diff --git a/packages/cli/src/commands/extensions/install.test.ts b/packages/cli/src/commands/extensions/install.test.ts
index 8b3f8c5807f..4db78548a20 100644
--- a/packages/cli/src/commands/extensions/install.test.ts
+++ b/packages/cli/src/commands/extensions/install.test.ts
@@ -227,14 +227,16 @@ describe('handleInstall', () => {
   });
 
   it('throws an error from an unknown source', async () => {
-    mockInferInstallMetadata.mockRejectedValue(
-      new Error('Install source not found.'),
-    );
+    const errorMessage =
+      'Install source not found: "test://google.com". If you are installing ' +
+      'from GitHub, provide the full repository URL ' +
+      '(e.g. "https://github.com/owner/repo").';
+    mockInferInstallMetadata.mockRejectedValue(new Error(errorMessage));
     await handleInstall({
       source: 'test://google.com',
     });
 
-    expect(debugErrorSpy).toHaveBeenCalledWith('Install source not found.');
+    expect(debugErrorSpy).toHaveBeenCalledWith(errorMessage);
     expect(processSpy).toHaveBeenCalledWith(1);
   });
 
diff --git a/packages/cli/src/config/extension-manager.test.ts b/packages/cli/src/config/extension-manager.test.ts
index 33c335c16ba..5547da1f6b3 100644
--- a/packages/cli/src/config/extension-manager.test.ts
+++ b/packages/cli/src/config/extension-manager.test.ts
@@ -8,7 +8,7 @@ import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import * as fs from 'node:fs';
 import * as os from 'node:os';
 import * as path from 'node:path';
-import { ExtensionManager } from './extension-manager.js';
+import { ExtensionManager, inferInstallMetadata } from './extension-manager.js';
 import { createTestMergedSettings, type MergedSettings } from './settings.js';
 import { createExtension } from '../test-utils/createExtension.js';
 import { EXTENSIONS_DIRECTORY_NAME } from './extensions/variables.js';
@@ -637,4 +637,31 @@ describe('ExtensionManager', () => {
       );
     });
   });
+
+  describe('inferInstallMetadata', () => {
+    it('throws a helpful error when a local install source is not found', async () => {
+      const missingSource = path.join(tempHomeDir, 'does-not-exist');
+
+      await expect(inferInstallMetadata(missingSource)).rejects.toThrow(
+        /Install source not found/,
+      );
+    });
+
+    it('suggests the GitHub URL form and an auth hint in the error', async () => {
+      const missingSource = path.join(tempHomeDir, 'does-not-exist');
+
+      let thrown: Error | undefined;
+      try {
+        await inferInstallMetadata(missingSource);
+      } catch (e) {
+        thrown = e as Error;
+      }
+
+      expect(thrown).toBeDefined();
+      expect(thrown!.message).toContain(missingSource);
+      expect(thrown!.message).toContain('https://github.com/owner/repo');
+      expect(thrown!.message.toLowerCase()).toContain('authenticated');
+      expect(thrown!.message).toContain('sso://');
+    });
+  });
 });
diff --git a/packages/cli/src/config/extension-manager.ts b/packages/cli/src/config/extension-manager.ts
index ded72510fc7..5585bb2d9ac 100644
--- a/packages/cli/src/config/extension-manager.ts
+++ b/packages/cli/src/config/extension-manager.ts
@@ -1325,7 +1325,14 @@ export async function inferInstallMetadata(
         type: 'local',
       };
     } catch {
-      throw new Error('Install source not found.');
+      throw new Error(
+        `Install source not found: "${source}". ` +
+          `If you are installing from GitHub, provide the full repository URL ` +
+          `(e.g. "https://github.com/owner/repo"). ` +
+          `For private or SSO-protected repositories, make sure you are ` +
+          `authenticated (e.g. via an SSH key with the appropriate SSO ` +
+          `authorization, or by using an "sso://" URL).`,
+      );
     }
   }
 }