improvement(execution): replace the remote-workers flag and its env fallback with a local-workers kill switch

A configured worker host is now used by default; the AppConfig-only
isolated-vm-local-workers kill switch forces local workers during rollout.
Drops IVM_REMOTE_WORKERS, and an unreachable AppConfig keeps the host in use.

Author: waleedlatif1

.claude/rules/sim-sandbox.md

@@ -88,8 +88,8 @@ payload or `ivm.Reference` wrapper in the worker:
 ## Where the worker runs
 
 The pool in `isolated-vm.ts` opens each worker either as a local child process
-or, when `IVM_REMOTE_WORKER_URL` is set and the `isolated-vm-remote-workers`
-flag is on, as a WebSocket connection to `isolated-vm-host.cjs`, which runs the
+or, when `IVM_REMOTE_WORKER_URL` is set (and the `isolated-vm-local-workers`
+kill switch is off), as a WebSocket connection to `isolated-vm-host.cjs`, which runs the
 same worker in a dedicated service that holds no platform credentials. Every
 rule in this file applies in both modes.
 

.cursor/rules/sim-sandbox.mdc

@@ -84,8 +84,8 @@ payload or `ivm.Reference` wrapper in the worker:
 ## Where the worker runs
 
 The pool in `isolated-vm.ts` opens each worker either as a local child process
-or, when `IVM_REMOTE_WORKER_URL` is set and the `isolated-vm-remote-workers`
-flag is on, as a WebSocket connection to `isolated-vm-host.cjs`, which runs the
+or, when `IVM_REMOTE_WORKER_URL` is set (and the `isolated-vm-local-workers`
+kill switch is off), as a WebSocket connection to `isolated-vm-host.cjs`, which runs the
 same worker in a dedicated service that holds no platform credentials. Every
 rule in this file applies in both modes.
 

apps/sim/lib/core/config/env.ts

@@ -454,9 +454,8 @@ export const env = createEnv({
     IVM_MAX_BROKER_ARGS_JSON_CHARS:        z.string().optional().default('262144'),  // Max JSON payload size for sandbox task broker args (isolate→host)
     IVM_MAX_BROKER_RESULT_JSON_CHARS:      z.string().optional().default('16777216'),// Max JSON payload size for sandbox task broker results (host→isolate)
     IVM_MAX_BROKERS_PER_EXECUTION:         z.string().optional().default('1000'),    // Max broker calls per sandbox task execution
-    IVM_REMOTE_WORKER_URL:                 z.string().optional(),                    // ws(s)://<host>/v1/worker of a dedicated worker host; used only while the isolated-vm-remote-workers flag is on
+    IVM_REMOTE_WORKER_URL:                 z.string().optional(),                    // ws(s)://<host>/v1/worker of a dedicated worker host; workers run there unless the isolated-vm-local-workers kill switch is on
     IVM_REMOTE_WORKER_TOKEN:               z.string().optional(),                    // Token the worker host expects (its IVM_HOST_TOKEN, >= 32 chars); required with IVM_REMOTE_WORKER_URL
-    IVM_REMOTE_WORKERS:                    z.string().optional(),                    // Fallback for the isolated-vm-remote-workers feature flag when AppConfig is not the source of truth
 
     // Knowledge Base Processing Configuration - Shared across all processing methods
     KB_CONFIG_MAX_DURATION:                z.number().optional().default(600),     // Max processing duration in seconds (10 minutes)

apps/sim/lib/core/config/feature-flags.ts

@@ -121,13 +121,13 @@ const FEATURE_FLAGS = {
       'targeting. Off-AppConfig falls back to KNOWLEDGE_MEMBER_ACCESS.',
     fallback: 'KNOWLEDGE_MEMBER_ACCESS',
   },
-  'isolated-vm-remote-workers': {
+  'isolated-vm-local-workers': {
     description:
-      'Run JavaScript sandbox workers on the dedicated worker host at IVM_REMOTE_WORKER_URL ' +
-      'instead of as local child processes. Global on/off only; no effect where ' +
-      'IVM_REMOTE_WORKER_URL is unset. While on, an unreachable host fails executions ' +
-      'instead of falling back to local workers. Off-AppConfig falls back to IVM_REMOTE_WORKERS.',
-    fallback: 'IVM_REMOTE_WORKERS',
+      'Kill switch: run JavaScript sandbox workers as local child processes even where ' +
+      'IVM_REMOTE_WORKER_URL names a dedicated worker host. Global on/off only; off by default, ' +
+      'so a configured host is used unless this is turned on. AppConfig-only: when AppConfig is ' +
+      'unreachable the host stays in use.',
+    fallback: null,
   },
   'mothership-abandoned-run-takeover-disabled': {
     description:

apps/sim/lib/execution/isolated-vm.test.ts

@@ -357,10 +357,7 @@ describe('isolated-vm scheduler', () => {
     }
   })
 
-  it('never falls back to a local worker while remote workers are enabled', async () => {
-    featureFlagsMockFns.mockIsFeatureEnabled.mockImplementation(
-      async (flag) => flag === 'isolated-vm-remote-workers'
-    )
+  it('never falls back to a local worker while a worker host is configured', async () => {
     vi.stubGlobal('WebSocket', UnreachableWebSocket)
     try {
       const { executeInIsolatedVM, spawnMock } = await loadExecutionModule({
@@ -383,6 +380,36 @@ describe('isolated-vm scheduler', () => {
       expect(result.result).toBeNull()
       expect(result.error?.isSystemError).toBe(true)
       expect(spawnMock).not.toHaveBeenCalled()
+    } finally {
+      vi.unstubAllGlobals()
+    }
+  })
+
+  it('runs local workers while the kill switch is on, even with a worker host configured', async () => {
+    featureFlagsMockFns.mockIsFeatureEnabled.mockImplementation(
+      async (flag) => flag === 'isolated-vm-local-workers'
+    )
+    vi.stubGlobal('WebSocket', UnreachableWebSocket)
+    try {
+      const { executeInIsolatedVM, spawnMock } = await loadExecutionModule({
+        envOverrides: {
+          IVM_REMOTE_WORKER_URL: 'ws://worker-host.internal:3100/v1/worker',
+          IVM_REMOTE_WORKER_TOKEN: 'remote-worker-token-of-at-least-32-chars',
+        },
+        spawns: [() => createReadyProc('ran locally')],
+      })
+
+      const result = await executeInIsolatedVM({
+        code: 'return "ran locally"',
+        params: {},
+        envVars: {},
+        contextVariables: {},
+        timeoutMs: 100,
+        requestId: 'req-kill-switch',
+      })
+
+      expect(result.result).toBe('ran locally')
+      expect(spawnMock).toHaveBeenCalledTimes(1)
     } finally {
       vi.unstubAllGlobals()
       featureFlagsMockFns.mockIsFeatureEnabled.mockReset().mockResolvedValue(false)

apps/sim/lib/execution/isolated-vm.ts

@@ -250,9 +250,10 @@ let spawnInProgress = 0
 let queueDrainRetryTimeout: ReturnType<typeof setTimeout> | null = null
 
 /**
- * Where workers run: local child processes, or connections to the dedicated
- * worker host (`isolated-vm-host.cjs`). Remote needs both
- * `IVM_REMOTE_WORKER_URL` and the `isolated-vm-remote-workers` flag.
+ * Where workers run: connections to the dedicated worker host
+ * (`isolated-vm-host.cjs`) when `IVM_REMOTE_WORKER_URL` is set, otherwise
+ * local child processes. The `isolated-vm-local-workers` kill switch forces
+ * local workers during rollout.
  */
 type WorkerTarget = { kind: 'local' } | { kind: 'remote'; url: string; token: string }
 
@@ -265,7 +266,7 @@ function workerTargetKey(target: WorkerTarget): string {
 
 async function resolveWorkerTarget(): Promise<WorkerTarget> {
   const url = env.IVM_REMOTE_WORKER_URL?.trim()
-  if (!url || !(await isFeatureEnabled('isolated-vm-remote-workers'))) return LOCAL_WORKER_TARGET
+  if (!url || (await isFeatureEnabled('isolated-vm-local-workers'))) return LOCAL_WORKER_TARGET
   return { kind: 'remote', url, token: env.IVM_REMOTE_WORKER_TOKEN ?? '' }
 }
 

apps/sim/scripts/test-isolated-vm-remote-e2e.ts

@@ -12,8 +12,7 @@ import { generateRandomHex } from '@sim/utils/random'
 
 /**
  * Runs the isolated-vm pool against a real `isolated-vm-host.cjs` over real
- * WebSockets, the topology used when `IVM_REMOTE_WORKER_URL` is set and the
- * `isolated-vm-remote-workers` flag is on. This process plays the app; the
+ * WebSockets, the topology used when `IVM_REMOTE_WORKER_URL` is set. This process plays the app; the
  * hosts are separate Node processes.
  *
  * Usage: bun run apps/sim/scripts/test-isolated-vm-remote-e2e.ts
@@ -241,7 +240,6 @@ async function run() {
 
   process.env.IVM_REMOTE_WORKER_URL = `ws://127.0.0.1:${host.port}/v1/worker`
   process.env.IVM_REMOTE_WORKER_TOKEN = host.token
-  process.env.IVM_REMOTE_WORKERS = 'true'
   process.env.IVM_MAX_STDOUT_CHARS = String(STDOUT_LIMIT_CHARS)
   process.env.IVM_QUEUE_TIMEOUT_MS = '5000'
   process.env.IVM_POOL_SIZE = '2'