diff --git a/README.md b/README.md
index 31e80bb..b0ebcb9 100644
--- a/README.md
+++ b/README.md
@@ -73,7 +73,8 @@ Until the first release tag exists, the root mirrors the preview build.
 
 To cut a release:
 
-1. Bump `version` in `package.json` (and the `.app-version` badge in `index.html`).
+1. Bump `version` in `package.json` — the only place it lives. The build injects
+   it into the in-app badge (`scripts/build.js`), and the E2E test derives it.
 2. Commit and merge to `main`.
 3. Tag the merge commit and push the tag:
    ```bash
diff --git a/doc/arc42.md b/doc/arc42.md
index 2dc389b..57f5487 100644
--- a/doc/arc42.md
+++ b/doc/arc42.md
@@ -300,7 +300,8 @@ flowchart TD
 
 ### Release process
 
-1. Bump `version` in `package.json` and the `.app-version` badge in `index.html`.
+1. Bump `version` in `package.json` (single source — the build injects it into
+   the badge and the E2E test derives it).
 2. Merge to `main` (updates `/preview`).
 3. `git tag vX.Y.Z && git push origin vX.Y.Z` → version-check gate → deploy to root.
 
@@ -321,8 +322,11 @@ flowchart TD
   notifications; the singleton is reset in tests for order-independence.
 - **Validation.** Structural validator with no external deps; CI validates all
   `examples/*.ocf.json`.
-- **Versioning.** Version lives in `package.json`, the in-app badge, and the
-  version test; the CI gate keeps a release tag consistent with `package.json`.
+- **Versioning.** The version lives solely in `package.json`. `scripts/build.js`
+  injects it into the bundle at build time (esbuild `define` → `__APP_VERSION__`,
+  read from `$npm_package_version`), `main.js` writes it into the badge, and the
+  E2E test derives the expected value from `package.json`. The CI gate keeps a
+  release tag consistent with `package.json`.
 
 ---
 
diff --git a/index.html b/index.html
index d3c53dd..527126f 100644
--- a/index.html
+++ b/index.html
@@ -23,7 +23,7 @@
         </svg>
         <h1 class="app-title">OCF<span class="app-title-thin">editor</span></h1>
       </div>
-      <span class="app-version">v0.2.1</span>
+      <span class="app-version"></span>
     </div>
     <div class="header-center">
       <div class="hdr-group">
diff --git a/package.json b/package.json
index 3375a03..7274b92 100644
--- a/package.json
+++ b/package.json
@@ -16,8 +16,8 @@
     "node": ">=22"
   },
   "scripts": {
-    "build": "esbuild src/main.js --bundle --outfile=ocf-bundle.js --format=iife --global-name=OCFEditor --sourcemap",
-    "watch": "esbuild src/main.js --bundle --outfile=ocf-bundle.js --format=iife --global-name=OCFEditor --sourcemap --watch",
+    "build": "node scripts/build.js",
+    "watch": "node scripts/build.js --watch",
     "validate-examples": "node scripts/validate-examples.js",
     "test": "npx playwright test",
     "test:ui": "npx playwright test --ui"
diff --git a/scripts/build-pages.sh b/scripts/build-pages.sh
index 4271cc4..ec70065 100755
--- a/scripts/build-pages.sh
+++ b/scripts/build-pages.sh
@@ -15,36 +15,35 @@ set -euo pipefail
 
 OUT="${1:-_site}"
 ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
-ESBUILD="$ROOT/node_modules/.bin/esbuild"
 
 # Files that make up a deployable site (everything index.html references).
 STATIC_FILES=(style.css opencoachingformat-v1.schema.json)
 
-# Build one variant from a git ref into a target directory, stamping the version.
-# $1 = git ref (tag or branch), $2 = target dir, $3 = version label for the badge
+# Build one variant from a git ref into a target directory.
+# $1 = git ref (tag or branch), $2 = target dir, $3 = version label (optional;
+# empty → scripts/build.js derives "v<version>" from that ref's package.json).
 build_variant() {
-  local ref="$1" dest="$2" version="$3"
+  local ref="$1" dest="$2" label="$3"
   local src
   src="$(mktemp -d)"
-  echo "→ building '$ref' as version '$version' into $dest"
+  echo "→ building '$ref' (version: ${label:-from package.json}) into $dest"
   git archive "$ref" | tar -x -C "$src"
 
-  mkdir -p "$dest"
-  "$ESBUILD" "$src/src/main.js" --bundle \
-    --outfile="$dest/ocf-bundle.js" --format=iife \
-    --global-name=OCFEditor --sourcemap
+  # Build with this checkout's scripts/build.js (it has esbuild via node_modules)
+  # against the extracted ref as the working directory, so the version is taken
+  # from the ref's own package.json — or from APP_VERSION when we pass a label.
+  # Older refs whose main.js predates build-time injection simply keep their
+  # static badge, which for a release tag already equals the tag version.
+  ( cd "$src" && APP_VERSION="$label" node "$ROOT/scripts/build.js" )
 
+  mkdir -p "$dest"
+  cp "$src/ocf-bundle.js" "$src/ocf-bundle.js.map" "$dest"/
   cp "$src/index.html" "$dest/index.html"
   for f in "${STATIC_FILES[@]}"; do
     [ -e "$src/$f" ] && cp "$src/$f" "$dest/"
   done
   [ -d "$src/examples" ] && cp -r "$src/examples" "$dest/examples"
 
-  # Stamp the in-app version badge so the deployed build self-reports its version.
-  if [ -n "$version" ]; then
-    sed -i "s#<span class=\"app-version\">[^<]*</span>#<span class=\"app-version\">${version}</span>#" "$dest/index.html"
-  fi
-
   rm -rf "$src"
 }
 
@@ -65,7 +64,8 @@ fi
 LATEST_TAG="$(git tag -l 'v*' --sort=-v:refname | head -n1 || true)"
 
 if [ -n "$LATEST_TAG" ]; then
-  build_variant "$LATEST_TAG" "$OUT" "$LATEST_TAG"
+  # Empty label → build.js stamps "v<version>" from the tag's package.json.
+  build_variant "$LATEST_TAG" "$OUT" ""
 else
   # No release yet → root shows the preview build so the site is never empty.
   echo "→ no v* tag found; root will mirror the preview build"
diff --git a/scripts/build.js b/scripts/build.js
new file mode 100644
index 0000000..7668cce
--- /dev/null
+++ b/scripts/build.js
@@ -0,0 +1,50 @@
+#!/usr/bin/env node
+//
+// Build the OCF Editor bundle with esbuild.
+//
+// The app version is injected at build time from a single source of truth —
+// package.json — so it never has to be kept in sync by hand. npm exposes the
+// version as $npm_package_version when this runs via `npm run build`; when the
+// script is invoked directly (e.g. by build-pages.sh against a checked-out
+// release) it falls back to the package.json in the current working directory.
+//
+// The display string can be overridden with the APP_VERSION env var (used by
+// build-pages.sh to label the /preview build), otherwise it is `v<version>`.
+//
+// Usage: node scripts/build.js [--watch]
+'use strict';
+
+const path = require('path');
+const esbuild = require('esbuild');
+
+// Resolve project files relative to the working directory, so this script can
+// build either the repo root (npm run build) or an extracted release tree.
+const cwd = process.cwd();
+const pkg = require(path.join(cwd, 'package.json'));
+
+const version =
+  process.env.APP_VERSION || `v${process.env.npm_package_version || pkg.version}`;
+
+const options = {
+  entryPoints: [path.join(cwd, 'src/main.js')],
+  bundle: true,
+  outfile: path.join(cwd, 'ocf-bundle.js'),
+  format: 'iife',
+  globalName: 'OCFEditor',
+  sourcemap: true,
+  define: { __APP_VERSION__: JSON.stringify(version) },
+};
+
+(async () => {
+  if (process.argv.includes('--watch')) {
+    const ctx = await esbuild.context(options);
+    await ctx.watch();
+    console.log(`esbuild: watching for changes (version ${version})…`);
+  } else {
+    await esbuild.build(options);
+    console.log(`esbuild: built ocf-bundle.js (version ${version})`);
+  }
+})().catch((err) => {
+  console.error(err);
+  process.exit(1);
+});
diff --git a/src/main.js b/src/main.js
index 8e8ea08..3e15e60 100644
--- a/src/main.js
+++ b/src/main.js
@@ -58,6 +58,11 @@ export function init() {
   playerControlsEl = document.getElementById('player-controls');
   modeToggle = document.getElementById('mode-toggle');
 
+  // Show the app version, injected at build time from package.json (see
+  // scripts/build.js). Single source of truth — no hand-maintained copies.
+  const versionEl = document.querySelector('.app-version');
+  if (versionEl) versionEl.textContent = __APP_VERSION__;
+
   // Init UI components
   new Palette(paletteEl, editorState);
   new Toolbar(toolbarEl, editorState);
diff --git a/tests/editor.spec.js b/tests/editor.spec.js
index 0af141c..67e467e 100644
--- a/tests/editor.spec.js
+++ b/tests/editor.spec.js
@@ -4,6 +4,11 @@
  */
 
 import { test, expect } from '@playwright/test';
+import { readFileSync } from 'node:fs';
+
+// Single source of truth: the version the app should display comes from
+// package.json (injected into the bundle at build time by scripts/build.js).
+const { version } = JSON.parse(readFileSync('package.json', 'utf8'));
 
 // ─── Helpers ─────────────────────────────────────────────────────────────────
 
@@ -31,7 +36,7 @@ test.describe('App loads', () => {
   test('page title and version are correct', async ({ page }) => {
     await openEditor(page);
     await expect(page).toHaveTitle(/OCF Editor/);
-    await expect(page.locator('.app-version')).toHaveText('v0.2.1');
+    await expect(page.locator('.app-version')).toHaveText(`v${version}`);
   });
 
   test('court SVG is rendered (no 404 on bundle)', async ({ page }) => {