diff --git a/docs/billing-forecast-endpoint.md b/docs/billing-forecast-endpoint.md
index a44ede57..418e6e43 100644
--- a/docs/billing-forecast-endpoint.md
+++ b/docs/billing-forecast-endpoint.md
@@ -6,11 +6,11 @@
 
 ## Overview
 
-The `/api/billing/forecast` endpoint allows authenticated developers to forecast their upcoming billing amount based on their actual current run rate over a configurable historical lookback window.
+The `/api/billing/forecast` endpoint (mounted via `createBillingForecastRouter` in `src/routes/index.ts` alongside `/api/forecast` from `createForecastRouter`) allows authenticated developers to forecast their upcoming billing amount based on their actual current run rate over a configurable historical lookback window.
 
 ## Authentication
 
-Requires Bearer JWT token or `x-user-id` header (via `requireAuth` middleware). Unauthenticated requests return `401 Unauthorized`.
+Requires Bearer JWT token (via `requireAuth` middleware). Unauthenticated requests return `401 Unauthorized`.
 
 ## Query Parameters
 
diff --git a/src/routes/billing/forecast.ts b/src/routes/billing/forecast.ts
index 2c722f9e..8fb44bbf 100644
--- a/src/routes/billing/forecast.ts
+++ b/src/routes/billing/forecast.ts
@@ -82,7 +82,7 @@ export interface BillingForecastRouterDeps {
 }
 
 /**
- * Creates the router for /api/billing/forecast.
+ * Creates the router for /api/billing/forecast (mounted by createApiRouter in src/routes/index.ts).
  */
 export function createBillingForecastRouter(deps: BillingForecastRouterDeps = {}): Router {
   const router = Router();
diff --git a/src/routes/forecast.test.ts b/src/routes/forecast.test.ts
index a3b12621..bebca693 100644
--- a/src/routes/forecast.test.ts
+++ b/src/routes/forecast.test.ts
@@ -13,7 +13,12 @@ import { forecastLogger } from '../middleware/forecastAccessLog.js';
 import jwt from 'jsonwebtoken';
 
 // Mock auditService to capture audit calls
-jest.mock('../services/auditService.js');
+jest.mock('../services/auditService.js', () => ({
+  defaultAuditService: {
+    record: jest.fn(),
+  },
+  appendAuditRow: jest.fn(),
+}));
 const mockAuditService = defaultAuditService as jest.Mocked<typeof defaultAuditService>;
 
 // Mock JWT to simulate authenticated users
@@ -823,3 +828,42 @@ describe('Forecast access log integration', () => {
     expect(payload.status).toBe(500);
   });
 });
+
+// =============================================================================
+// API Index Router Mount Integration (Issue #1215)
+// =============================================================================
+
+describe('createApiRouter forecast mounting (#1215)', () => {
+  it('mounts both /api/forecast and /api/billing/forecast on createApiRouter', async () => {
+    const { createApiRouter } = await import('./index.js');
+    const apiApp = express();
+    apiApp.use(express.json());
+    apiApp.use(requestIdMiddleware);
+    apiApp.use(auditEnrichMiddleware);
+    apiApp.use('/api', createApiRouter());
+    apiApp.use(errorHandler);
+
+    const listRes = await request(apiApp).get('/api/forecast?limit=5');
+    expect(listRes.status).toBe(200);
+    expect(listRes.body.success).toBe(true);
+    expect(listRes.body.data.items).toHaveLength(5);
+    expect(listRes.body.data.total).toBe(24);
+
+    const createRes = await request(apiApp)
+      .post('/api/forecast')
+      .set('Authorization', 'Bearer mock-token-dev-index-1')
+      .send({ name: 'Mounted Forecast', description: 'Created via createApiRouter' });
+    expect(createRes.status).toBe(201);
+    expect(createRes.body.data.name).toBe('Mounted Forecast');
+
+    const billingRes = await request(apiApp)
+      .get('/api/billing/forecast?lookbackDays=15&period=week')
+      .set('Authorization', 'Bearer mock-token-dev-index-1');
+    expect(billingRes.status).toBe(200);
+    expect(billingRes.body.userId).toBe('dev-index-1');
+    expect(billingRes.body.lookbackDays).toBe(15);
+    expect(billingRes.body.forecastPeriod).toBe('week');
+    expect(billingRes.body.forecastDays).toBe(7);
+  });
+});
+
diff --git a/src/routes/forecast.ts b/src/routes/forecast.ts
index 430b33fc..b8b60fc1 100644
--- a/src/routes/forecast.ts
+++ b/src/routes/forecast.ts
@@ -101,6 +101,10 @@ export interface Forecast {
 // In-memory store for forecast data (simulated persistence)
 const forecastStore = new Map<string, Forecast>();
 
+export function resetForecastStoreForTests(): void {
+  forecastStore.clear();
+}
+
 // -----------------------------------------------------------------------
 // Helper functions
 // -----------------------------------------------------------------------
diff --git a/src/routes/index.ts b/src/routes/index.ts
index a81e1670..694379c1 100644
--- a/src/routes/index.ts
+++ b/src/routes/index.ts
@@ -32,6 +32,7 @@ import type { SubscriptionRepository } from "../repositories/subscriptionReposit
 import type { DeveloperRepository } from "../repositories/developerRepository.js";
 import type { ApiRepository } from "../repositories/apiRepository.js";
 import { createForecastRouter } from "./forecast.js";
+import { createBillingForecastRouter } from "./billing/forecast.js";
 import { createPlansRouter } from "./plans.js";
 import { createCreditsRouter } from "./credits.js";
 import type { CreditsRepository } from "../repositories/creditsRepository.js";
@@ -76,6 +77,7 @@ export function createApiRouter(deps: ApiRouterDeps = {}): Router {
   const router = Router();
 
   router.use("/health", healthRouter);
+  router.use("/forecast", createForecastRouter());
   router.use("/plans", createPlansRouter());
   // Hot-path credits lookup (idx_credits_lookup_hot) — see migrations/credits_index.sql
   router.use(
@@ -258,6 +260,11 @@ export function createApiRouter(deps: ApiRouterDeps = {}): Router {
   }
 
   if (billingMiddlewares.length > 0) {
+    router.use(
+      "/billing/forecast",
+      ...billingMiddlewares,
+      createBillingForecastRouter(),
+    );
     router.use("/billing", ...billingMiddlewares, billingRouter);
     router.use(
       "/billing/portal",
@@ -265,6 +272,7 @@ export function createApiRouter(deps: ApiRouterDeps = {}): Router {
       createBillingPortalRouter(),
     );
   } else {
+    router.use("/billing/forecast", createBillingForecastRouter());
     router.use("/billing", billingRouter);
     router.use("/billing/portal", createBillingPortalRouter());
   }