diff --git a/src/server/clickhouse/client.ts b/src/server/clickhouse/client.ts
index a8d8f43..1a7ef0f 100644
--- a/src/server/clickhouse/client.ts
+++ b/src/server/clickhouse/client.ts
@@ -1,8 +1,11 @@
 import { createClient } from '@clickhouse/client'
 
-export const clickhouse = createClient({
-  url: process.env.CLICKHOUSE_URL,
-  username: process.env.CLICKHOUSE_USER,
-  password: process.env.CLICKHOUSE_PASSWORD,
-  database: 'newsletter',
-})
+const url = process.env.CLICKHOUSE_URL
+const username = process.env.CLICKHOUSE_USER
+const password = process.env.CLICKHOUSE_PASSWORD
+
+if (!url || !username || !password) {
+  throw new Error('CLICKHOUSE_URL, CLICKHOUSE_USER und CLICKHOUSE_PASSWORD müssen gesetzt sein')
+}
+
+export const clickhouse = createClient({ url, username, password, database: 'newsletter' })
diff --git a/src/server/db/tenant.test.ts b/src/server/db/tenant.test.ts
index e2e79e9..84e8615 100644
--- a/src/server/db/tenant.test.ts
+++ b/src/server/db/tenant.test.ts
@@ -1,36 +1,65 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest'
 
-vi.mock('./client', () => ({
-  db: {
-    execute: vi.fn(),
-    query: vi.fn().mockResolvedValue([{ id: '1' }]),
-  },
+// Pool mocken — gibt einen fake PoolClient zurück
+const mockClient = vi.hoisted(() => ({
+  query: vi.fn(),
+  release: vi.fn(),
+}))
+
+vi.mock('pg', () => ({
+  Pool: vi.fn().mockImplementation(() => ({
+    connect: vi.fn().mockResolvedValue(mockClient),
+  })),
 }))
 
 import { withTenant } from './tenant'
-import { db } from './client'
 
 describe('withTenant', () => {
-  beforeEach(() => vi.clearAllMocks())
+  beforeEach(() => {
+    vi.clearAllMocks()
+    mockClient.query.mockResolvedValue({ rows: [] })
+  })
 
   it('setzt search_path auf tenant-Schema', async () => {
-    await withTenant('abc123', () => db.query('SELECT 1', []))
-    expect(db.execute).toHaveBeenCalledWith('SET search_path = tenant_abc123, public')
+    await withTenant('abc123', async (client) => {
+      await client.query('SELECT 1', [])
+    })
+    expect(mockClient.query).toHaveBeenCalledWith('SET search_path = tenant_abc123, public')
   })
 
   it('setzt search_path zurück auf public nach Ausführung', async () => {
-    await withTenant('abc123', () => db.query('SELECT 1', []))
-    expect(db.execute).toHaveBeenLastCalledWith('SET search_path = public')
+    await withTenant('abc123', async (client) => {
+      await client.query('SELECT 1', [])
+    })
+    expect(mockClient.query).toHaveBeenLastCalledWith('SET search_path = public')
   })
 
   it('setzt search_path zurück auch bei Fehler', async () => {
-    const fn = vi.fn().mockRejectedValue(new Error('DB-Fehler'))
-    await expect(withTenant('abc123', fn)).rejects.toThrow('DB-Fehler')
-    expect(db.execute).toHaveBeenLastCalledWith('SET search_path = public')
+    await expect(
+      withTenant('abc123', async () => { throw new Error('DB-Fehler') })
+    ).rejects.toThrow('DB-Fehler')
+    expect(mockClient.query).toHaveBeenLastCalledWith('SET search_path = public')
+    expect(mockClient.release).toHaveBeenCalled()
   })
 
   it('gibt Rückgabewert der Funktion zurück', async () => {
     const result = await withTenant('abc123', async () => 'testdata')
     expect(result).toBe('testdata')
   })
+
+  it('wirft bei ungültiger tenantId', async () => {
+    await expect(
+      withTenant('invalid-tenant!', async () => 'x')
+    ).rejects.toThrow('Ungültige tenantId')
+  })
+
+  it('gibt dieselbe Client-Instanz an fn weiter (Connection-Isolation)', async () => {
+    let capturedClient: unknown
+    await withTenant('tenant1', async (client) => {
+      capturedClient = client
+    })
+    // Client muss nach withTenant released worden sein
+    expect(mockClient.release).toHaveBeenCalledOnce()
+    expect(capturedClient).toBeDefined()
+  })
 })
diff --git a/src/server/db/tenant.ts b/src/server/db/tenant.ts
index 63554de..6320ca9 100644
--- a/src/server/db/tenant.ts
+++ b/src/server/db/tenant.ts
@@ -1,11 +1,31 @@
-import { db } from './client'
+import { Pool, type QueryResultRow } from 'pg'
 
-export async function withTenant<T>(tenantId: string, fn: () => Promise<T>): Promise<T> {
+const pool = new Pool({ connectionString: process.env.DATABASE_URL })
+
+interface TenantClient {
+  execute: (sql: string) => Promise<void>
+  query: <T extends QueryResultRow = Record<string, unknown>>(sql: string, params: unknown[]) => Promise<T[]>
+}
+
+export async function withTenant<T>(
+  tenantId: string,
+  fn: (client: TenantClient) => Promise<T>
+): Promise<T> {
+  if (!/^[a-z0-9_]+$/.test(tenantId)) {
+    throw new Error(`Ungültige tenantId: ${tenantId}`)
+  }
   const schema = `tenant_${tenantId}`
-  await db.execute(`SET search_path = ${schema}, public`)
+  const client = await pool.connect()
   try {
-    return await fn()
+    await client.query(`SET search_path = ${schema}, public`)
+    const tenantClient: TenantClient = {
+      execute: async (sql) => { await client.query(sql) },
+      query: async <T extends QueryResultRow = Record<string, unknown>>(sql: string, params: unknown[]) =>
+        client.query<T>(sql, params).then((r) => r.rows),
+    }
+    return await fn(tenantClient)
   } finally {
-    await db.execute(`SET search_path = public`)
+    await client.query('SET search_path = public')
+    client.release()
   }
 }