(({ init }) => {
      init({ theme: { primary: "#AC2323" } });
    });
  }
</script>

7. TanStack Start

// root.tsx
import { Meta, Scripts } from "@tanstack/react-router";

export function Root() {
  return (
    <html lang="en">
      <head>
        <Meta />
        {process.env.NODE_ENV === "development" && (
          <script
            src="https://unpkg.com/@oyerinde/caliper/dist/index.global.js"
            data-config={JSON.stringify({ theme: { primary: "#AC2323" } })}
            async
          />
        )}
      </head>
      <body>
        <Scripts />
      </body>
    </html>
  );
}

Agent Bridge Installation 🤖

The Agent Bridge enables AI agents (like Claude or Cursor) to communicate with Caliper. It is available as a sub-export of the main package.

1. Vite & Module Bundlers

If you've installed @oyerinde/caliper via npm, you can initialize the bridge using the plugin pattern:

import { init } from "@oyerinde/caliper";
import { CaliperBridge } from "@oyerinde/caliper/bridge";

const caliper = init();

caliper.use(
  CaliperBridge({
    enabled: true,
    wsPort: 9876,
  })
);

2. Standalone (CDN/IIFE)

If you are using the global script tag, the bridge is automatically included in the default bundle. Use the minified lite version if you don't need bridge support:

<!-- Full version (Includes Agent Bridge) -->
<script
  src="https://unpkg.com/@oyerinde/caliper/dist/index.global.js"
  data-config='{"bridge": {"enabled": true}}'
></script>

<!-- Lite Version (Core only, No Bridge) -->
<script src="https://unpkg.com/@oyerinde/caliper/dist/index.global.min.js"></script>

3. Next.js (App Router)

Enable the bridge directly in your configuration block:

<Script
  src="https://unpkg.com/@oyerinde/caliper/dist/index.global.js"
  data-config={JSON.stringify({
    bridge: { enabled: true },
  })}
  strategy="afterInteractive"
/>

Configuration 🛠️

Caliper can be customized to fit your specific design system and workflow. init() automatically mounts the overlay in the browser.

import { init } from "@oyerinde/caliper";

init({
  theme: {
    primary: "#18A0FB", // Main brand color
    secondary: "#F24E1E", // Accent color (for highlights)
    calcBg: "rgba(30,30,30,0.95)",
    calcShadow: "rgba(0,0,0,0.5)",
    calcOpHighlight: "#18A0FB", // Operator pulse color
    calcText: "#FFFFFF",
    text: "#FFFFFF",
    projection: "#9B51E4", // Edge projection lines
    ruler: "#18A0FB", // Ruler/guideline color
  },
  commands: {
    activate: "Alt", // Reveal overlay
    freeze: " ", // Key to lock lines
    select: "Control", // Key to select (held)
    clear: "Escape", // Clear measurements
    ruler: "r", // Ruler (Shift+r)
    selectionHoldDuration: 250, // Select hold-time (ms)
    calculator: {
      top: "t",
      right: "r",
      bottom: "b",
      left: "l",
      distance: "g",
    },
    projection: {
      top: "w",
      left: "a",
      bottom: "s",
      right: "d",
    },
  },
  animation: {
    enabled: true, // Smooth hover box
    lerpFactor: 0.25, // Fluidity (low = slower)
  },
});

Excluding Elements

To prevent Caliper from measuring specific elements (like sidebars, floating buttons, or decorative overlays), add the data-caliper-ignore attribute:

<div data-caliper-ignore>
  <!-- This element and its children will be ignored by Caliper -->
</div>

Stable Markers

For AI agents to reliably rediscover elements across re-renders (like HMR), we recommend adding stable markers to critical UI components. Use the caliperProps helper to add these attributes only in non-production environments.

Usage:

<div {...caliperProps("main-sidebar")}>...</div>

Interaction Guide ⌨️

Measurements

• Cmd/Ctrl + Click + Hold (250ms) — Select an element.
• Right-Click — Copy element metadata (selector, ID, text) when selected.
• Hover — View relative distances to target.
• Option/Alt — Hold to reveal the overlay.
• Space — Freeze the current state.

Analysis tools

• W / A / S / D — Trigger edge projections.
• Shift + R — Create a pair of vertical and horizontal guidelines at the cursor.
• Numeric Keypad — Type numbers while projecting to set specific edge distances.
• Escape — Clear all active measurements, rulers, and projections.

Ruler Precision

• Arrow Keys — Nudge selected ruler lines by 1px.
• Shift + Arrows — Nudge by 10px.
• Option/Alt + Arrows — Nudge by 0.1px for sub-pixel auditing.
• Magnetic Snap — Rulers automatically snap to active projection lines.
• Chained Measurement — Link multiple parallel rulers with Shift + Click to reveal gaps.
• Auto-Focus — Typing operators (+ - * / .) instantly switches focus to the calculator.

License ⚖️

This project is licensed under the MIT License.

Author & Acknowledgement 👤

Caliper is built and maintained with precision by Daniel Oyerinde.

If you find this tool helpful in your design-to-code workflow, consider giving it a star on GitHub.

Copyright © 2026 Daniel Oyerinde.