diff --git a/packages/api/src/app/service.spec.ts b/packages/api/src/app/service.spec.ts
index b692aa6eb2..248d42dacd 100644
--- a/packages/api/src/app/service.spec.ts
+++ b/packages/api/src/app/service.spec.ts
@@ -327,6 +327,51 @@ describe('createAppConfigService', () => {
 
         expect(deps.getApplicableConfigs).toHaveBeenCalledWith([]);
       });
+
+      it('scopes the override cache key to the ALS tenant when no tenantId param is given', async () => {
+        const { tenantStorage } = jest.requireActual('@librechat/data-schemas');
+        const deps = createDeps({
+          getApplicableConfigs: jest
+            .fn()
+            .mockResolvedValue([{ priority: 10, overrides: { x: 1 }, isActive: true }]),
+        });
+        const { getAppConfig } = createAppConfigService(deps);
+
+        await tenantStorage.run({ tenantId: 'tenant-a' }, async () =>
+          getAppConfig({ role: 'USER' }),
+        );
+
+        const overrideKey = [...deps._cache._store.keys()].find((k: string) =>
+          k.includes('_OVERRIDE_:'),
+        );
+        expect(overrideKey).toBe('app_config:_OVERRIDE_:tenant-a:USER');
+        expect(overrideKey).not.toContain('__default__');
+      });
+
+      it('does not serve one tenant a cached config built for another tenant', async () => {
+        const { tenantStorage, getTenantId } = jest.requireActual('@librechat/data-schemas');
+        // Each tenant's DB overrides carry a marker derived from the active ALS tenant.
+        const deps = createDeps({
+          getApplicableConfigs: jest
+            .fn()
+            .mockImplementation(async () => [
+              { priority: 10, overrides: { whoami: getTenantId() }, isActive: true },
+            ]),
+        });
+        const { getAppConfig } = createAppConfigService(deps);
+
+        const configA = (await tenantStorage.run({ tenantId: 'tenant-a' }, async () =>
+          getAppConfig({ role: 'USER' }),
+        )) as TestConfig & { whoami?: string };
+        const configB = (await tenantStorage.run({ tenantId: 'tenant-b' }, async () =>
+          getAppConfig({ role: 'USER' }),
+        )) as TestConfig & { whoami?: string };
+
+        expect(configA.whoami).toBe('tenant-a');
+        expect(configB.whoami).toBe('tenant-b');
+        // A cache collision would short-circuit the second tenant's DB read.
+        expect(deps.getApplicableConfigs).toHaveBeenCalledTimes(2);
+      });
     });
 
     it('does not cache on buildPrincipals error — retries on next request', async () => {
diff --git a/packages/api/src/app/service.ts b/packages/api/src/app/service.ts
index 613c428fca..658056503f 100644
--- a/packages/api/src/app/service.ts
+++ b/packages/api/src/app/service.ts
@@ -73,7 +73,10 @@ export function _resetOverrideStrictCache(): void {
 }
 
 function overrideCacheKey(role?: string, userId?: string, tenantId?: string): string {
-  const tenant = tenantId || '__default__';
+  // Fall back to the ALS tenant context before `__default__`: callers that rely on the
+  // tenant middleware (the common path) pass no explicit tenantId, so without this the
+  // entry is keyed under the shared `__default__` bucket and leaks across tenants.
+  const tenant = tenantId || getTenantId() || '__default__';
   if (userId && role) {
     return `_OVERRIDE_:${tenant}:${role}:${userId}`;
   }
@@ -174,16 +177,16 @@ export function createAppConfigService(deps: AppConfigServiceDeps) {
       return baseConfig;
     }
 
-    // Strict-isolation + no tenant (param or ALS) = pathological path (middleware bypass or
-    // unauthenticated startup). Pre-tenant calls use baseOnly:true; admin calls carry tenantId.
-    // If ALS has a tenant, Mongoose scopes queries to that tenant's overrides — must fall through.
-    // Not cached: the cache key doesn't include ALS context, so a cached __default__ entry would
-    // be served to later ALS-scoped calls that share the same param-derived key.
+    // Strict isolation + no tenant anywhere (neither param nor ALS) is pathological: a
+    // middleware bypass or an unauthenticated startup call. Pre-tenant calls should use
+    // baseOnly:true and admin calls carry an explicit tenantId. Return the base config
+    // without caching it under the shared `__default__` bucket. When ALS has a tenant,
+    // overrideCacheKey scopes the key to it, so we fall through and cache per-tenant.
     if (principals.length === 0 && !tenantId && !getTenantId() && isStrictOverrideMode()) {
       return baseConfig;
     }
 
-    if (!tenantId && isStrictOverrideMode() && !_warnedNoTenantInStrictMode) {
+    if (!tenantId && !getTenantId() && isStrictOverrideMode() && !_warnedNoTenantInStrictMode) {
       _warnedNoTenantInStrictMode = true;
       logger.warn(
         '[getAppConfig] No tenantId in strict mode — falling back to __default__. ' +