diff --git a/apps/sim/lib/execution/isolated-vm-worker-hardening.test.ts b/apps/sim/lib/execution/isolated-vm-worker-hardening.test.ts
new file mode 100644
index 00000000000..7eb9797c8ca
--- /dev/null
+++ b/apps/sim/lib/execution/isolated-vm-worker-hardening.test.ts
@@ -0,0 +1,80 @@
+/**
+ * @vitest-environment node
+ *
+ * Guards the isolate hardening contract in `isolated-vm-worker.cjs`: no raw
+ * `ivm.Reference` host bridge may survive as an isolate global once user code
+ * runs. Each bootstrap must capture its bridges in a closure and list their
+ * global names in its own `undefined_globals`.
+ *
+ * The two execution paths harden independently, so every assertion is scoped to
+ * one path's source slice — a bridge installed by `executeCode` but undefined
+ * only by `executeTask` must still fail.
+ */
+import { readFileSync } from 'node:fs'
+import { join } from 'node:path'
+import { describe, expect, it } from 'vitest'
+
+const WORKER_SOURCE = readFileSync(join(__dirname, 'isolated-vm-worker.cjs'), 'utf8')
+
+const EXECUTE_CODE_MARKER = 'async function executeCode('
+const EXECUTE_TASK_MARKER = 'async function executeTask('
+
+/**
+ * Source of one execution path, from its function declaration to the start of
+ * the next one (or end of file for the last path).
+ */
+function pathSource(marker: string, nextMarker?: string): string {
+  const start = WORKER_SOURCE.indexOf(marker)
+  expect(start, `${marker} not found — worker layout changed`).toBeGreaterThan(-1)
+  const end = nextMarker ? WORKER_SOURCE.indexOf(nextMarker, start) : WORKER_SOURCE.length
+  expect(end, `${nextMarker} not found — worker layout changed`).toBeGreaterThan(start)
+  return WORKER_SOURCE.slice(start, end)
+}
+
+const EXECUTION_PATHS = [
+  { name: 'executeCode', source: pathSource(EXECUTE_CODE_MARKER, EXECUTE_TASK_MARKER) },
+  { name: 'executeTask', source: pathSource(EXECUTE_TASK_MARKER) },
+]
+
+/**
+ * Globals bound to a raw `ivm.Reference`. The worker names these `__*Ref`;
+ * `ivm.Callback` bridges (`__log`, `__textEncode`, …) are plain isolate
+ * functions that expose no host handle and are deliberately not matched.
+ */
+function referenceBridges(source: string): string[] {
+  return [...source.matchAll(/jail\.set\('(__\w+Ref)'/g)].map((match) => match[1])
+}
+
+function hardeningList(source: string): string {
+  const list = source.match(/const undefined_globals = \[[\s\S]*?\]/)
+  expect(list, 'no undefined_globals hardening list in this execution path').not.toBeNull()
+  return (list as RegExpMatchArray)[0]
+}
+
+describe('isolated-vm worker hardening', () => {
+  it.each(EXECUTION_PATHS)(
+    '$name undefines every ivm.Reference bridge it installs',
+    ({ name, source }) => {
+      const bridges = referenceBridges(source)
+      expect(
+        bridges.length,
+        `${name} installs no __*Ref bridges — detection is stale`
+      ).toBeGreaterThan(0)
+
+      const list = hardeningList(source)
+      for (const bridge of bridges) {
+        expect(
+          list.includes(`'${bridge}'`),
+          `${name} sets ${bridge} as an isolate global but its hardening list omits it`
+        ).toBe(true)
+      }
+    }
+  )
+
+  it.each(EXECUTION_PATHS)('$name undefines the isolated-vm escape globals', ({ source }) => {
+    const list = hardeningList(source)
+    for (const name of ['Isolate', 'Context', 'Script', 'Reference', 'ExternalCopy']) {
+      expect(list).toContain(`'${name}'`)
+    }
+  })
+})
diff --git a/apps/sim/lib/execution/isolated-vm-worker.cjs b/apps/sim/lib/execution/isolated-vm-worker.cjs
index 0ccc4387943..ac19d487b21 100644
--- a/apps/sim/lib/execution/isolated-vm-worker.cjs
+++ b/apps/sim/lib/execution/isolated-vm-worker.cjs
@@ -310,8 +310,12 @@ async function executeCode(request, executionId) {
         info: (...args) => __log(...args),
       };
 
-      // Set up fetch function that uses the host's secure fetch
-      async function fetch(url, options) {
+      // Set up fetch function that uses the host's secure fetch. The raw
+      // host bridge is captured in this closure so the hardening step below
+      // can undefine the global without breaking fetch().
+      (() => {
+      const __fetch = globalThis.__fetchRef;
+      const fetchImpl = async function fetch(url, options) {
         let optionsJson;
         if (options) {
           try {
@@ -323,7 +327,7 @@ async function executeCode(request, executionId) {
             throw new Error('fetch options exceed maximum payload size');
           }
         }
-        const resultJson = await __fetchRef.apply(undefined, [url, optionsJson], { result: { promise: true } });
+        const resultJson = await __fetch.apply(undefined, [url, optionsJson], { result: { promise: true } });
         let result;
         try {
           result = JSON.parse(resultJson);
@@ -355,7 +359,16 @@ async function executeCode(request, executionId) {
           blob: async () => { throw new Error('blob() not supported in sandbox'); },
           arrayBuffer: async () => { throw new Error('arrayBuffer() not supported in sandbox'); },
         };
-      }
+      };
+      // Same property attributes a top-level \`function fetch\` declaration
+      // produced, so user code sees an unchanged global.
+      Object.defineProperty(global, 'fetch', {
+        value: fetchImpl,
+        writable: true,
+        enumerable: true,
+        configurable: false
+      });
+      })();
 
       const sim = (() => {
         const broker = __brokerRef;
@@ -408,7 +421,7 @@ async function executeCode(request, executionId) {
       const undefined_globals = [
         'Isolate', 'Context', 'Script', 'Module', 'Callback', 'Reference',
         'ExternalCopy', 'process', 'require', 'module', 'exports', '__dirname', '__filename',
-        '__brokerRef', '__broker', '__callSimBroker'
+        '__fetchRef', '__brokerRef', '__broker', '__callSimBroker'
       ];
       for (const name of undefined_globals) {
         try {