diff --git a/packages/cli/src/modules/start/commands/repo/start.test.ts b/packages/cli/src/modules/start/commands/repo/start.test.ts
new file mode 100644
index 0000000000..2950552209
--- /dev/null
+++ b/packages/cli/src/modules/start/commands/repo/start.test.ts
@@ -0,0 +1,221 @@
+/*
+ * Copyright 2025 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { PackageGraph } from '@backstage/cli-node';
+import { findTargetPackages } from './start';
+import { posix } from 'path';
+import { paths } from '../../../../lib/paths';
+
+const mocks = {
+  app: {
+    packageJson: {
+      name: 'app',
+      version: '0',
+      backstage: { role: 'frontend' },
+    },
+    dir: '/root/packages/app',
+  },
+  backend: {
+    packageJson: {
+      name: 'backend',
+      version: '0',
+      backstage: { role: 'backend' },
+    },
+    dir: '/root/packages/backend',
+  },
+  appNext: {
+    packageJson: {
+      name: 'app-next',
+      version: '0',
+      backstage: { role: 'frontend' },
+    },
+    dir: '/root/packages/app-next',
+  },
+  backendNext: {
+    packageJson: {
+      name: 'backend-next',
+      version: '0',
+      backstage: { role: 'backend' },
+    },
+    dir: '/root/packages/backend-next',
+  },
+  otherApp: {
+    packageJson: {
+      name: 'other-app',
+      version: '0',
+      backstage: { role: 'frontend' },
+    },
+    dir: '/root/packages/other-app',
+  },
+  pluginX: {
+    packageJson: {
+      name: 'plugin-x',
+      version: '0',
+      backstage: { role: 'frontend-plugin', pluginId: 'x' },
+    },
+    dir: '/root/plugins/plugin-x',
+  },
+  pluginXBackend: {
+    packageJson: {
+      name: 'plugin-x-backend',
+      version: '0',
+      backstage: { role: 'backend-plugin', pluginId: 'x' },
+    },
+    dir: '/root/plugins/plugin-x-backend',
+  },
+  pluginY: {
+    packageJson: {
+      name: 'plugin-y',
+      version: '0',
+      backstage: { role: 'frontend-plugin', pluginId: 'y' },
+    },
+    dir: '/root/plugins/plugin-y',
+  },
+  pluginYBackend: {
+    packageJson: {
+      name: 'plugin-y-backend',
+      version: '0',
+      backstage: { role: 'backend-plugin', pluginId: 'y' },
+    },
+    dir: '/root/plugins/plugin-y-backend',
+  },
+} as const;
+
+describe('findTargetPackages', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+    jest
+      .spyOn(paths, 'resolveTargetRoot')
+      .mockImplementation((...parts: string[]) => {
+        return posix.resolve('/root', ...parts);
+      });
+  });
+
+  it('should select default packages', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue(Object.values(mocks));
+    const result = await findTargetPackages([], []);
+    expect(result).toEqual([mocks.app, mocks.backend]);
+  });
+
+  it('should select packages by plugin ID', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue(Object.values(mocks));
+    const result = await findTargetPackages([], ['x']);
+    expect(result).toEqual([mocks.pluginX, mocks.pluginXBackend]);
+  });
+
+  it('should throw an error if no packages match the plugin ID', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue(Object.values(mocks));
+    await expect(
+      findTargetPackages([], ['nonexistent-plugin']),
+    ).rejects.toThrow(
+      "Unable to find any plugin packages with plugin ID 'nonexistent-plugin'. Make sure backstage.pluginId is set in your package.json files by running 'yarn fix --publish'.",
+    );
+  });
+
+  it('should select packages by explicit names', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue(Object.values(mocks));
+    const result = await findTargetPackages(['other-app'], []);
+    expect(result).toEqual([mocks.otherApp]);
+  });
+
+  it('should throw an error if no package matches the explicit name', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue(Object.values(mocks));
+    await expect(
+      findTargetPackages(['nonexistent-package'], []),
+    ).rejects.toThrow("Unable to find package by name 'nonexistent-package'");
+  });
+
+  it('should select packages by relative path', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue(Object.values(mocks));
+    const result = await findTargetPackages(
+      ['packages/app', 'packages/backend-next'],
+      [],
+    );
+    expect(result).toEqual([mocks.app, mocks.backendNext]);
+  });
+
+  it('should throw an error if no package matches the relative path', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue(Object.values(mocks));
+    await expect(findTargetPackages(['nonexistent/path'], [])).rejects.toThrow(
+      "Unable to find package by name 'nonexistent/path'",
+    );
+  });
+
+  it('should select a single frontend or backend package if no arguments are provided', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue([mocks.app]);
+    const result = await findTargetPackages([], []);
+    expect(result).toEqual([mocks.app]);
+  });
+
+  it('should throw an error if multiple frontend packages other than packages/app are found without explicit selection', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue([mocks.otherApp, mocks.appNext]);
+    await expect(findTargetPackages([], [])).rejects.toThrow(
+      "Found multiple packages with role 'frontend' but none of the use the default path '/root/packages/app',choose which packages you want to run by passing the package names explicitly as arguments, for example 'yarn backstage-cli repo start my-app my-backend'.",
+    );
+  });
+
+  it('should select a single plugin package if no app or backend packages are found', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue([mocks.pluginX]);
+    const result = await findTargetPackages([], []);
+    expect(result).toEqual([mocks.pluginX]);
+  });
+
+  it('should select a pair of plugin packages if no app or backend packages are found', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue([mocks.pluginX, mocks.pluginXBackend]);
+    const result = await findTargetPackages([], []);
+    expect(result).toEqual([mocks.pluginX, mocks.pluginXBackend]);
+  });
+
+  // Right now we're not validating this because it requires backstage.pluginId to be set, and it's a strange case anyway
+  it('should select a pair of plugin packages even if they are from different plugins', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue([mocks.pluginX, mocks.pluginYBackend]);
+    const result = await findTargetPackages([], []);
+    expect(result).toEqual([mocks.pluginX, mocks.pluginYBackend]);
+  });
+
+  it('should throw an error if multiple plugin packages are found without explicit selection', async () => {
+    jest
+      .spyOn(PackageGraph, 'listTargetPackages')
+      .mockResolvedValue([mocks.pluginX, mocks.pluginY]);
+    await expect(findTargetPackages([], [])).rejects.toThrow(
+      "Found multiple packages with role 'frontend-plugin', please choose which packages you want to run by passing the package names explicitly as arguments, for example 'yarn backstage-cli repo start my-plugin my-plugin-backend'.",
+    );
+  });
+});
diff --git a/packages/cli/src/modules/start/commands/repo/start.ts b/packages/cli/src/modules/start/commands/repo/start.ts
index b25054bbfc..b0bb8f5c01 100644
--- a/packages/cli/src/modules/start/commands/repo/start.ts
+++ b/packages/cli/src/modules/start/commands/repo/start.ts
@@ -61,7 +61,10 @@ export async function command(namesOrPaths: string[], options: CommandOptions) {
   await Promise.all(packageOptions.map(entry => startPackage(entry.options)));
 }
 
-async function findTargetPackages(namesOrPaths: string[], pluginIds: string[]) {
+export async function findTargetPackages(
+  namesOrPaths: string[],
+  pluginIds: string[],
+) {
   const targetPackages = new Array<BackstagePackage>();
 
   const packages = await PackageGraph.listTargetPackages();
@@ -75,7 +78,7 @@ async function findTargetPackages(namesOrPaths: string[], pluginIds: string[]) {
         ACCEPTED_PACKAGE_ROLES.includes(pkg.packageJson.backstage.role)
       );
     });
-    if (!matchingPackages) {
+    if (matchingPackages.length === 0) {
       throw new Error(
         `Unable to find any plugin packages with plugin ID '${pluginId}'. Make sure backstage.pluginId is set in your package.json files by running 'yarn fix --publish'.`,
       );
@@ -145,7 +148,7 @@ async function findTargetPackages(namesOrPaths: string[], pluginIds: string[]) {
     );
     if (matchingPackages.length > 1) {
       throw new Error(
-        `Found multiple packages with role '${role}', please choose which packages you want` +
+        `Found multiple packages with role '${role}', please choose which packages you want ` +
           `to run by passing the package names explicitly as arguments, for example ` +
           `'yarn backstage-cli repo start my-plugin my-plugin-backend'.`,
       );