diff --git a/src/modules/ai-runtime/platform-budget.service.spec.ts b/src/modules/ai-runtime/platform-budget.service.spec.ts
new file mode 100644
index 0000000..3b04b18
--- /dev/null
+++ b/src/modules/ai-runtime/platform-budget.service.spec.ts
@@ -0,0 +1,247 @@
+import { BadRequestException } from '@nestjs/common';
+import { PlatformBudgetService } from './platform-budget.service';
+
+describe('PlatformBudgetService', () => {
+  let service: PlatformBudgetService;
+  let mockBudgetFindUnique: jest.Mock;
+  let mockBudgetCreate: jest.Mock;
+  let mockBudgetUpsert: jest.Mock;
+  let mockJobCount: jest.Mock;
+
+  const healthyBudget = {
+    localDate: new Date(),
+    provider: 'deepseek',
+    model: 'deepseek-chat',
+    totalTokens: 1000,
+    costEstimate: 100,
+    jobCount: 5,
+    failedCount: 0,
+    circuitBreakerStatus: 'closed',
+    circuitBreakerReason: null,
+  };
+
+  const today = () => {
+    const d = new Date();
+    return new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
+  };
+
+  beforeEach(() => {
+    mockBudgetFindUnique = jest.fn();
+    mockBudgetCreate = jest.fn();
+    mockBudgetUpsert = jest.fn();
+    mockJobCount = jest.fn();
+
+    const mockPrisma = {
+      platformAiBudgetDaily: {
+        findUnique: mockBudgetFindUnique,
+        create: mockBudgetCreate,
+        upsert: mockBudgetUpsert,
+      },
+      aiRuntimeJob: { count: mockJobCount },
+    } as any;
+
+    service = new PlatformBudgetService(mockPrisma);
+  });
+
+  // ═══════════════════════════════════════════════════════════════════
+  // checkPlatformBudget
+  // ═══════════════════════════════════════════════════════════════════
+
+  describe('checkPlatformBudget', () => {
+    it('passes when budget is healthy', async () => {
+      mockBudgetFindUnique.mockResolvedValue(healthyBudget);
+
+      await expect(service.checkPlatformBudget('deepseek', 'deepseek-chat')).resolves.toBeUndefined();
+    });
+
+    it('auto-creates budget on first call of day', async () => {
+      mockBudgetFindUnique.mockResolvedValue(null);
+      mockBudgetCreate.mockResolvedValue(healthyBudget);
+
+      await expect(service.checkPlatformBudget('deepseek', 'deepseek-chat')).resolves.toBeUndefined();
+      expect(mockBudgetCreate).toHaveBeenCalledWith(
+        expect.objectContaining({ data: expect.objectContaining({ provider: 'deepseek', model: 'deepseek-chat' }) }),
+      );
+    });
+
+    it('throws PLATFORM_CIRCUIT_OPEN when circuit is open', async () => {
+      mockBudgetFindUnique.mockResolvedValue({
+        ...healthyBudget,
+        circuitBreakerStatus: 'open',
+        circuitBreakerReason: 'Consecutive failures reached 10',
+      });
+
+      await expect(service.checkPlatformBudget('deepseek', 'deepseek-chat')).rejects.toMatchObject({
+        response: expect.objectContaining({ errorCode: 'PLATFORM_CIRCUIT_OPEN' }),
+      });
+    });
+
+    it('throws PLATFORM_CIRCUIT_HALF_OPEN when half_open + active jobs >= limit', async () => {
+      mockBudgetFindUnique.mockResolvedValue({
+        ...healthyBudget,
+        circuitBreakerStatus: 'half_open',
+      });
+      mockJobCount.mockResolvedValue(2); // halfOpenMaxJobs = 2
+
+      await expect(service.checkPlatformBudget('deepseek', 'deepseek-chat')).rejects.toMatchObject({
+        response: expect.objectContaining({ errorCode: 'PLATFORM_CIRCUIT_HALF_OPEN' }),
+      });
+    });
+
+    it('passes half_open when active jobs under limit', async () => {
+      mockBudgetFindUnique.mockResolvedValue({
+        ...healthyBudget,
+        circuitBreakerStatus: 'half_open',
+      });
+      mockJobCount.mockResolvedValue(1); // 1 < 2 (halfOpenMaxJobs)
+
+      await expect(service.checkPlatformBudget('deepseek', 'deepseek-chat')).resolves.toBeUndefined();
+    });
+
+    it('throws PLATFORM_TOKEN_BUDGET_EXCEEDED', async () => {
+      mockBudgetFindUnique.mockResolvedValue({
+        ...healthyBudget,
+        totalTokens: 10_000_000, // at limit
+      });
+
+      await expect(service.checkPlatformBudget('deepseek', 'deepseek-chat')).rejects.toMatchObject({
+        response: expect.objectContaining({ errorCode: 'PLATFORM_TOKEN_BUDGET_EXCEEDED' }),
+      });
+    });
+
+    it('throws PLATFORM_COST_BUDGET_EXCEEDED', async () => {
+      mockBudgetFindUnique.mockResolvedValue({
+        ...healthyBudget,
+        costEstimate: 50_000, // at limit ($500)
+      });
+
+      await expect(service.checkPlatformBudget('deepseek', 'deepseek-chat')).rejects.toMatchObject({
+        response: expect.objectContaining({ errorCode: 'PLATFORM_COST_BUDGET_EXCEEDED' }),
+      });
+    });
+  });
+
+  // ═══════════════════════════════════════════════════════════════════
+  // recordSuccess
+  // ═══════════════════════════════════════════════════════════════════
+
+  describe('recordSuccess', () => {
+    it('upserts with usage data and resets circuit to closed', async () => {
+      const localDate = today();
+      mockBudgetUpsert.mockResolvedValue({});
+
+      await service.recordSuccess('deepseek', 'deepseek-chat', 100, 50, 150, 0.002);
+
+      expect(mockBudgetUpsert).toHaveBeenCalledWith({
+        where: { localDate_provider_model: { localDate, provider: 'deepseek', model: 'deepseek-chat' } },
+        create: expect.objectContaining({
+          inputTokens: 100, outputTokens: 50, totalTokens: 150, costEstimate: 0.002,
+        }),
+        update: expect.objectContaining({
+          circuitBreakerStatus: 'closed',
+          circuitBreakerReason: null,
+          failedCount: 0,
+        }),
+      });
+    });
+  });
+
+  // ═══════════════════════════════════════════════════════════════════
+  // recordFailure
+  // ═══════════════════════════════════════════════════════════════════
+
+  describe('recordFailure', () => {
+    it('increments failedCount', async () => {
+      mockBudgetFindUnique.mockResolvedValue(healthyBudget); // failedCount=0
+      mockBudgetUpsert.mockResolvedValue({});
+
+      await service.recordFailure('deepseek', 'deepseek-chat', 'MODEL_TIMEOUT');
+
+      const call = mockBudgetUpsert.mock.calls[0][0];
+      expect(call.update.failedCount).toBe(1);
+    });
+
+    it('opens circuit breaker when failedCount reaches threshold', async () => {
+      mockBudgetFindUnique.mockResolvedValue({
+        ...healthyBudget,
+        failedCount: 9, // threshold is 10 → this failure = 10
+      });
+      mockBudgetUpsert.mockResolvedValue({});
+
+      await service.recordFailure('deepseek', 'deepseek-chat', 'MODEL_TIMEOUT');
+
+      const call = mockBudgetUpsert.mock.calls[0][0];
+      expect(call.update.circuitBreakerStatus).toBe('open');
+      expect(call.update.circuitBreakerReason).toContain('MODEL_TIMEOUT');
+    });
+
+    it('creates budget on first failure of day', async () => {
+      mockBudgetFindUnique.mockResolvedValue(null);
+      mockBudgetCreate.mockResolvedValue(healthyBudget);
+      mockBudgetUpsert.mockResolvedValue({});
+
+      await service.recordFailure('deepseek', 'deepseek-chat', 'ERROR');
+
+      expect(mockBudgetUpsert).toHaveBeenCalled();
+    });
+  });
+
+  // ═══════════════════════════════════════════════════════════════════
+  // transitionToHalfOpen / closeCircuit
+  // ═══════════════════════════════════════════════════════════════════
+
+  describe('admin circuit control', () => {
+    it('transitionToHalfOpen upserts with half_open status', async () => {
+      mockBudgetUpsert.mockResolvedValue({});
+      await service.transitionToHalfOpen('deepseek', 'deepseek-chat');
+      const call = mockBudgetUpsert.mock.calls[0][0];
+      expect(call.update.circuitBreakerStatus).toBe('half_open');
+    });
+
+    it('closeCircuit upserts with closed status + reset failedCount', async () => {
+      mockBudgetUpsert.mockResolvedValue({});
+      await service.closeCircuit('deepseek', 'deepseek-chat');
+      const call = mockBudgetUpsert.mock.calls[0][0];
+      expect(call.update.circuitBreakerStatus).toBe('closed');
+      expect(call.update.failedCount).toBe(0);
+    });
+  });
+
+  // ═══════════════════════════════════════════════════════════════════
+  // getBudgetState
+  // ═══════════════════════════════════════════════════════════════════
+
+  describe('getBudgetState', () => {
+    it('returns budget state with limits', async () => {
+      mockBudgetFindUnique.mockResolvedValue({
+        ...healthyBudget,
+        provider: 'deepseek',
+        model: 'deepseek-chat',
+        totalTokens: 500000,
+        costEstimate: 25000,
+        jobCount: 12,
+        failedCount: 2,
+        circuitBreakerStatus: 'closed',
+        circuitBreakerReason: null,
+      });
+
+      const result = await service.getBudgetState('deepseek', 'deepseek-chat');
+
+      expect(result.provider).toBe('deepseek');
+      expect(result.totalTokens).toBe(500000);
+      expect(result.costEstimateCents).toBe(25000);
+      expect(result.circuitBreakerStatus).toBe('closed');
+      expect(result.limits.maxDailyTokens).toBe(10_000_000);
+      expect(result.limits.maxDailyCostCents).toBe(50_000);
+    });
+
+    it('auto-creates budget when none exists', async () => {
+      mockBudgetFindUnique.mockResolvedValue(null);
+      mockBudgetCreate.mockResolvedValue(healthyBudget);
+
+      const result = await service.getBudgetState('deepseek', 'deepseek-chat');
+
+      expect(result.provider).toBe('deepseek');
+    });
+  });
+});