diff --git a/packages/angular/cli/src/commands/update/cli.ts b/packages/angular/cli/src/commands/update/cli.ts
index de6d7f53fea0..fe4620870327 100644
--- a/packages/angular/cli/src/commands/update/cli.ts
+++ b/packages/angular/cli/src/commands/update/cli.ts
@@ -231,6 +231,17 @@ export default class UpdateCommandModule extends CommandModule<UpdateCommandArgs
     logger.info('Collecting installed dependencies...');
 
     const rootDependencies = await packageManager.getProjectDependencies();
+
+    // In npm/pnpm/yarn workspace setups the package manager's `list` command is
+    // executed against the workspace root, so it may only surface the root
+    // workspace's direct dependencies.  When the Angular project lives inside a
+    // workspace member its own `package.json` entries (e.g. `@angular/core`) will
+    // be absent from that list.  To preserve the pre-v21 behaviour we supplement
+    // the map with any packages declared in the Angular project root's
+    // `package.json` that are resolvable from `node_modules` but were not already
+    // returned by the package manager.
+    await supplementWithLocalDependencies(rootDependencies, this.context.root);
+
     logger.info(`Found ${rootDependencies.size} dependencies.`);
 
     const workflow = new NodeWorkflow(this.context.root, {
@@ -675,22 +686,64 @@ export default class UpdateCommandModule extends CommandModule<UpdateCommandArgs
   }
 }
 
-async function readPackageManifest(manifestPath: string): Promise<PackageManifest | undefined> {
-  try {
-    const content = await fs.readFile(manifestPath, 'utf8');
+/**
+ * Supplements the given dependency map with packages that are declared in the
+ * Angular project root's `package.json` but were not returned by the package
+ * manager's `list` command.
+ *
+ * In npm/pnpm/yarn workspace setups the package manager runs against the
+ * workspace root, which may not include dependencies that only appear in a
+ * workspace member's `package.json`.  Reading the member's `package.json`
+ * directly and resolving the installed version from `node_modules` restores
+ * the behaviour that was present before the package-manager abstraction was
+ * introduced in v21.
+ *
+ * @param dependencies The map to supplement in place.
+ * @param projectRoot The root directory of the Angular project (workspace member).
+ */
+export async function supplementWithLocalDependencies(
+  dependencies: Map<string, InstalledPackage>,
+  projectRoot: string,
+): Promise<void> {
+  const localManifest = await readPackageManifest(path.join(projectRoot, 'package.json'));
+  if (!localManifest) {
+    return;
+  }
 
-    return JSON.parse(content) as PackageManifest;
-  } catch {
-    return undefined;
+  const localDeps: Record<string, string> = {
+    ...localManifest.dependencies,
+    ...localManifest.devDependencies,
+    ...localManifest.peerDependencies,
+  };
+
+  const projectRequire = createRequire(path.join(projectRoot, 'package.json'));
+
+  for (const depName of Object.keys(localDeps)) {
+    if (dependencies.has(depName)) {
+      continue;
+    }
+    let pkgJsonPath: string;
+    try {
+      pkgJsonPath = projectRequire.resolve(`${depName}/package.json`);
+    } catch {
+      continue;
+    }
+    const installed = await readPackageManifest(pkgJsonPath);
+    if (installed?.version) {
+      dependencies.set(depName, {
+        name: depName,
+        version: installed.version,
+        path: path.dirname(pkgJsonPath),
+      });
+    }
   }
 }
 
-function findPackageJson(workspaceDir: string, packageName: string): string | undefined {
+async function readPackageManifest(manifestPath: string): Promise<PackageManifest | undefined> {
   try {
-    const projectRequire = createRequire(path.join(workspaceDir, 'package.json'));
-    const packageJsonPath = projectRequire.resolve(`${packageName}/package.json`);
+    const content = await fs.readFile(manifestPath, 'utf8');
 
-    return packageJsonPath;
+    return JSON.parse(content) as PackageManifest;
   } catch {
     return undefined;
   }
diff --git a/packages/angular/cli/src/commands/update/cli_spec.ts b/packages/angular/cli/src/commands/update/cli_spec.ts
new file mode 100644
index 000000000000..24719548576d
--- /dev/null
+++ b/packages/angular/cli/src/commands/update/cli_spec.ts
@@ -0,0 +1,150 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import * as fs from 'node:fs/promises';
+import * as os from 'node:os';
+import * as path from 'node:path';
+import type { InstalledPackage } from '../../package-managers';
+import { supplementWithLocalDependencies } from './cli';
+
+/**
+ * Creates a minimal on-disk fixture that simulates an npm workspace member:
+ *
+ *   <projectRoot>/
+ *     package.json          ← Angular project manifest (workspace member)
+ *     node_modules/
+ *       <depName>/
+ *         package.json      ← installed package manifest
+ */
+async function createWorkspaceMemberFixture(options: {
+  projectDeps: Record<string, string>;
+  installedPackages: Array<{ name: string; version: string }>;
+}): Promise<string> {
+  const projectRoot = await fs.mkdtemp(path.join(os.tmpdir(), 'ng-update-spec-'));
+
+  // Write the Angular project's package.json
+  await fs.writeFile(
+    path.join(projectRoot, 'package.json'),
+    JSON.stringify({
+      name: 'test-app',
+      version: '0.0.0',
+      dependencies: options.projectDeps,
+    }),
+  );
+
+  // Write each installed package into node_modules
+  for (const pkg of options.installedPackages) {
+    // Support scoped packages like @angular/core
+    const pkgDir = path.join(projectRoot, 'node_modules', ...pkg.name.split('/'));
+    await fs.mkdir(pkgDir, { recursive: true });
+    await fs.writeFile(
+      path.join(pkgDir, 'package.json'),
+      JSON.stringify({ name: pkg.name, version: pkg.version }),
+    );
+  }
+
+  return projectRoot;
+}
+
+describe('supplementWithLocalDependencies', () => {
+  let tmpDir: string;
+
+  afterEach(async () => {
+    if (tmpDir) {
+      await fs.rm(tmpDir, { recursive: true, force: true });
+    }
+  });
+
+  it('should add packages from the local package.json that are missing from the dependency map', async () => {
+    // Simulates an npm workspace member where `npm list` (run against the
+    // workspace root) did not return `@angular/core`, even though it is
+    // declared in the member's package.json and installed in node_modules.
+    tmpDir = await createWorkspaceMemberFixture({
+      projectDeps: { '@angular/core': '^21.0.0' },
+      installedPackages: [{ name: '@angular/core', version: '21.2.4' }],
+    });
+
+    const deps = new Map<string, InstalledPackage>();
+
+    await supplementWithLocalDependencies(deps, tmpDir);
+
+    expect(deps.has('@angular/core')).toBeTrue();
+    expect(deps.get('@angular/core')?.version).toBe('21.2.4');
+  });
+
+  it('should not overwrite a package that is already present in the dependency map', async () => {
+    tmpDir = await createWorkspaceMemberFixture({
+      projectDeps: { '@angular/core': '^21.0.0' },
+      installedPackages: [{ name: '@angular/core', version: '21.2.4' }],
+    });
+
+    // The package manager already returned a version for @angular/core.
+    const existingEntry: InstalledPackage = { name: '@angular/core', version: '21.0.0' };
+    const deps = new Map<string, InstalledPackage>([['@angular/core', existingEntry]]);
+
+    await supplementWithLocalDependencies(deps, tmpDir);
+
+    // The existing entry must not be overwritten.
+    expect(deps.get('@angular/core')).toBe(existingEntry);
+    expect(deps.get('@angular/core')?.version).toBe('21.0.0');
+  });
+
+  it('should skip packages that are declared in package.json but not installed in node_modules', async () => {
+    tmpDir = await createWorkspaceMemberFixture({
+      projectDeps: { 'not-installed': '^1.0.0' },
+      installedPackages: [],
+    });
+
+    const deps = new Map<string, InstalledPackage>();
+
+    await supplementWithLocalDependencies(deps, tmpDir);
+
+    // Package is not installed; should not be added.
+    expect(deps.has('not-installed')).toBeFalse();
+  });
+
+  it('should handle devDependencies and peerDependencies in addition to dependencies', async () => {
+    tmpDir = await createWorkspaceMemberFixture({
+      projectDeps: {},
+      installedPackages: [
+        { name: 'rxjs', version: '7.8.2' },
+        { name: 'zone.js', version: '0.15.0' },
+      ],
+    });
+
+    // Write a package.json that uses devDependencies and peerDependencies.
+    await fs.writeFile(
+      path.join(tmpDir, 'package.json'),
+      JSON.stringify({
+        name: 'test-app',
+        version: '0.0.0',
+        devDependencies: { 'zone.js': '~0.15.0' },
+        peerDependencies: { rxjs: '~7.8.0' },
+      }),
+    );
+
+    const deps = new Map<string, InstalledPackage>();
+
+    await supplementWithLocalDependencies(deps, tmpDir);
+
+    expect(deps.has('zone.js')).toBeTrue();
+    expect(deps.get('zone.js')?.version).toBe('0.15.0');
+    expect(deps.has('rxjs')).toBeTrue();
+    expect(deps.get('rxjs')?.version).toBe('7.8.2');
+  });
+
+  it('should do nothing when the project root has no package.json', async () => {
+    tmpDir = await fs.mkdtemp(path.join(os.tmpdir(), 'ng-update-spec-'));
+
+    const deps = new Map<string, InstalledPackage>();
+
+    // Should resolve without throwing.
+    await expectAsync(supplementWithLocalDependencies(deps, tmpDir)).toBeResolved();
+    expect(deps.size).toBe(0);
+  });
+});