diff --git a/mcp-examples/pocket-cep/src/__tests__/integration/api/sa-config-route.test.ts b/mcp-examples/pocket-cep/src/__tests__/integration/api/sa-config-route.test.ts
index b047802..343428b 100644
--- a/mcp-examples/pocket-cep/src/__tests__/integration/api/sa-config-route.test.ts
+++ b/mcp-examples/pocket-cep/src/__tests__/integration/api/sa-config-route.test.ts
@@ -98,6 +98,17 @@ describe("/api/auth/sa-config", () => {
       expect(res.status).toBe(400);
     });
 
+    it("returns 400 when customerId format is invalid", async () => {
+      const req = new NextRequest("http://localhost:3000/api/auth/sa-config", {
+        method: "POST",
+        body: JSON.stringify({ customerId: "invalid-id" }),
+      });
+      const res = await POST(req);
+      expect(res.status).toBe(400);
+      const body = await res.json();
+      expect(body.error).toContain("Invalid customerId format");
+    });
+
     it("returns 400 and blocks cookie saving when DWD token minting fails", async () => {
       mockMintToken.mockRejectedValue(new Error("unauthorized_client: client not authorized"));
       const req = new NextRequest("http://localhost:3000/api/auth/sa-config", {
diff --git a/mcp-examples/pocket-cep/src/__tests__/integration/api/users-route.test.ts b/mcp-examples/pocket-cep/src/__tests__/integration/api/users-route.test.ts
index 0f5154c..4528984 100644
--- a/mcp-examples/pocket-cep/src/__tests__/integration/api/users-route.test.ts
+++ b/mcp-examples/pocket-cep/src/__tests__/integration/api/users-route.test.ts
@@ -29,6 +29,8 @@ vi.mock("next/headers", () => ({
   cookies: async () => ({
     getAll: () => [],
     delete: vi.fn(),
+    has: vi.fn().mockReturnValue(false),
+    get: vi.fn().mockReturnValue(undefined),
   }),
 }));
 
diff --git a/mcp-examples/pocket-cep/src/__tests__/unit/auth-errors.test.ts b/mcp-examples/pocket-cep/src/__tests__/unit/auth-errors.test.ts
index d4e1486..8e88e3b 100644
--- a/mcp-examples/pocket-cep/src/__tests__/unit/auth-errors.test.ts
+++ b/mcp-examples/pocket-cep/src/__tests__/unit/auth-errors.test.ts
@@ -166,7 +166,9 @@ describe("toAuthError", () => {
       const err = new Error("Invalid Customer Id");
       const result = toAuthError(err, "admin-sdk");
       expect(result?.code).toBe("invalid_customer_id");
-      expect(result?.remedy).toContain("Ensure the Customer ID parameter passed to the tool is correct");
+      expect(result?.remedy).toContain(
+        "Ensure the Customer ID parameter passed to the tool is correct",
+      );
       expect(result?.remedy).not.toContain("/sa-setup");
     });
   });
diff --git a/mcp-examples/pocket-cep/src/__tests__/unit/cache-key.test.ts b/mcp-examples/pocket-cep/src/__tests__/unit/cache-key.test.ts
new file mode 100644
index 0000000..079b493
--- /dev/null
+++ b/mcp-examples/pocket-cep/src/__tests__/unit/cache-key.test.ts
@@ -0,0 +1,55 @@
+/**
+ * @file Unit tests for buildCallerCacheKey.
+ *
+ * Verifies that cache keys are correctly generated and partitioned by
+ * both caller identity (token hash or "sa") and tenant (customerId)
+ * to prevent cross-tenant data leakage.
+ */
+
+import { describe, it, expect } from "vitest";
+import { createHash } from "node:crypto";
+import { buildCallerCacheKey } from "@/lib/cache-key";
+
+describe("buildCallerCacheKey", () => {
+  const serverUrl = "http://localhost:4000/mcp";
+  const token = "mock-oauth-token-123";
+  const tokenHash = createHash("sha256").update(token).digest("hex").slice(0, 16);
+
+  it("generates correct key for service_account mode without customerId", () => {
+    const key = buildCallerCacheKey(serverUrl, undefined);
+    expect(key).toBe(`${serverUrl}|sa`);
+  });
+
+  it("generates correct key for service_account mode with customerId", () => {
+    const key = buildCallerCacheKey(serverUrl, undefined, "C0111111");
+    expect(key).toBe(`${serverUrl}|sa|c:C0111111`);
+  });
+
+  it("generates correct key for user_oauth mode without customerId", () => {
+    const key = buildCallerCacheKey(serverUrl, token);
+    expect(key).toBe(`${serverUrl}|u:${tokenHash}`);
+  });
+
+  it("generates correct key for user_oauth mode with customerId", () => {
+    const key = buildCallerCacheKey(serverUrl, token, "C0222222");
+    expect(key).toBe(`${serverUrl}|u:${tokenHash}|c:C0222222`);
+  });
+
+  it("varies key when token changes", () => {
+    const token2 = "another-token";
+    const tokenHash2 = createHash("sha256").update(token2).digest("hex").slice(0, 16);
+
+    const key1 = buildCallerCacheKey(serverUrl, token);
+    const key2 = buildCallerCacheKey(serverUrl, token2);
+
+    expect(key1).not.toBe(key2);
+    expect(key2).toBe(`${serverUrl}|u:${tokenHash2}`);
+  });
+
+  it("varies key when customerId changes", () => {
+    const key1 = buildCallerCacheKey(serverUrl, undefined, "C0111111");
+    const key2 = buildCallerCacheKey(serverUrl, undefined, "C0222222");
+
+    expect(key1).not.toBe(key2);
+  });
+});
diff --git a/mcp-examples/pocket-cep/src/__tests__/unit/mcp-tools.test.ts b/mcp-examples/pocket-cep/src/__tests__/unit/mcp-tools.test.ts
index 4de5638..2006655 100644
--- a/mcp-examples/pocket-cep/src/__tests__/unit/mcp-tools.test.ts
+++ b/mcp-examples/pocket-cep/src/__tests__/unit/mcp-tools.test.ts
@@ -20,6 +20,7 @@ vi.mock("@/lib/mcp-client", () => ({
 
 vi.mock("@/lib/sa-session", () => ({
   getServiceAccountConfig: vi.fn().mockResolvedValue(null),
+  getActiveCustomerId: vi.fn().mockResolvedValue(undefined),
 }));
 
 import { getMcpToolsForAiSdk, invalidateToolCatalog } from "@/lib/mcp-tools";
diff --git a/mcp-examples/pocket-cep/src/app/api/auth/sa-config/route.ts b/mcp-examples/pocket-cep/src/app/api/auth/sa-config/route.ts
index 8412081..58580d1 100644
--- a/mcp-examples/pocket-cep/src/app/api/auth/sa-config/route.ts
+++ b/mcp-examples/pocket-cep/src/app/api/auth/sa-config/route.ts
@@ -18,6 +18,7 @@ import {
   DwdScopeVerificationError,
   mintServiceAccountTokenOrThrow,
 } from "@/lib/access-token";
+import { CUSTOMER_ID_REGEX } from "@/lib/constants";
 
 /**
  * Returns the currently configured Service Account tenant credentials.
@@ -61,6 +62,13 @@ export async function POST(request: NextRequest) {
     return NextResponse.json({ error: "customerId is required" }, { status: 400 });
   }
 
+  if (!CUSTOMER_ID_REGEX.test(customerId)) {
+    return NextResponse.json(
+      { error: "Invalid customerId format. Must start with 'C' (e.g. C01234567)." },
+      { status: 400 },
+    );
+  }
+
   const impersonatedUser = body.impersonatedUser?.trim() || "";
 
   clearServiceAccountTokenCache();
diff --git a/mcp-examples/pocket-cep/src/app/api/insights/risky-activity/route.ts b/mcp-examples/pocket-cep/src/app/api/insights/risky-activity/route.ts
index 77a49f2..b8c9586 100644
--- a/mcp-examples/pocket-cep/src/app/api/insights/risky-activity/route.ts
+++ b/mcp-examples/pocket-cep/src/app/api/insights/risky-activity/route.ts
@@ -14,6 +14,8 @@ import { getGoogleAccessToken } from "@/lib/access-token";
 import { getEnv } from "@/lib/env";
 import { callMcpTool } from "@/lib/mcp-client";
 import { requireSession } from "@/lib/session";
+import { getActiveCustomerId } from "@/lib/sa-session";
+import { buildCallerCacheKey } from "@/lib/cache-key";
 import { isAuthError, toAuthError } from "@/lib/auth-errors";
 import { CACHE_TAGS, getOrFetch } from "@/lib/server-cache";
 import { summarizeChromeActivity } from "@/lib/activity-summarizer";
@@ -38,9 +40,10 @@ export async function POST(request: Request) {
   const selectedUser = body.selectedUser ?? "";
   const config = getEnv();
   const accessToken = await getGoogleAccessToken();
-
   try {
-    const cacheKey = `insights:risky-activity:${selectedUser}`;
+    const customerId = await getActiveCustomerId();
+    const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken, customerId);
+    const cacheKey = `insights:risky-activity:${callerKey}:${selectedUser}`;
     const summary = await getOrFetch({
       key: cacheKey,
       ttlMs: INSIGHT_TTL_MS,
diff --git a/mcp-examples/pocket-cep/src/app/api/prompts/route.ts b/mcp-examples/pocket-cep/src/app/api/prompts/route.ts
index cc1fbb8..f03deac 100644
--- a/mcp-examples/pocket-cep/src/app/api/prompts/route.ts
+++ b/mcp-examples/pocket-cep/src/app/api/prompts/route.ts
@@ -15,6 +15,7 @@ import { getEnv } from "@/lib/env";
 import { getMcpPrompt, listMcpPrompts } from "@/lib/mcp-client";
 import { buildCallerCacheKey } from "@/lib/cache-key";
 import { requireSession } from "@/lib/session";
+import { getActiveCustomerId } from "@/lib/sa-session";
 import { LOG_TAGS } from "@/lib/constants";
 import { getErrorMessage } from "@/lib/errors";
 import { getOrFetch, CACHE_TAGS } from "@/lib/server-cache";
@@ -27,9 +28,9 @@ export async function GET() {
 
   const config = getEnv();
   const accessToken = await getGoogleAccessToken();
-  const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken);
-
   try {
+    const customerId = await getActiveCustomerId();
+    const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken, customerId);
     const prompts = await getOrFetch({
       key: `prompts:${callerKey}`,
       ttlMs: CATALOG_TTL_MS,
diff --git a/mcp-examples/pocket-cep/src/app/api/tools/route.ts b/mcp-examples/pocket-cep/src/app/api/tools/route.ts
index e939554..ab45410 100644
--- a/mcp-examples/pocket-cep/src/app/api/tools/route.ts
+++ b/mcp-examples/pocket-cep/src/app/api/tools/route.ts
@@ -24,6 +24,7 @@ import { getGoogleAccessToken } from "@/lib/access-token";
 import { getEnv } from "@/lib/env";
 import { buildCallerCacheKey } from "@/lib/cache-key";
 import { requireSession } from "@/lib/session";
+import { getActiveCustomerId } from "@/lib/sa-session";
 import { LOG_TAGS } from "@/lib/constants";
 import { getErrorMessage } from "@/lib/errors";
 import { getOrFetch, CACHE_TAGS } from "@/lib/server-cache";
@@ -45,9 +46,9 @@ export async function GET() {
 
   const config = getEnv();
   const accessToken = await getGoogleAccessToken();
-  const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken);
-
   try {
+    const customerId = await getActiveCustomerId();
+    const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken, customerId);
     const tools = await getOrFetch({
       key: `tools:${callerKey}`,
       ttlMs: TOOLS_TTL_MS,
diff --git a/mcp-examples/pocket-cep/src/app/api/users/route.ts b/mcp-examples/pocket-cep/src/app/api/users/route.ts
index c0df4c2..e47f09d 100644
--- a/mcp-examples/pocket-cep/src/app/api/users/route.ts
+++ b/mcp-examples/pocket-cep/src/app/api/users/route.ts
@@ -19,6 +19,7 @@ import { getGoogleAccessToken } from "@/lib/access-token";
 import { searchUsers, buildAdminQuery, type DirectoryUser } from "@/lib/admin-sdk";
 import { buildCallerCacheKey } from "@/lib/cache-key";
 import { requireSession } from "@/lib/session";
+import { getActiveCustomerId } from "@/lib/sa-session";
 import { conditionalJson } from "@/lib/http-cache";
 import { getOrFetch, CACHE_TAGS } from "@/lib/server-cache";
 import { respondWithApiError, unauthenticatedResponse } from "@/lib/api-response";
@@ -38,9 +39,9 @@ export async function GET(request: NextRequest) {
   const config = getEnv();
   const accessToken = await getGoogleAccessToken();
   const adminQuery = buildAdminQuery(query);
-  const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken);
-
   try {
+    const customerId = await getActiveCustomerId();
+    const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken, customerId);
     const users = await getOrFetch({
       key: `users:${callerKey}:${query.trim().toLowerCase()}`,
       ttlMs: USERS_TTL_MS,
diff --git a/mcp-examples/pocket-cep/src/lib/activity-data.ts b/mcp-examples/pocket-cep/src/lib/activity-data.ts
index 564be1d..2cdd4f1 100644
--- a/mcp-examples/pocket-cep/src/lib/activity-data.ts
+++ b/mcp-examples/pocket-cep/src/lib/activity-data.ts
@@ -67,7 +67,8 @@ export async function getCachedActivity(
   }
   const saConfig = config.AUTH_MODE === "service_account" ? await getServiceAccountConfig() : null;
   const impersonatedUser = saConfig?.impersonatedUser;
-  const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken);
+  const customerId = saConfig?.customerId;
+  const callerKey = buildCallerCacheKey(config.MCP_SERVER_URL, accessToken, customerId);
 
   return getOrFetch({
     key: `activity:${callerKey}:${days}`,
@@ -100,7 +101,11 @@ export async function getActivitySafe(days: number = DEFAULT_ACTIVITY_DAYS): Pro
  * Pulls and groups Chrome audit events for the given caller, scoped to
  * `days` of history. Pagination stops at {@link ACTIVITY_MAX_EVENTS}.
  */
-async function fetchActivity(tokenToUse: string, days: number, impersonatedUser?: string): Promise<ActivityMap> {
+async function fetchActivity(
+  tokenToUse: string,
+  days: number,
+  impersonatedUser?: string,
+): Promise<ActivityMap> {
   const requestHeaders = await buildGoogleApiHeaders(tokenToUse);
 
   const baseUrl = new URL(
diff --git a/mcp-examples/pocket-cep/src/lib/cache-key.ts b/mcp-examples/pocket-cep/src/lib/cache-key.ts
index df128a5..ced5e1f 100644
--- a/mcp-examples/pocket-cep/src/lib/cache-key.ts
+++ b/mcp-examples/pocket-cep/src/lib/cache-key.ts
@@ -1,26 +1,31 @@
 /**
- * @file Shared cache-key builder for per-caller in-process caches.
+ * @file Shared cache-key builder for per-caller and per-tenant in-process caches.
  *
  * Several server-side caches (MCP tool catalog, MCP prompt catalog,
- * Admin Reports activity) isolate entries by caller identity so that
- * user_oauth sessions don't share cached data with one another or with
- * service_account (ADC) flows.
+ * Admin Reports activity) isolate entries by caller identity and tenant so that
+ * user_oauth sessions and service_account sessions don't share cached data
+ * across different Google Workspace customers (domains) or users.
  *
- * service_account callers share a single `|sa` entry (no token). Each
+ * service_account callers partition by `|sa|c:<customerId>`. Each
  * user_oauth caller gets a per-token entry keyed by a truncated
- * SHA-256 — the raw access token never lands in the Map.
+ * SHA-256, optionally partitioned by `customerId` if available.
  */
 
 import { createHash } from "node:crypto";
 
 /**
- * Builds a cache key of the form `${serverUrl}|sa` (service_account) or
- * `${serverUrl}|u:<16-char-hash>` (user_oauth). The hash length is
+ * Builds a cache key of the form `${serverUrl}|sa|c:${customerId}` (service_account) or
+ * `${serverUrl}|u:${hash}|c:${customerId}` (user_oauth). The hash length is
  * enough to make collisions astronomically unlikely while keeping keys
  * short in logs and heap dumps.
  */
-export function buildCallerCacheKey(serverUrl: string, accessToken: string | undefined): string {
-  if (!accessToken) return `${serverUrl}|sa`;
+export function buildCallerCacheKey(
+  serverUrl: string,
+  accessToken: string | undefined,
+  customerId?: string,
+): string {
+  const customerPart = customerId ? `|c:${customerId}` : "";
+  if (!accessToken) return `${serverUrl}|sa${customerPart}`;
   const hash = createHash("sha256").update(accessToken).digest("hex").slice(0, 16);
-  return `${serverUrl}|u:${hash}`;
+  return `${serverUrl}|u:${hash}${customerPart}`;
 }
diff --git a/mcp-examples/pocket-cep/src/lib/constants.ts b/mcp-examples/pocket-cep/src/lib/constants.ts
index cef0927..fd0d2be 100644
--- a/mcp-examples/pocket-cep/src/lib/constants.ts
+++ b/mcp-examples/pocket-cep/src/lib/constants.ts
@@ -14,6 +14,12 @@
  */
 export const SA_EMAIL_DOMAIN = "service-account.local";
 
+/**
+ * Regex pattern for validating Google Workspace Customer IDs.
+ * Must start with 'C' followed by alphanumeric characters (e.g. C01234567).
+ */
+export const CUSTOMER_ID_REGEX = /^C[a-zA-Z0-9]+$/;
+
 /**
  * DOM id for the header's user-search input. Referenced from the app
  * bar (focus button), dashboard (`/` keyboard shortcut), and the
diff --git a/mcp-examples/pocket-cep/src/lib/env.ts b/mcp-examples/pocket-cep/src/lib/env.ts
index c3f1654..a70ac12 100644
--- a/mcp-examples/pocket-cep/src/lib/env.ts
+++ b/mcp-examples/pocket-cep/src/lib/env.ts
@@ -10,7 +10,7 @@
  */
 
 import { z } from "zod";
-import { DEFAULT_MCP_URL } from "./constants";
+import { DEFAULT_MCP_URL, CUSTOMER_ID_REGEX } from "./constants";
 
 /**
  * Regex-validated Google OAuth client ID. The strict format check catches
@@ -47,7 +47,7 @@ const baseFields = {
     .or(z.literal("")),
   CEP_CUSTOMER_ID: z
     .string()
-    .regex(/^C[a-zA-Z0-9]+$/, "CEP_CUSTOMER_ID must start with 'C' (e.g. C01234567).")
+    .regex(CUSTOMER_ID_REGEX, "CEP_CUSTOMER_ID must start with 'C' (e.g. C01234567).")
     .optional()
     .or(z.literal("")),
 };
diff --git a/mcp-examples/pocket-cep/src/lib/mcp-tools.ts b/mcp-examples/pocket-cep/src/lib/mcp-tools.ts
index aa2d607..98cf5f1 100644
--- a/mcp-examples/pocket-cep/src/lib/mcp-tools.ts
+++ b/mcp-examples/pocket-cep/src/lib/mcp-tools.ts
@@ -17,7 +17,7 @@ import { callMcpTool, listMcpTools, type McpToolDefinition } from "./mcp-client"
 import { toAuthError } from "./auth-errors";
 import { buildCallerCacheKey } from "./cache-key";
 import { LOG_TAGS } from "./constants";
-import { getServiceAccountConfig } from "./sa-session";
+import { getServiceAccountConfig, getActiveCustomerId } from "./sa-session";
 
 const TOOL_CATALOG_TTL_MS = 5 * 60 * 1000;
 
@@ -30,8 +30,9 @@ const toolCatalogCache = new Map<string, { tools: McpToolDefinition[]; expiresAt
 async function getCachedToolCatalog(
   serverUrl: string,
   accessToken: string | undefined,
+  customerId?: string,
 ): Promise<McpToolDefinition[]> {
-  const key = buildCallerCacheKey(serverUrl, accessToken);
+  const key = buildCallerCacheKey(serverUrl, accessToken, customerId);
   const now = Date.now();
   const cached = toolCatalogCache.get(key);
   if (cached && cached.expiresAt > now) return cached.tools;
@@ -66,7 +67,7 @@ export async function getMcpToolsForAiSdk(
   accessToken?: string,
   customerId?: string,
 ): Promise<ToolSet> {
-  const mcpTools = await getCachedToolCatalog(serverUrl, accessToken);
+  const mcpTools = await getCachedToolCatalog(serverUrl, accessToken, customerId);
   const tools: ToolSet = {};
 
   for (const t of mcpTools) {
@@ -144,7 +145,8 @@ function extractErrorText(content: unknown): string {
  * for injection into reference prompts (e.g. follow-up suggestion brainstorming).
  */
 export async function getMcpToolsSummary(serverUrl: string, accessToken?: string): Promise<string> {
-  const mcpTools = await getCachedToolCatalog(serverUrl, accessToken);
+  const customerId = await getActiveCustomerId();
+  const mcpTools = await getCachedToolCatalog(serverUrl, accessToken, customerId);
   return mcpTools
     .map((t) => {
       const schema = t.inputSchema as