diff --git a/adminforth/documentation/docs/tutorial/03-Customization/12-security.md b/adminforth/documentation/docs/tutorial/03-Customization/12-security.md
@@ -296,6 +296,8 @@ So to completely hide the email field from all users apart superadmins, you shou
 
 So if you will configure the email column in user resource like this, only superadmin will be able to see emails, and only in the list view.
 
+> ☝️ `backendOnly` cannot be set on a primary key column (neither `true` nor a function): frontend needs primary key values to build record links, so AdminForth refuses such config on startup.
+
 ## Blocking login attempts before credentials check
 
 `auth.beforeLoginAttempt` hooks are called on every request to the login endpoint, before AdminForth looks the user up in the database.

diff --git a/adminforth/modules/configValidator.ts b/adminforth/modules/configValidator.ts
@@ -1352,6 +1352,13 @@ export default class ConfigValidator implements IConfigValidator {
 
   postProcessAfterDiscover(resource: AdminForthResource) {
     resource.columns.forEach((column) => {
+      // checked after discovery, because primaryKey may come from the database schema.
+      // frontend builds record links from primary key values, so they must always reach it.
+      // a backendOnly function is rejected too: it can hide the key only for some users
+      if (column.primaryKey && column.backendOnly) {
+        throw new Error(`Resource "${resource.resourceId}" column "${column.name}" is a primary key and cannot be backendOnly`);
+      }
+
       // if db/user says column is required in boolean, expand
       if (typeof column.required === 'boolean') {
         column.required = { create: column.required, edit: column.required };

diff --git a/adminforth/types/Common.ts b/adminforth/types/Common.ts
@@ -1037,6 +1037,7 @@ export interface AdminForthResourceColumnInputCommon {
 
   /**
    * if true field will !not be passed to UI under no circumstances, but will be presented in hooks
+   * Not allowed on primary key columns.
    */
   backendOnly?: boolean,
 

diff --git a/tests/jest_tests/primary_key_backend_only.test.ts b/tests/jest_tests/primary_key_backend_only.test.ts
@@ -0,0 +1,104 @@
+import { mkdtempSync, rmSync } from 'node:fs';
+import { createRequire } from 'node:module';
+import os from 'node:os';
+import path from 'node:path';
+import { fileURLToPath, pathToFileURL } from 'node:url';
+
+const currentDir = path.dirname(fileURLToPath(import.meta.url));
+const applicationDir = path.resolve(currentDir, '../application');
+
+// resolves to the BUILT package (adminforth/dist) the test application runs on;
+// rebuild it (`npx tsc` in adminforth/) after editing adminforth sources or the assertions below test stale code
+const require = createRequire(import.meta.url);
+// jest's resolver does not know the `node:sqlite` builtin, so it is required directly
+const { DatabaseSync } = require('node:sqlite');
+const adminforthEntry = require.resolve('adminforth', { paths: [applicationDir] });
+const { default: AdminForth } = await import(pathToFileURL(adminforthEntry).href);
+
+// own fixture, so the primary key is discovered from a known schema
+const fixtureDir = mkdtempSync(path.join(os.tmpdir(), 'af-pk-backend-only-'));
+const dbPath = path.join(fixtureDir, 'db.sqlite');
+const db = new DatabaseSync(dbPath);
+db.exec(`
+  CREATE TABLE users (id INTEGER PRIMARY KEY, email TEXT NOT NULL, password_hash TEXT NOT NULL);
+  INSERT INTO users VALUES (1, 'pk@example.test', 'hidden');
+`);
+db.close();
+
+process.env.ADMINFORTH_SECRET ??= 'x'.repeat(64);
+
+afterAll(() => {
+  rmSync(fixtureDir, { recursive: true, force: true });
+});
+
+// constructor validation and database discovery together, as on real startup
+async function startApp(idColumn: Record<string, unknown>) {
+  // AdminForth allows one instance per process; each test needs a fresh one
+  delete (globalThis as any).adminforth;
+  const app = new AdminForth({
+    baseUrl: '',
+    auth: {
+      usersResourceId: 'users',
+      usernameField: 'email',
+      passwordHashField: 'password_hash',
+    },
+    dataSources: [{ id: 'sqlite', url: `sqlite://${dbPath}` }],
+    resources: [{
+      resourceId: 'users',
+      table: 'users',
+      dataSource: 'sqlite',
+      columns: [
+        { name: 'id', ...idColumn },
+        { name: 'email' },
+        { name: 'password_hash', backendOnly: true },
+      ],
+    }],
+    menu: [{ label: 'Users', resourceId: 'users' }],
+  });
+  await app.discoverDatabases();
+  return app;
+}
+
+async function listUsers(app: any) {
+  const endpoints: any[] = [];
+  app.restApi.registerEndpoints({ endpoint: (config: any) => endpoints.push(config) });
+  const { handler } = endpoints.find((endpoint) => endpoint.path === '/get_resource_data');
+  return handler({
+    body: { resourceId: 'users', source: 'list', limit: 10, offset: 0, filters: [], sort: [] },
+    adminUser: { pk: '1', username: 'pk@example.test', dbUser: {} },
+    headers: {},
+    query: {},
+    cookies: {},
+    requestUrl: '/get_resource_data',
+    abortSignal: new AbortController().signal,
+  });
+}
+
+const pkBackendOnlyError = /column "id" is a primary key and cannot be backendOnly/;
+
+// `primaryKey` is omitted, not set to undefined: undefined would override the discovered value on merge
+describe.each([
+  ['declared', { primaryKey: true }],
+  ['discovered from database', {}],
+])('%s primary key', (_, pkFlag) => {
+  it.each([
+    ['true', true],
+    ['function returning true', () => true],
+    ['function returning false', () => false],
+  ])('rejects startup with backendOnly %s', async (_, backendOnly) => {
+    await expect(startApp({ ...pkFlag, backendOnly })).rejects.toThrow(pkBackendOnlyError);
+  });
+
+  it.each([
+    ['false', { backendOnly: false }],
+    ['omitted', {}],
+  ])('starts with backendOnly %s and keeps the id in list rows', async (_, backendOnlyFlag) => {
+    const app = await startApp({ ...pkFlag, ...backendOnlyFlag });
+    const result = await listUsers(app);
+
+    expect(result.data).toEqual([expect.objectContaining({ id: 1, email: 'pk@example.test' })]);
+    expect(result.recordIds).toEqual([1]);
+    // non-primary-key backendOnly column stays allowed and hidden
+    expect(result.data[0]).not.toHaveProperty('password_hash');
+  });
+});
-Original file line number
+Diff line change
@@ Expand Up / @@ -1037,6 +1037,7 @@ export interface AdminForthResourceColumnInputCommon { @@
       /**
        * if true field will !not be passed to UI under no circumstances, but will be presented in hooks
+       * Not allowed on primary key columns.
        */
       backendOnly?: boolean,
@@ Expand Down @@