diff --git a/tests/grok-adapter.test.ts b/tests/grok-adapter.test.ts
new file mode 100644
index 0000000..bb459b2
--- /dev/null
+++ b/tests/grok-adapter.test.ts
@@ -0,0 +1,74 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+// ---------------------------------------------------------------------------
+// Mock OpenAI constructor (must be hoisted for Vitest)
+// ---------------------------------------------------------------------------
+const OpenAIMock = vi.hoisted(() => vi.fn())
+
+vi.mock('openai', () => ({
+  default: OpenAIMock,
+}))
+
+import { GrokAdapter } from '../src/llm/grok.js'
+import { createAdapter } from '../src/llm/adapter.js'
+
+// ---------------------------------------------------------------------------
+// GrokAdapter tests
+// ---------------------------------------------------------------------------
+
+describe('GrokAdapter', () => {
+  beforeEach(() => {
+    OpenAIMock.mockClear()
+  })
+
+  it('has name "grok"', () => {
+    const adapter = new GrokAdapter()
+    expect(adapter.name).toBe('grok')
+  })
+
+  it('uses XAI_API_KEY by default', () => {
+    const original = process.env['XAI_API_KEY']
+    process.env['XAI_API_KEY'] = 'xai-test-key-123'
+
+    try {
+      new GrokAdapter()
+      expect(OpenAIMock).toHaveBeenCalledWith(
+        expect.objectContaining({
+          apiKey: 'xai-test-key-123',
+          baseURL: 'https://api.x.ai/v1',
+        })
+      )
+    } finally {
+      if (original === undefined) {
+        delete process.env['XAI_API_KEY']
+      } else {
+        process.env['XAI_API_KEY'] = original
+      }
+    }
+  })
+
+  it('uses official xAI baseURL by default', () => {
+    new GrokAdapter('some-key')
+    expect(OpenAIMock).toHaveBeenCalledWith(
+      expect.objectContaining({
+        apiKey: 'some-key',
+        baseURL: 'https://api.x.ai/v1',
+      })
+    )
+  })
+
+  it('allows overriding apiKey and baseURL', () => {
+    new GrokAdapter('custom-key', 'https://custom.endpoint/v1')
+    expect(OpenAIMock).toHaveBeenCalledWith(
+      expect.objectContaining({
+        apiKey: 'custom-key',
+        baseURL: 'https://custom.endpoint/v1',
+      })
+    )
+  })
+
+  it('createAdapter("grok") returns GrokAdapter instance', async () => {
+    const adapter = await createAdapter('grok')
+    expect(adapter).toBeInstanceOf(GrokAdapter)
+  })
+})
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