diff --git a/Dockerfile b/Dockerfile
index 39a95f9..1146815 100755
--- a/Dockerfile
+++ b/Dockerfile
@@ -10,7 +10,19 @@ WORKDIR /app/backend
 COPY backend/package.json ./
 RUN npm install
 COPY backend/ ./
-RUN npm run build
+# Only this stage ever sees .git, and the final image below copies just `dist`,
+# so no repository history reaches the deployed container. The build reads the
+# commit out of it directly rather than shelling out to git, because this base
+# image has no git binary and adding an apt layer so the image can print seven
+# characters is a poor trade. See #233.
+#
+# There is no .dockerignore, so .git is in the build context here and in
+# Portainer, which clones the repository before building.
+COPY .git ./.git
+# writeBuildInfo runs against the compiled output, so it has to follow tsc. It
+# warns rather than fails when .git is unreadable: a deploy must never be
+# stopped by the thing whose only job is to say which deploy it is.
+RUN npm run build && node dist/writeBuildInfo.js
 
 FROM node:20-bookworm-slim
 WORKDIR /app
diff --git a/backend/src/app.ts b/backend/src/app.ts
index a0027da..5c3ccd9 100755
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -9,6 +9,7 @@ import adminSettingsRouter from './routes/adminSettings';
 import adminEmailTemplatesRouter from './routes/adminEmailTemplates';
 import adminCategoriesRouter from './routes/adminCategories';
 import adminTagsRouter from './routes/adminTags';
+import adminVersionRouter from './routes/adminVersion';
 import filtersRouter from './routes/filters';
 import customersRouter from './routes/customers';
 import publicRouter from './routes/public';
@@ -78,6 +79,7 @@ app.use('/api/admin/settings', requireAdminGate, adminSettingsRouter);
 app.use('/api/admin/email-templates', requireAdminGate, adminEmailTemplatesRouter);
 app.use('/api/admin/categories', requireAdminGate, adminCategoriesRouter);
 app.use('/api/admin/tags', requireAdminGate, adminTagsRouter);
+app.use('/api/admin/version', requireAdminGate, adminVersionRouter);
 app.use('/api/admin', requireAdminGate, adminRouter);
 app.use('/api/customers/me/addresses', shippingAddressesRouter);
 app.use('/api/customers', customersRouter);
diff --git a/backend/src/buildInfo.ts b/backend/src/buildInfo.ts
new file mode 100644
index 0000000..bc53067
--- /dev/null
+++ b/backend/src/buildInfo.ts
@@ -0,0 +1,157 @@
+import { readFileSync } from 'fs';
+import path from 'path';
+
+/**
+ * Which build is running, so a deployed environment can say so.
+ *
+ * There was previously no way to tell. On 2026-08-29 a QA container kept
+ * serving a pre-merge image after its stack was rebuilt, and the only thing
+ * that revealed it was npm happening to echo an old script line — had the
+ * change been anywhere other than a package.json script, the container would
+ * have looked healthy while running the wrong code. See #233.
+ *
+ * The commit is read out of `.git` directly rather than by shelling out to
+ * git: `node:20-bookworm-slim` has no git binary, and adding an apt layer to
+ * this image so that it can print seven characters is a poor trade.
+ *
+ * Everything here fails to `unknown` rather than throwing. This runs during a
+ * Docker build, and a version stamp must never be the thing that stops a
+ * deploy.
+ */
+
+export const UNKNOWN_COMMIT = 'unknown';
+
+/** Full 40-character object name, which is what both HEAD and refs contain. */
+const SHA_PATTERN = /^[0-9a-f]{40}$/;
+
+const SHORT_LENGTH = 7;
+
+/**
+ * The `.git` files this needs, as content rather than paths.
+ *
+ * Passed in rather than read here so the resolution rules are pure and can be
+ * tested without a repository on disk — the same reasoning `uploadTypes.ts`
+ * and `keyByCallerAndEmail` are shaped by.
+ */
+export interface GitSource {
+  /** `.git/HEAD`, or null when there is no `.git` at all. */
+  head: string | null;
+  /** `.git/<ref>` for a symbolic HEAD, or null when the ref is packed. */
+  readRef(ref: string): string | null;
+  /** `.git/packed-refs`, or null when the repository has none. */
+  packedRefs: string | null;
+}
+
+/**
+ * Finds `<sha> <ref>` in a packed-refs file.
+ *
+ * Lines beginning `#` are the header and lines beginning `^` are the object an
+ * annotated tag points at — neither is a ref, and treating a `^` line as one
+ * would return the wrong commit for any tag.
+ */
+function fromPackedRefs(packedRefs: string, ref: string): string | null {
+  for (const line of packedRefs.split('\n')) {
+    if (line.startsWith('#') || line.startsWith('^')) continue;
+    const [sha, name] = line.trim().split(/\s+/);
+    if (name === ref && sha && SHA_PATTERN.test(sha)) return sha;
+  }
+  return null;
+}
+
+/**
+ * The short commit for a checkout, or `unknown`.
+ *
+ * Two shapes of HEAD are possible and both occur here: a detached HEAD holds
+ * the object name directly, which is what a checkout of a specific ref
+ * produces, and a symbolic HEAD holds `ref: refs/heads/<name>`, which is what
+ * a working clone has. The ref may be loose or packed, and a fresh clone
+ * commonly packs it.
+ */
+export function resolveCommit(source: GitSource): string {
+  const head = source.head?.trim();
+  if (!head) return UNKNOWN_COMMIT;
+
+  if (SHA_PATTERN.test(head)) {
+    return head.slice(0, SHORT_LENGTH);
+  }
+
+  if (!head.startsWith('ref:')) {
+    // Neither a ref line nor an object name. Returning it verbatim would put
+    // whatever the file happened to contain onto the admin screen.
+    return UNKNOWN_COMMIT;
+  }
+
+  const ref = head.slice('ref:'.length).trim();
+  if (!ref) return UNKNOWN_COMMIT;
+
+  const loose = source.readRef(ref)?.trim();
+  if (loose && SHA_PATTERN.test(loose)) {
+    return loose.slice(0, SHORT_LENGTH);
+  }
+
+  const packed = source.packedRefs ? fromPackedRefs(source.packedRefs, ref) : null;
+  return packed ? packed.slice(0, SHORT_LENGTH) : UNKNOWN_COMMIT;
+}
+
+/** Reads a file, treating any failure as absence. */
+function readOrNull(filePath: string): string | null {
+  try {
+    return readFileSync(filePath, 'utf8');
+  } catch {
+    return null;
+  }
+}
+
+/** A `GitSource` backed by a real `.git` directory. */
+export function gitSourceAt(gitDir: string): GitSource {
+  return {
+    head: readOrNull(path.join(gitDir, 'HEAD')),
+    // The ref is a repository-relative path with forward slashes; join splits
+    // it correctly on both platforms.
+    readRef: (ref) => readOrNull(path.join(gitDir, ...ref.split('/'))),
+    packedRefs: readOrNull(path.join(gitDir, 'packed-refs'))
+  };
+}
+
+export interface BuildInfo {
+  commit: string;
+  builtAt: string | null;
+}
+
+/** Where the build writes its stamp, and where the server reads it back. */
+export const BUILD_INFO_PATH = path.join(__dirname, 'buildInfo.json');
+
+const MISSING: BuildInfo = { commit: UNKNOWN_COMMIT, builtAt: null };
+
+let cached: BuildInfo | null = null;
+
+/**
+ * The stamp written at build time.
+ *
+ * Read once and cached: it cannot change while the process lives, and this is
+ * on a request path. Absent in local development, where nothing has been
+ * built — reported as unknown rather than treated as an error, so `npm run
+ * dev` is unaffected.
+ */
+export function readBuildInfo(): BuildInfo {
+  if (cached) return cached;
+
+  const raw = readOrNull(BUILD_INFO_PATH);
+  if (!raw) {
+    cached = MISSING;
+    return cached;
+  }
+
+  try {
+    const parsed = JSON.parse(raw) as Partial<BuildInfo>;
+    cached = {
+      commit: typeof parsed.commit === 'string' ? parsed.commit : UNKNOWN_COMMIT,
+      builtAt: typeof parsed.builtAt === 'string' ? parsed.builtAt : null
+    };
+  } catch {
+    // A malformed stamp is not worth failing a boot over.
+    cached = MISSING;
+  }
+
+  return cached;
+}
diff --git a/backend/src/routes/adminVersion.ts b/backend/src/routes/adminVersion.ts
new file mode 100644
index 0000000..450855f
--- /dev/null
+++ b/backend/src/routes/adminVersion.ts
@@ -0,0 +1,23 @@
+import { Router, Request, Response } from 'express';
+import { readBuildInfo } from '../buildInfo';
+
+const router = Router();
+
+/**
+ * What build this environment is running (#233).
+ *
+ * Behind `requireAdminGate` like every other admin router, and deliberately
+ * not folded into `/api/config`. That endpoint is public — the storefront
+ * fetches it on every load — and a commit hash there would tell anyone exactly
+ * which revision of a public repository is deployed, which is free help to
+ * someone matching known issues against it. Nothing here is needed by a
+ * customer.
+ *
+ * Not wrapped in asyncRoute because the handler is synchronous: the stamp is
+ * read from disk once and cached, so there is no promise to reject.
+ */
+router.get('/', (_req: Request, res: Response) => {
+  res.json(readBuildInfo());
+});
+
+export default router;
diff --git a/backend/src/writeBuildInfo.ts b/backend/src/writeBuildInfo.ts
new file mode 100644
index 0000000..ea15e88
--- /dev/null
+++ b/backend/src/writeBuildInfo.ts
@@ -0,0 +1,67 @@
+/**
+ * Writes the build stamp that the admin reads back (#233).
+ *
+ * Runs once, at the end of the Docker build, against the compiled output:
+ *
+ *   npm run build && node dist/writeBuildInfo.js
+ *
+ * It has to run after `tsc` because `tsc` writes into `dist/` and would not
+ * remove a JSON file placed there first — but ordering it explicitly means the
+ * stamp is never left over from a previous build.
+ *
+ * Never fails the build. A missing or unreadable `.git` produces a stamp
+ * saying `unknown`, which is a worse answer than a commit and a much better
+ * one than a deploy that stopped. `.git` is absent from the final image by
+ * design; only this build stage sees it.
+ */
+
+import { writeFileSync, existsSync } from 'fs';
+import path from 'path';
+import { resolveCommit, gitSourceAt, BUILD_INFO_PATH, UNKNOWN_COMMIT, BuildInfo } from './buildInfo';
+
+/**
+ * Where `.git` is, relative to wherever this was run from.
+ *
+ * Two layouts, both real. In the container the repository's `.git` is copied
+ * beside the backend, so it sits in the working directory. Locally the backend
+ * is a subdirectory of the repository, so it is one level up. An explicit
+ * argument wins over both, which is what makes this testable by hand.
+ */
+function findGitDir(explicit?: string): string | null {
+  const candidates = [
+    explicit,
+    path.join(process.cwd(), '.git'),
+    path.join(process.cwd(), '..', '.git')
+  ].filter((candidate): candidate is string => typeof candidate === 'string');
+
+  return candidates.find((candidate) => existsSync(candidate)) ?? null;
+}
+
+export function buildStamp(gitDir: string | null): BuildInfo {
+  return {
+    commit: gitDir ? resolveCommit(gitSourceAt(gitDir)) : UNKNOWN_COMMIT,
+    // Whole seconds: this is read by a person comparing it to when they
+    // pressed a button, not by anything that needs precision.
+    builtAt: new Date().toISOString().replace(/\.\d{3}Z$/, 'Z')
+  };
+}
+
+// Guarded so that importing this module cannot rewrite the stamp of a running
+// deployment — the same reasoning as backfillImageReencode.ts (#231).
+if (require.main === module) {
+  const gitDir = findGitDir(process.argv[2]);
+  const stamp = buildStamp(gitDir);
+
+  if (stamp.commit === UNKNOWN_COMMIT) {
+    // Loud, because a deploy that cannot say what it is defeats the point of
+    // the stamp — but a warning, not a failure.
+    const where = gitDir ? ` at ${gitDir}` : '';
+    console.warn(
+      `[build-info] no readable .git found${where} — ` +
+        `the admin will report the commit as "${UNKNOWN_COMMIT}"`
+    );
+  }
+
+  writeFileSync(BUILD_INFO_PATH, `${JSON.stringify(stamp, null, 2)}\n`, 'utf8');
+  console.info(`[build-info] ${stamp.commit} built ${stamp.builtAt}`);
+}
diff --git a/backend/tests/integration/adminVersion.integration.test.ts b/backend/tests/integration/adminVersion.integration.test.ts
new file mode 100644
index 0000000..1347b31
--- /dev/null
+++ b/backend/tests/integration/adminVersion.integration.test.ts
@@ -0,0 +1,65 @@
+import request from 'supertest';
+import app from '../../src/app';
+import { pool } from '../../src/db';
+import { resetDb, closeDb } from './setup/testDb';
+
+// ADMIN_GATE_SECRET is read per request rather than at import, so it can be set
+// here without reloading the app, and is restored afterwards so this file
+// cannot change how anything later in the same process behaves. Same shape as
+// adminGate.integration.test.ts.
+const SECRET = 'integration-version-secret';
+const original = process.env.ADMIN_GATE_SECRET;
+let warn: jest.SpyInstance;
+
+beforeEach(async () => {
+  await resetDb();
+  warn = jest.spyOn(console, 'warn').mockImplementation(() => undefined);
+});
+
+afterEach(() => {
+  warn.mockRestore();
+  if (original === undefined) {
+    delete process.env.ADMIN_GATE_SECRET;
+  } else {
+    process.env.ADMIN_GATE_SECRET = original;
+  }
+});
+
+afterAll(async () => {
+  await pool.end();
+  await closeDb();
+});
+
+describe('reporting which build is running', () => {
+  it('returns a commit and a build time', async () => {
+    const res = await request(app).get('/api/admin/version');
+
+    expect(res.status).toBe(200);
+    expect(typeof res.body.commit).toBe('string');
+    expect(res.body.commit.length).toBeGreaterThan(0);
+    // Null under test, where nothing has been built. The contract is that the
+    // key is always present, so the admin never has to distinguish "absent"
+    // from "unbuilt".
+    expect(res.body).toHaveProperty('builtAt');
+  });
+
+  // The whole point is that this is not on public /api/config. A commit hash
+  // there would tell any visitor which revision of a public repository is
+  // deployed.
+  it('is behind the admin gate', async () => {
+    process.env.ADMIN_GATE_SECRET = SECRET;
+
+    const res = await request(app).get('/api/admin/version');
+
+    expect(res.status).toBe(403);
+    expect(res.body.error).toBe('forbidden');
+  });
+
+  it('does not leak the commit through the public config endpoint', async () => {
+    const res = await request(app).get('/api/config');
+
+    expect(res.status).toBe(200);
+    expect(res.body).not.toHaveProperty('commit');
+    expect(res.body).not.toHaveProperty('builtAt');
+  });
+});
diff --git a/backend/tests/unit/buildInfo.test.ts b/backend/tests/unit/buildInfo.test.ts
new file mode 100644
index 0000000..fd453fa
--- /dev/null
+++ b/backend/tests/unit/buildInfo.test.ts
@@ -0,0 +1,109 @@
+import { resolveCommit, GitSource, UNKNOWN_COMMIT } from '../../src/buildInfo';
+
+// A source with nothing in it, so each test states only the files it cares
+// about. Every field is deliberately explicit — a missing `.git` is a normal
+// case here, not an edge one, and it has to be as easy to write as the others.
+function source(overrides: Partial<GitSource> = {}): GitSource {
+  return {
+    head: null,
+    readRef: () => null,
+    packedRefs: null,
+    ...overrides
+  };
+}
+
+const SHA = '846646f7ec8ec0087ec1d89bcee8ab1865b57a7a';
+
+describe('resolveCommit', () => {
+  // What a fresh clone at a specific commit looks like — Portainer checks out
+  // a ref, and a detached HEAD holds the SHA with no indirection at all.
+  it('reads a detached HEAD directly', () => {
+    expect(resolveCommit(source({ head: `${SHA}\n` }))).toBe('846646f');
+  });
+
+  // What a working checkout looks like, including this repository.
+  it('follows a symbolic HEAD to its ref file', () => {
+    expect(
+      resolveCommit(
+        source({
+          head: 'ref: refs/heads/main\n',
+          readRef: (ref) => (ref === 'refs/heads/main' ? `${SHA}\n` : null)
+        })
+      )
+    ).toBe('846646f');
+  });
+
+  // A ref that has been packed has no loose file. Without this the common case
+  // of a freshly cloned repository resolves to unknown.
+  it('falls back to packed-refs when the ref file is absent', () => {
+    expect(
+      resolveCommit(
+        source({
+          head: 'ref: refs/heads/main\n',
+          packedRefs: `# pack-refs with: peeled fully-peeled sorted\n${SHA} refs/heads/main\n`
+        })
+      )
+    ).toBe('846646f');
+  });
+
+  it('picks the right entry out of a packed-refs with many', () => {
+    const other = 'ffffffffffffffffffffffffffffffffffffffff';
+    expect(
+      resolveCommit(
+        source({
+          head: 'ref: refs/heads/main\n',
+          packedRefs:
+            `# pack-refs with: peeled fully-peeled sorted\n` +
+            `${other} refs/heads/other\n` +
+            `${SHA} refs/heads/main\n` +
+            `${other} refs/tags/v1\n`
+        })
+      )
+    ).toBe('846646f');
+  });
+
+  // Annotated tags write a second `^<sha>` line for the object the tag points
+  // at. It is not a ref line and must not be mistaken for one.
+  it('ignores peeled tag lines in packed-refs', () => {
+    expect(
+      resolveCommit(
+        source({
+          head: 'ref: refs/tags/v1\n',
+          packedRefs:
+            `${SHA} refs/tags/v1\n` + `^ffffffffffffffffffffffffffffffffffffffff\n`
+        })
+      )
+    ).toBe('846646f');
+  });
+
+  // Every failure below returns UNKNOWN rather than throwing. This runs during
+  // a Docker build; a version stamp must never be the thing that stops a deploy.
+  it('is unknown when there is no .git at all', () => {
+    expect(resolveCommit(source())).toBe(UNKNOWN_COMMIT);
+  });
+
+  it('is unknown when the ref resolves to nothing', () => {
+    expect(resolveCommit(source({ head: 'ref: refs/heads/main\n' }))).toBe(UNKNOWN_COMMIT);
+  });
+
+  it('is unknown when packed-refs does not mention the ref', () => {
+    expect(
+      resolveCommit(
+        source({
+          head: 'ref: refs/heads/gone\n',
+          packedRefs: `${SHA} refs/heads/main\n`
+        })
+      )
+    ).toBe(UNKNOWN_COMMIT);
+  });
+
+  // A HEAD that is neither a ref line nor a SHA. Returning it verbatim would
+  // put arbitrary file contents on the admin screen.
+  it('is unknown when HEAD is not something it recognises', () => {
+    expect(resolveCommit(source({ head: 'not a sha and not a ref\n' }))).toBe(UNKNOWN_COMMIT);
+  });
+
+  it('is unknown when HEAD is empty', () => {
+    expect(resolveCommit(source({ head: '   \n' }))).toBe(UNKNOWN_COMMIT);
+  });
+});
diff --git a/frontend/src/admin/Admin.tsx b/frontend/src/admin/Admin.tsx
index 23a0a44..475548e 100755
--- a/frontend/src/admin/Admin.tsx
+++ b/frontend/src/admin/Admin.tsx
@@ -33,6 +33,7 @@ import Emails from './Emails';
 import Settings from './Settings';
 import Categories from './Categories';
 import Tags from './Tags';
+import BuildStamp from './BuildStamp';
 import CategoryTreeSelect from './CategoryTreeSelect';
 import ItemCard from '../components/ItemCard';
 import InventoryFilters from './InventoryFilters';
@@ -379,6 +380,7 @@ export default function Admin() {
       <Header className="site-header" style={{ background: token.colorBgContainer, borderBottom: `1px solid ${token.colorBorderSecondary}` }}>
         <Title level={3} className="site-header-title" style={{ color: token.colorText }}>Admin</Title>
         <div className="site-header-actions">
+          <BuildStamp />
           <Switch checked={mode === 'dark'} onChange={toggle} checkedChildren="Dark" unCheckedChildren="Light" />
         </div>
       </Header>
diff --git a/frontend/src/admin/BuildStamp.tsx b/frontend/src/admin/BuildStamp.tsx
new file mode 100644
index 0000000..828ac71
--- /dev/null
+++ b/frontend/src/admin/BuildStamp.tsx
@@ -0,0 +1,61 @@
+import React, { useEffect, useState } from 'react';
+import Typography from 'antd/es/typography';
+
+const { Text } = Typography;
+
+interface BuildInfo {
+  commit: string;
+  builtAt: string | null;
+}
+
+/**
+ * Which build this environment is running (#233).
+ *
+ * There was previously no way to tell from the screen, and a stack that had
+ * been redeployed could still be serving the previous image. Shown in the admin
+ * header rather than tucked into a settings tab, because its whole value is
+ * being visible without going to look for it.
+ *
+ * Served from the gated /api/admin/version rather than the public /api/config,
+ * so a commit hash is never handed to a storefront visitor.
+ */
+export default function BuildStamp() {
+  const [info, setInfo] = useState<BuildInfo | null>(null);
+
+  useEffect(() => {
+    let cancelled = false;
+
+    void fetch('/api/admin/version')
+      .then((res) => (res.ok ? res.json() : null))
+      .then((data: BuildInfo | null) => {
+        if (!cancelled) setInfo(data);
+      })
+      // Renders nothing on failure. A version stamp is a convenience, and it
+      // should not put an error in front of someone doing something else.
+      .catch(() => undefined);
+
+    return () => {
+      cancelled = true;
+    };
+  }, []);
+
+  if (!info) return null;
+
+  // Local time and no seconds: this is read by a person comparing it against
+  // when they pressed redeploy, not by anything that needs precision.
+  const built = info.builtAt
+    ? new Date(info.builtAt).toLocaleString(undefined, {
+        day: 'numeric',
+        month: 'short',
+        hour: '2-digit',
+        minute: '2-digit'
+      })
+    : null;
+
+  return (
+    <Text type="secondary" style={{ fontSize: 12, fontFamily: 'monospace' }}>
+      {info.commit}
+      {built ? ` · built ${built}` : ''}
+    </Text>
+  );
+}