diff --git a/products/05-aws-cost-anomaly/src/api/anomalies.ts b/products/05-aws-cost-anomaly/src/api/anomalies.ts
new file mode 100644
index 0000000..42f8d23
--- /dev/null
+++ b/products/05-aws-cost-anomaly/src/api/anomalies.ts
@@ -0,0 +1,100 @@
+import type { FastifyInstance } from 'fastify';
+import { z } from 'zod';
+import { withTenant } from '../data/db.js';
+
+const listQuerySchema = z.object({
+  page: z.coerce.number().min(1).default(1),
+  limit: z.coerce.number().min(1).max(100).default(20),
+  status: z.enum(['open', 'acknowledged', 'snoozed', 'expected', 'resolved']).optional(),
+  account_id: z.string().optional(),
+  min_score: z.coerce.number().min(0).max(100).optional(),
+});
+
+export function registerAnomalyRoutes(app: FastifyInstance) {
+  // List anomalies
+  app.get('/api/v1/anomalies', async (req, reply) => {
+    const query = listQuerySchema.parse(req.query);
+    const tenantId = (req as any).tenantId;
+    const offset = (query.page - 1) * query.limit;
+
+    const result = await withTenant(tenantId, async (client) => {
+      let sql = 'SELECT * FROM anomalies WHERE 1=1';
+      const params: any[] = [];
+      let idx = 1;
+
+      if (query.status) { sql += ` AND status = $${idx++}`; params.push(query.status); }
+      if (query.account_id) { sql += ` AND account_id = $${idx++}`; params.push(query.account_id); }
+      if (query.min_score) { sql += ` AND score >= $${idx++}`; params.push(query.min_score); }
+
+      sql += ` ORDER BY detected_at DESC LIMIT $${idx++} OFFSET $${idx++}`;
+      params.push(query.limit, offset);
+
+      return client.query(sql, params);
+    });
+
+    return { anomalies: result.rows, page: query.page, limit: query.limit };
+  });
+
+  // Acknowledge anomaly
+  app.post('/api/v1/anomalies/:id/acknowledge', async (req, reply) => {
+    const { id } = req.params as { id: string };
+    const tenantId = (req as any).tenantId;
+
+    await withTenant(tenantId, async (client) => {
+      await client.query("UPDATE anomalies SET status = 'acknowledged' WHERE id = $1 AND status = 'open'", [id]);
+    });
+    return { status: 'acknowledged' };
+  });
+
+  // Snooze anomaly
+  app.post('/api/v1/anomalies/:id/snooze', async (req, reply) => {
+    const { id } = req.params as { id: string };
+    const { hours } = z.object({ hours: z.coerce.number().min(1).max(168).default(24) }).parse(req.body ?? {});
+    const tenantId = (req as any).tenantId;
+
+    await withTenant(tenantId, async (client) => {
+      await client.query(
+        "UPDATE anomalies SET status = 'snoozed', snoozed_until = now() + $1 * interval '1 hour' WHERE id = $2",
+        [hours, id],
+      );
+    });
+    return { status: 'snoozed', until_hours: hours };
+  });
+
+  // Mark as expected (recurring cost)
+  app.post('/api/v1/anomalies/:id/expected', async (req, reply) => {
+    const { id } = req.params as { id: string };
+    const tenantId = (req as any).tenantId;
+
+    await withTenant(tenantId, async (client) => {
+      await client.query("UPDATE anomalies SET status = 'expected' WHERE id = $1", [id]);
+    });
+    return { status: 'expected' };
+  });
+
+  // Dashboard summary
+  app.get('/api/v1/dashboard', async (req, reply) => {
+    const tenantId = (req as any).tenantId;
+
+    const result = await withTenant(tenantId, async (client) => {
+      const openCount = await client.query("SELECT COUNT(*)::int as count FROM anomalies WHERE status = 'open'");
+      const topResources = await client.query(`
+        SELECT resource_type, COUNT(*)::int as anomaly_count, AVG(score)::numeric(5,2) as avg_score
+        FROM anomalies WHERE status = 'open'
+        GROUP BY resource_type ORDER BY anomaly_count DESC LIMIT 10
+      `);
+      const recentTrend = await client.query(`
+        SELECT date_trunc('hour', detected_at) as hour, COUNT(*)::int as count
+        FROM anomalies WHERE detected_at > now() - interval '24 hours'
+        GROUP BY hour ORDER BY hour
+      `);
+      return {
+        open_anomalies: openCount.rows[0]?.count ?? 0,
+        top_resources: topResources.rows,
+        hourly_trend: recentTrend.rows,
+      };
+    });
+
+    return result;
+  });
+}
diff --git a/products/05-aws-cost-anomaly/src/api/baselines.ts b/products/05-aws-cost-anomaly/src/api/baselines.ts
new file mode 100644
index 0000000..70dd620
--- /dev/null
+++ b/products/05-aws-cost-anomaly/src/api/baselines.ts
@@ -0,0 +1,38 @@
+import type { FastifyInstance } from 'fastify';
+import { withTenant } from '../data/db.js';
+
+export function registerBaselineRoutes(app: FastifyInstance) {
+  // List baselines
+  app.get('/api/v1/baselines', async (req, reply) => {
+    const tenantId = (req as any).tenantId;
+
+    const result = await withTenant(tenantId, async (client) => {
+      return client.query(`
+        SELECT account_id, resource_type, welford_count as sample_count,
+               welford_mean::numeric(12,4) as mean,
+               CASE WHEN welford_count > 1
+                    THEN sqrt(welford_m2 / welford_count)::numeric(12,4)
+                    ELSE 0 END as stddev,
+               updated_at
+        FROM baselines ORDER BY account_id, resource_type
+      `);
+    });
+
+    return { baselines: result.rows };
+  });
+
+  // Reset baseline for a specific resource
+  app.delete('/api/v1/baselines/:accountId/:resourceType', async (req, reply) => {
+    const { accountId, resourceType } = req.params as { accountId: string; resourceType: string };
+    const tenantId = (req as any).tenantId;
+
+    await withTenant(tenantId, async (client) => {
+      await client.query(
+        'DELETE FROM baselines WHERE account_id = $1 AND resource_type = $2',
+        [accountId, resourceType],
+      );
+    });
+
+    return { status: 'reset', accountId, resourceType };
+  });
+}
diff --git a/products/05-aws-cost-anomaly/src/api/governance.ts b/products/05-aws-cost-anomaly/src/api/governance.ts
new file mode 100644
index 0000000..b12b83e
--- /dev/null
+++ b/products/05-aws-cost-anomaly/src/api/governance.ts
@@ -0,0 +1,59 @@
+import type { FastifyInstance } from 'fastify';
+import { withTenant } from '../data/db.js';
+import { GovernanceEngine } from '../governance/engine.js';
+
+const engine = new GovernanceEngine();
+
+export function registerGovernanceRoutes(app: FastifyInstance) {
+  // Get governance status
+  app.get('/api/v1/governance', async (req, reply) => {
+    const tenantId = (req as any).tenantId;
+
+    const result = await withTenant(tenantId, async (client) => {
+      return client.query('SELECT governance_mode, governance_started_at FROM tenants WHERE id = $1', [tenantId]);
+    });
+
+    const tenant = result.rows[0];
+    if (!tenant) return reply.status(404).send({ error: 'Tenant not found' });
+
+    const daysInMode = Math.floor((Date.now() - new Date(tenant.governance_started_at).getTime()) / (86400 * 1000));
+
+    return {
+      mode: tenant.governance_mode,
+      days_in_mode: daysInMode,
+      started_at: tenant.governance_started_at,
+    };
+  });
+
+  // Check promotion eligibility
+  app.get('/api/v1/governance/promotion', async (req, reply) => {
+    const tenantId = (req as any).tenantId;
+
+    const result = await withTenant(tenantId, async (client) => {
+      const tenant = await client.query('SELECT governance_mode, governance_started_at FROM tenants WHERE id = $1', [tenantId]);
+      const t = tenant.rows[0];
+      if (!t) return null;
+
+      // Calculate FP rate from last 30 days
+      const fpStats = await client.query(`
+        SELECT
+          COUNT(*) FILTER (WHERE status = 'expected')::float /
+          NULLIF(COUNT(*)::float, 0) as fp_rate
+        FROM anomalies
+        WHERE detected_at > now() - interval '30 days'
+      `);
+
+      return { tenant: t, fpRate: fpStats.rows[0]?.fp_rate ?? 0 };
+    });
+
+    if (!result) return reply.status(404).send({ error: 'Tenant not found' });
+
+    const daysInMode = Math.floor((Date.now() - new Date(result.tenant.governance_started_at).getTime()) / (86400 * 1000));
+    const evaluation = engine.evaluatePromotion(tenantId, {
+      fpRate: result.fpRate,
+      daysInCurrentMode: daysInMode,
+    });
+
+    return { ...evaluation, current_mode: result.tenant.governance_mode, fp_rate: result.fpRate, days_in_mode: daysInMode };
+  });
+}
diff --git a/products/05-aws-cost-anomaly/src/api/ingestion.ts b/products/05-aws-cost-anomaly/src/api/ingestion.ts
new file mode 100644
index 0000000..9d26d9e
--- /dev/null
+++ b/products/05-aws-cost-anomaly/src/api/ingestion.ts
@@ -0,0 +1,103 @@
+import type { FastifyInstance } from 'fastify';
+import { z } from 'zod';
+import pino from 'pino';
+import { withTenant } from '../data/db.js';
+import { WelfordBaseline, scoreAnomaly, shouldAlert, type CostEvent } from '../detection/scorer.js';
+import { config } from '../config/index.js';
+
+const logger = pino({ name: 'api-ingestion' });
+
+const costEventSchema = z.object({
+  account_id: z.string(),
+  resource_type: z.string(),
+  hourly_cost: z.number().min(0),
+  region: z.string().default('us-east-1'),
+  tags: z.record(z.string()).default({}),
+});
+
+const batchSchema = z.object({
+  events: z.array(costEventSchema).min(1).max(100),
+});
+
+export function registerIngestionRoutes(app: FastifyInstance) {
+  // Ingest cost events (from agent or Cost Explorer poller)
+  app.post('/api/v1/ingest', async (req, reply) => {
+    const tenantId = (req as any).tenantId;
+    const { events } = batchSchema.parse(req.body);
+
+    const results = await withTenant(tenantId, async (client) => {
+      const anomalies = [];
+
+      for (const event of events) {
+        // Fetch or create baseline
+        const baselineRow = await client.query(
+          `SELECT welford_count, welford_mean, welford_m2, version
+           FROM baselines WHERE account_id = $1 AND resource_type = $2`,
+          [event.account_id, event.resource_type],
+        );
+
+        let baseline: WelfordBaseline;
+        let version: number;
+
+        if (baselineRow.rows[0]) {
+          baseline = WelfordBaseline.fromJSON({
+            count: baselineRow.rows[0].welford_count,
+            mean: parseFloat(baselineRow.rows[0].welford_mean),
+            m2: parseFloat(baselineRow.rows[0].welford_m2),
+          });
+          version = baselineRow.rows[0].version;
+        } else {
+          baseline = new WelfordBaseline();
+          version = 0;
+        }
+
+        // Score before updating baseline
+        const score = scoreAnomaly({
+          cost: event.hourly_cost,
+          mean: baseline.mean,
+          stddev: baseline.stddev,
+        });
+
+        const isAnomaly = shouldAlert(score, config.ANOMALY_THRESHOLD);
+
+        // Update baseline with optimistic locking (BMad must-have: concurrent Welford corruption)
+        baseline.update(event.hourly_cost);
+        const bj = baseline.toJSON();
+
+        if (version === 0) {
+          await client.query(
+            `INSERT INTO baselines (tenant_id, account_id, resource_type, welford_count, welford_mean, welford_m2, version)
+             VALUES ($1, $2, $3, $4, $5, $6, 1)
+             ON CONFLICT (tenant_id, account_id, resource_type) DO NOTHING`,
+            [tenantId, event.account_id, event.resource_type, bj.count, bj.mean, bj.m2],
+          );
+        } else {
+          const updated = await client.query(
+            `UPDATE baselines SET welford_count = $1, welford_mean = $2, welford_m2 = $3,
+                    version = version + 1, updated_at = now()
+             WHERE account_id = $4 AND resource_type = $5 AND version = $6`,
+            [bj.count, bj.mean, bj.m2, event.account_id, event.resource_type, version],
+          );
+          if (updated.rowCount === 0) {
+            logger.warn({ accountId: event.account_id, resourceType: event.resource_type },
+              'Optimistic lock conflict — baseline update skipped');
+          }
+        }
+
+        // Record anomaly if threshold exceeded
+        if (isAnomaly) {
+          await client.query(
+            `INSERT INTO anomalies (tenant_id, account_id, resource_type, region, hourly_cost, score, baseline_mean, baseline_stddev, tags)
+             VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)`,
+            [tenantId, event.account_id, event.resource_type, event.region, event.hourly_cost, score, baseline.mean, baseline.stddev, JSON.stringify(event.tags)],
+          );
+          anomalies.push({ ...event, score, isAnomaly: true });
+        }
+      }
+
+      return anomalies;
+    });
+
+    return { ingested: events.length, anomalies: results.length, details: results };
+  });
+}
diff --git a/products/05-aws-cost-anomaly/src/config/index.ts b/products/05-aws-cost-anomaly/src/config/index.ts
new file mode 100644
index 0000000..3dbc8ef
--- /dev/null
+++ b/products/05-aws-cost-anomaly/src/config/index.ts
@@ -0,0 +1,14 @@
+import { z } from 'zod';
+
+const envSchema = z.object({
+  PORT: z.coerce.number().default(3000),
+  DATABASE_URL: z.string().default('postgresql://localhost:5432/dd0c_cost'),
+  REDIS_URL: z.string().default('redis://localhost:6379'),
+  JWT_SECRET: z.string().min(32).default('dev-secret-change-me-in-production!!'),
+  CORS_ORIGIN: z.string().default('*'),
+  LOG_LEVEL: z.string().default('info'),
+  ANOMALY_THRESHOLD: z.coerce.number().default(50),
+});
+
+export const config = envSchema.parse(process.env);
+export type Config = z.infer<typeof envSchema>;
diff --git a/products/05-aws-cost-anomaly/src/data/db.ts b/products/05-aws-cost-anomaly/src/data/db.ts
new file mode 100644
index 0000000..85cd7f5
--- /dev/null
+++ b/products/05-aws-cost-anomaly/src/data/db.ts
@@ -0,0 +1,24 @@
+import pg from 'pg';
+import pino from 'pino';
+import { config } from '../config/index.js';
+
+const logger = pino({ name: 'data' });
+
+export const pool = new pg.Pool({ connectionString: config.DATABASE_URL });
+
+export async function withTenant<T>(tenantId: string, fn: (client: pg.PoolClient) => Promise<T>): Promise<T> {
+  const client = await pool.connect();
+  try {
+    await client.query('BEGIN');
+    await client.query(`SET LOCAL app.tenant_id = '${tenantId}'`);
+    const result = await fn(client);
+    await client.query('COMMIT');
+    return result;
+  } catch (err) {
+    await client.query('ROLLBACK');
+    throw err;
+  } finally {
+    await client.query('RESET app.tenant_id');
+    client.release();
+  }
+}
diff --git a/products/05-aws-cost-anomaly/src/index.ts b/products/05-aws-cost-anomaly/src/index.ts
new file mode 100644
index 0000000..92d9ba4
--- /dev/null
+++ b/products/05-aws-cost-anomaly/src/index.ts
@@ -0,0 +1,29 @@
+import Fastify from 'fastify';
+import cors from '@fastify/cors';
+import pino from 'pino';
+import { config } from './config/index.js';
+import { registerAnomalyRoutes } from './api/anomalies.js';
+import { registerBaselineRoutes } from './api/baselines.js';
+import { registerGovernanceRoutes } from './api/governance.js';
+import { registerIngestionRoutes } from './api/ingestion.js';
+
+const logger = pino({ name: 'dd0c-cost', level: config.LOG_LEVEL });
+
+const app = Fastify({ logger: true });
+
+await app.register(cors, { origin: config.CORS_ORIGIN });
+
+app.get('/health', async () => ({ status: 'ok', service: 'dd0c-cost' }));
+
+registerIngestionRoutes(app);
+registerAnomalyRoutes(app);
+registerBaselineRoutes(app);
+registerGovernanceRoutes(app);
+
+try {
+  await app.listen({ port: config.PORT, host: '0.0.0.0' });
+  logger.info({ port: config.PORT }, 'dd0c/cost started');
+} catch (err) {
+  logger.fatal(err, 'Failed to start');
+  process.exit(1);
+}