diff --git a/next.config.mjs b/next.config.mjs
index 5588e26..add6fdb 100644
--- a/next.config.mjs
+++ b/next.config.mjs
@@ -1,5 +1,28 @@
+import { execSync } from 'node:child_process';
+
+// The commit this build was made from, for the debug footer on every page.
+// The FULL sha: the footer shows the short form but copies this, so what
+// lands in the clipboard is directly usable with git. Vercel's build
+// container has no .git directory but provides the sha in an env var; local
+// builds ask git; anything else shows 'unknown' rather than failing the
+// build over a label.
+function resolveCommitSha() {
+  const fromPlatform = process.env.VERCEL_GIT_COMMIT_SHA;
+  if (fromPlatform) return fromPlatform;
+  try {
+    return execSync('git rev-parse HEAD', {
+      stdio: ['ignore', 'pipe', 'ignore'],
+    }).toString().trim();
+  } catch {
+    return 'unknown';
+  }
+}
+
 /** @type {import('next').NextConfig} */
 const nextConfig = {
+  env: {
+    NEXT_PUBLIC_COMMIT_SHA: resolveCommitSha(),
+  },
   // `next dev` and `next build` both write to the output directory, so a build
   // started while a dev server is serving makes that server read manifests the
   // build has just replaced. Leaving this unset keeps production on `.next`,
diff --git a/src/app/components/DebugFooter.js b/src/app/components/DebugFooter.js
new file mode 100644
index 0000000..723871c
--- /dev/null
+++ b/src/app/components/DebugFooter.js
@@ -0,0 +1,51 @@
+"use client";
+
+import { useEffect, useRef, useState } from 'react';
+
+// Build stamp rendered on every page: which commit is this deployment?
+//
+// Bottom-center, styled as a link; clicking copies the FULL sha (the label
+// shows the short form). z-40 keeps it under dialogs (z-50) and under the
+// game screen's action bar and map (z-500+), so on a phone mid-round it ducks
+// behind the controls. The wrapper stays click-transparent; only the button
+// itself takes the pointer.
+export default function DebugFooter() {
+  const sha = process.env.NEXT_PUBLIC_COMMIT_SHA || 'unknown';
+  const [copied, setCopied] = useState(false);
+  const resetTimerRef = useRef(null);
+
+  useEffect(() => () => clearTimeout(resetTimerRef.current), []);
+
+  const handleCopy = async () => {
+    try {
+      await navigator.clipboard.writeText(sha);
+      setCopied(true);
+      clearTimeout(resetTimerRef.current);
+      resetTimerRef.current = setTimeout(() => setCopied(false), 1500);
+    } catch (error) {
+      // Clipboard access can be denied (insecure context, permissions); the
+      // label is still readable, so failing quietly beats an alert.
+      console.error('Could not copy commit sha:', error);
+    }
+  };
+
+  return (
+    <div className="pointer-events-none fixed inset-x-0 bottom-[env(safe-area-inset-bottom)] z-40 flex justify-center">
+      {/* The visual is deliberately tiny, so the tap target comes from
+          padding: ~44px tall, well past the label's own box. min-w in ch
+          keeps the width stable when the 7-char sha swaps for the 7-char
+          'copied!', so the click never shifts the thing being clicked.
+          aria-live announces that swap to screen readers. */}
+      <button
+        type="button"
+        onClick={handleCopy}
+        aria-label="Copy build commit"
+        aria-live="polite"
+        title={sha}
+        className="pointer-events-auto flex min-h-11 min-w-[9ch] cursor-pointer select-none items-center justify-center rounded-md px-3 font-mono text-[11px] leading-none text-muted-foreground underline underline-offset-2 transition-colors hover:text-foreground active:text-foreground focus-visible:outline-none focus-visible:ring-[3px] focus-visible:ring-ring"
+      >
+        {copied ? 'copied!' : sha.slice(0, 7)}
+      </button>
+    </div>
+  );
+}
diff --git a/src/app/layout.js b/src/app/layout.js
index e5301af..db284ab 100644
--- a/src/app/layout.js
+++ b/src/app/layout.js
@@ -1,4 +1,5 @@
 import { THEME_STORAGE_KEY } from '../lib/theme';
+import DebugFooter from './components/DebugFooter';
 import { Analytics } from '@vercel/analytics/next';
 import { SpeedInsights } from '@vercel/speed-insights/next';
 import { Geist, Geist_Mono } from "next/font/google";
@@ -53,6 +54,7 @@ export default function RootLayout({ children }) {
         className={`${geistSans.variable} ${geistMono.variable} antialiased`}
       >
         {children}
+        <DebugFooter />
         <Analytics />
         <SpeedInsights />
       </body>
diff --git a/tests/e2e/home.spec.js b/tests/e2e/home.spec.js
index 0e5090b..81f3804 100644
--- a/tests/e2e/home.spec.js
+++ b/tests/e2e/home.spec.js
@@ -35,6 +35,20 @@ test('lists an uncovered district as disabled, with the reason', async ({ page }
   await expect(page.getByRole('link', { name: /^Cu Chi$/ })).toHaveCount(0);
 });
 
+test('shows the build commit in the debug footer and copies it on click', async ({ page, context }) => {
+  // The dev server resolves the sha from git, so this asserts the real wiring:
+  // config env -> layout -> footer -> clipboard.
+  const stamp = page.getByRole('button', { name: 'Copy build commit' });
+  await expect(stamp).toBeVisible();
+  await expect(stamp).toHaveText(/^[0-9a-f]{7}$/);
+
+  await context.grantPermissions(['clipboard-read', 'clipboard-write']);
+  await stamp.click();
+  await expect(stamp).toHaveText('copied!');
+  const copied = await page.evaluate(() => navigator.clipboard.readText());
+  expect(copied).toMatch(/^[0-9a-f]{40}$/);
+});
+
 test('shows the stubbed leaderboard in the modal', async ({ page }) => {
   await page.getByRole('button', { name: /Leaderboard/i }).click();
   await expect(page.getByText('top-player')).toBeVisible();