diff --git a/tests/api/incidents/rca-draft.test.ts b/tests/api/incidents/rca-draft.test.ts
new file mode 100644
index 0000000..484e03a
--- /dev/null
+++ b/tests/api/incidents/rca-draft.test.ts
@@ -0,0 +1,78 @@
+import { describe, it, expect, vi } from 'vitest'
+
+const mockIncident = {
+  id: 'inc-1',
+  incident_type: 'injury',
+  description: 'Forklift operator collided with racking in zone B, injuring left arm.',
+  severity: 3,
+  injury_involved: true,
+  medical_status: 'medical_treatment',
+  is_fatality: false,
+  is_serious_bodily_injury: false,
+  triage_notes: null,
+  sites: { name: 'Warehouse A' },
+  zones: { name: 'Zone B' },
+}
+
+vi.mock('@/lib/supabase/server', () => ({
+  createClient: vi.fn().mockResolvedValue({
+    auth: {
+      getUser: vi.fn().mockResolvedValue({ data: { user: { id: 'user-1' } }, error: null }),
+    },
+    from: vi.fn().mockReturnValue({
+      select: vi.fn().mockReturnThis(),
+      eq: vi.fn().mockReturnThis(),
+      single: vi.fn()
+        .mockResolvedValueOnce({ data: { role: 'hse' } })  // profile
+        .mockResolvedValueOnce({ data: mockIncident }),      // incident
+    }),
+    rpc: vi.fn().mockResolvedValue({ error: null }),
+  }),
+}))
+
+vi.mock('@/lib/claude/client', () => ({
+  createAnthropicClient: vi.fn().mockReturnValue({
+    messages: {
+      create: vi.fn().mockResolvedValue({
+        content: [{
+          type: 'tool_use',
+          name: 'draft_rca',
+          input: {
+            five_why_steps: [
+              { why: 'Why did the incident happen?', answer: 'Forklift entered zone B without checking for pedestrians.' },
+              { why: 'Why was there no check?', answer: 'No pedestrian exclusion zone marked at zone B entrance.' },
+              { why: 'Why was it not marked?', answer: 'Site hazard assessment did not include zone B forklift route.' },
+            ],
+            root_cause_summary: 'Absence of pedestrian exclusion zone and forklift route hazard assessment in zone B.',
+            capa_suggestions: [
+              'Mark pedestrian exclusion zones at all forklift routes in zone B within 7 days.',
+              'Update site hazard assessment to include forklift routes in all zones.',
+              'Conduct forklift safety refresher for all operators within 30 days.',
+            ],
+          },
+        }],
+      }),
+    },
+  }),
+}))
+
+vi.mock('@/lib/settings', () => ({
+  getApiKey: vi.fn().mockResolvedValue('sk-test-key'),
+}))
+
+describe('POST /api/incidents/[id]/ai/rca-draft', () => {
+  it('returns five_why_steps, root_cause_summary, and capa_suggestions', async () => {
+    const { POST } = await import('@/app/api/incidents/[id]/ai/rca-draft/route')
+    const req = new Request('http://localhost/api/incidents/inc-1/ai/rca-draft', { method: 'POST' })
+    const res = await POST(req as never, { params: Promise.resolve({ id: 'inc-1' }) })
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    expect(body).toHaveProperty('five_why_steps')
+    expect(Array.isArray(body.five_why_steps)).toBe(true)
+    expect(body.five_why_steps.length).toBeGreaterThanOrEqual(1)
+    expect(body).toHaveProperty('root_cause_summary')
+    expect(typeof body.root_cause_summary).toBe('string')
+    expect(body).toHaveProperty('capa_suggestions')
+    expect(Array.isArray(body.capa_suggestions)).toBe(true)
+  })
+})