feat(function): add local data conversion APIs

apps/docs/content/docs/workflows/blocks/function.mdx

@@ -148,15 +148,42 @@ workspace path instead, or keep the secret out of the output.
 
 ## Language
 
-JavaScript without imports runs in a fast local sandbox. JavaScript with `import` or `require`, Python, and Shell run in the configured remote sandbox provider.
+JavaScript without imports, mounted files, or a selected sandbox runs in a fast local sandbox. JavaScript with `import` or `require`, Python, and Shell run in the configured remote sandbox provider.
 
 | Feature | JavaScript | Python | Shell |
 | --- | --- | --- | --- |
-| **Execution** | Local when there are no imports; remote with imports | Always remote | Always remote |
+| **Execution** | Local by default; remote with imports, mounted files, or a selected sandbox | Always remote | Always remote |
 | **Return a value** | `return { … }` | Assign `__sim_result__ = { … }` | Print `__SIM_RESULT__={…}` |
 | **HTTP requests** | `fetch()` built in | `requests` or `httpx` | `curl` or an installed CLI |
 | **Best for** | quick transforms and JSON | scripts, data science, charts, complex math | CLI workflows and system utilities |
 
+### Local JavaScript data APIs
+
+`Buffer`, `atob`, `btoa`, `TextEncoder`, and `TextDecoder` are available without
+imports. For example, encode a JSON payload as UTF-8 base64:
+
+```javascript
+const payload = { message: 'Hello 🌍' }
+return Buffer.from(JSON.stringify(payload), 'utf8').toString('base64')
+```
+
+`Buffer` is a Uint8Array-backed implementation of common Node Buffer operations,
+including UTF-8, base64, hex, concatenation, and numeric reads and writes. It does
+not provide every API from the latest Node release; import `node:buffer` to use
+the remote runtime's native implementation. All local Buffer allocations,
+including `allocUnsafe`, are zero-filled and count toward the isolate's memory
+limit.
+
+`TextEncoder` encodes UTF-8; `TextDecoder` supports encoding labels, streaming
+decoding, and fatal decoding errors. `atob` and `btoa` operate on binary strings,
+not Unicode text. Use `Buffer` for Unicode base64 conversion.
+
+These data APIs do not add filesystem access, Node modules, timers, streams, or
+`FormData` to the local runtime. Import the required Node module or select a
+remote sandbox when those capabilities are needed.
+
+### Remote runtimes
+
 <Callout type="info">
 Python and Shell require a remote sandbox. They are enabled by default on sim.ai;
 on a self-hosted instance, build and configure the provider's dedicated
@@ -236,8 +263,8 @@ Then open the block's advanced options and choose the sandbox under **Sandbox**.
 
 The default and custom behavior is intentionally explicit:
 
-- **JavaScript without imports** stays in the local isolated runtime for speed and
-  ignores the sandbox selection.
+- **JavaScript without imports** stays in the local isolated runtime for speed
+  unless a sandbox is selected or files are mounted.
 - **JavaScript with `import` or `require`** runs remotely. With no selection it
   uses the Function base; with a sandbox it gets that sandbox's npm packages and
   system packages and managed CLI tools.

apps/sim/blocks/blocks/function.ts

@@ -13,6 +13,7 @@ export const FunctionBlock: BlockConfig<CodeExecutionOutput> = {
     'This is a core workflow block. Execute custom JavaScript, Python, or Shell code within your workflow. JavaScript without imports runs locally for fast execution, while code with imports, Python, and Shell run in a remote sandbox.',
   bestPractices: `
   - JavaScript code without external imports runs in a local VM for fastest execution.
+  - Local JavaScript includes Buffer, atob, btoa, TextEncoder, and TextDecoder for data conversion without imports. Use Buffer.from(text, 'utf8').toString('base64') for Unicode text; btoa only accepts binary strings.
   - JavaScript code with import/require statements runs in a remote sandbox.
   - Python code always runs in a remote sandbox.
   - Shell code runs CLI commands in a remote sandbox.
@@ -63,7 +64,7 @@ IMPORTANT FORMATTING RULES:
 1. Reference Environment Variables: Use the exact syntax {{VARIABLE_NAME}}. In JavaScript and Python, prefer the unquoted form when the placeholder is the complete expression (for example, 'const apiKey = {{SERVICE_API_KEY}};'). Quoted and embedded string forms such as '"Bearer {{SERVICE_API_KEY}}"', template literals, and JavaScript regex literals are also supported. In Shell, prefer '"{{SERVICE_API_KEY}}"' when the secret should be one scalar argument; use a bare placeholder only when Bash word-splitting or pattern semantics are intentional. Sim binds the resolved value separately from the source at execution time, preserving its exact string contents.
 2. Reference Input Parameters/Workflow Variables: Use the exact syntax <variable_name>. Do NOT wrap it in quotes (e.g., use 'userId = <userId>;' not 'userId = "<userId>";'). This includes parameters defined in the block's schema and outputs from previous blocks.
 3. Function Body ONLY: Do NOT include the function signature (e.g., 'async function myFunction() {' or the surrounding '}').
-4. Imports: Standard Node.js built-in modules (e.g., 'crypto', 'fs') are always available. Third-party packages are available ONLY when the block has a sandbox selected — the sandbox's package list is appended below when one is. Never import a package that is not on that list.
+4. Runtime APIs: Buffer, atob, btoa, TextEncoder, and TextDecoder are available without imports. Use Buffer.from(text, 'utf8').toString('base64') for Unicode text; btoa only accepts binary strings. Importing Node.js built-in modules (e.g., 'crypto', 'fs') runs the code in a remote sandbox. Third-party packages are available ONLY when the block has a sandbox selected — the sandbox's package list is appended below when one is. Never import a package that is not on that list.
 5. Output: Ensure the code returns a value if the function is expected to produce output. Use 'return'.
 6. Clarity: Write clean, readable code.
 7. No Explanations: Do NOT include markdown formatting, comments explaining the rules, or any text other than the raw JavaScript code for the function body.

apps/sim/lib/execution/function-globals.smoke.test.ts

@@ -0,0 +1,84 @@
+/**
+ * @vitest-environment node
+ *
+ * Exercises the actual worker with SIM_HELPERS_SMOKE=1 and a compatible Node build.
+ */
+import { describe, expect, it } from 'vitest'
+import { executeInIsolatedVM } from '@/lib/execution/isolated-vm'
+
+function run(code: string, timeoutMs = 5000) {
+  return executeInIsolatedVM({
+    code,
+    params: {},
+    envVars: {},
+    contextVariables: {},
+    timeoutMs,
+    requestId: 'function-globals-smoke',
+  })
+}
+
+describe.skipIf(process.env.SIM_HELPERS_SMOKE !== '1')('Function globals in a real isolate', () => {
+  it('uses Buffer and text codecs without imports or a remote sandbox', async () => {
+    const result = await run(`
+      const payload = { text: 'Hello 🌍' }
+      const encoded = Buffer.from(JSON.stringify(payload), 'utf8').toString('base64')
+      const decoded = new TextDecoder().decode(new TextEncoder().encode(payload.text))
+      return { encoded, decoded, binary: atob(btoa('hello')), zero: Buffer.allocUnsafe(32).every(b => b === 0) }
+    `)
+    expect(result.error).toBeUndefined()
+    expect(result.result).toEqual({
+      encoded: Buffer.from(JSON.stringify({ text: 'Hello 🌍' }), 'utf8').toString('base64'),
+      decoded: 'Hello 🌍',
+      binary: 'hello',
+      zero: true,
+    })
+  })
+
+  it('keeps prototypes and host capabilities isolated between executions', async () => {
+    const first = await run(`
+      Buffer.prototype.polluted = true
+      TextEncoder.prototype.polluted = true
+      return Buffer.from.constructor('return typeof process + ":" + typeof require')()
+    `)
+    expect(first.error).toBeUndefined()
+    expect(first.result).toBe('undefined:undefined')
+    const next = await run(
+      'return [Buffer.prototype.polluted === undefined, TextEncoder.prototype.polluted === undefined]'
+    )
+    expect(next.error).toBeUndefined()
+    expect(next.result).toEqual([true, true])
+  })
+
+  it('preserves user-code error locations and catches invalid conversions', async () => {
+    const result = await run("const data = Buffer.from('hello')\nthrow new Error('expected error')")
+    expect(result.error).toMatchObject({
+      line: 2,
+      lineContent: "throw new Error('expected error')",
+    })
+    const caught = await run("try { btoa('🌍'); return false } catch { return true }")
+    expect(caught.error).toBeUndefined()
+    expect(caught.result).toBe(true)
+  })
+
+  it('retains timeout enforcement and allows subsequent executions', async () => {
+    const result = await run('while (true) { new TextEncoder().encode("hello") }', 100)
+    expect(result.error).toBeDefined()
+    expect(result.termination).toBe('timeout')
+    const next = await run('return Buffer.from("ok").toString()')
+    expect(next.error).toBeUndefined()
+    expect(next.result).toBe('ok')
+  })
+
+  it('charges Buffer allocations to the existing isolate memory limit', async () => {
+    const result = await run(
+      'const buffers = []; while (true) { buffers.push(Buffer.alloc(16 * 1024 * 1024)) }'
+    )
+    expect(result.error).toBeDefined()
+    expect(result.error?.message).toMatch(
+      /memory|disposed|cancelled|Array buffer allocation failed/i
+    )
+    const next = await run('return Buffer.alloc(4).length')
+    expect(next.error).toBeUndefined()
+    expect(next.result).toBe(4)
+  })
+})

apps/sim/lib/execution/isolated-vm-worker.cjs

@@ -20,6 +20,7 @@ const MAX_FETCH_OPTIONS_JSON_CHARS =
 
 const SANDBOX_BUNDLE_DIR = path.join(__dirname, 'sandbox', 'bundles')
 const SANDBOX_BUNDLE_FILES = {
+  'function-globals': 'function-globals.cjs',
   pptxgenjs: 'pptxgenjs.cjs',
   docx: 'docx.cjs',
   'pdf-lib': 'pdf-lib.cjs',
@@ -199,6 +200,7 @@ async function executeCode(request, executionId) {
 
   let context = null
   let bootstrapScript = null
+  let globalsScript = null
   let runtimeBindingsScript = null
   let userScript = null
   let logCallback = null
@@ -215,6 +217,10 @@ async function executeCode(request, executionId) {
 
     await jail.set('global', jail.derefInto())
 
+    /** Evaluate pure JavaScript inside this isolate; never share host constructors. */
+    globalsScript = await isolate.compileScript(getBundleSource('function-globals').source)
+    await globalsScript.run(context, { timeout: timeoutMs })
+
     logCallback = new ivm.Callback((...args) => {
       const message = args.map((arg) => stringifyLogValue(arg)).join(' ')
       appendStdout(`${message}\n`)
@@ -567,6 +573,7 @@ async function executeCode(request, executionId) {
       userScript,
       runtimeBindingsScript,
       bootstrapScript,
+      globalsScript,
       ...externalCopies,
       fetchCallback,
       brokerCallback,

apps/sim/lib/execution/sandbox/bundles/build.ts

@@ -1,11 +1,11 @@
 #!/usr/bin/env bun
 /**
- * Builds isolate-compatible bundles for the document-generation libraries.
+ * Builds isolate-compatible bundles for Function globals and document libraries.
  *
- * Each library is bundled with `target=browser, format=iife` so it can be
- * evaluated inside a V8 isolate that has no Node APIs (`require`, `process`,
- * `fs`). The emitted files attach their exports to `globalThis.__bundles[name]`
- * and are checked in so production images don't need the bundler at runtime.
+ * Document libraries target browsers and register on `globalThis.__bundles`.
+ * Function globals use neutral resolution so dependencies provide pure JavaScript
+ * fallbacks instead of assuming native browser codecs. Both emit IIFEs checked
+ * in so production images don't need the bundler at runtime.
  *
  * Every bundle is evaluated in a bare context before it is written: the
  * bundler can emit a reference to a runtime helper it never defines (Bun does
@@ -19,7 +19,11 @@ import { mkdirSync, rmSync, writeFileSync } from 'node:fs'
 import { dirname, join } from 'node:path'
 import { fileURLToPath } from 'node:url'
 import { createLogger } from '@sim/logger'
-import { evaluateSandboxBundle } from '@/lib/execution/sandbox/bundles/verify'
+import { build } from 'esbuild'
+import {
+  evaluateFunctionGlobals,
+  evaluateSandboxBundle,
+} from '@/lib/execution/sandbox/bundles/verify'
 import type { SandboxBundleName } from '@/lib/execution/sandbox/types'
 
 const logger = createLogger('SandboxBundleBuild')
@@ -45,11 +49,11 @@ const ENTRIES_DIR = join(HERE, '.entries')
 const APP_SIM_ROOT = join(HERE, '..', '..', '..', '..')
 
 interface BundleSpec {
-  /** Key on `globalThis.__bundles`. */
-  name: SandboxBundleName
+  /** Bundle identity; document libraries register on `globalThis.__bundles`. */
+  name: SandboxBundleName | 'function-globals'
   /** Short filename written under `bundles/<file>.cjs`. */
   outFile: string
-  /** Source of the entry file bun will bundle. */
+  /** Source of the entry file to bundle. */
   entry: string
 }
 
@@ -65,6 +69,17 @@ if (typeof globalThis.process === 'undefined') globalThis.process = __processPol
 `
 
 const BUNDLES: ReadonlyArray<BundleSpec> = [
+  {
+    name: 'function-globals',
+    outFile: 'function-globals.cjs',
+    entry: `
+import { Buffer } from 'buffer/'
+import { TextEncoder, TextDecoder } from '@exodus/bytes/encoding.js'
+import atob from 'core-js-pure/actual/atob'
+import btoa from 'core-js-pure/actual/btoa'
+Object.assign(globalThis, { Buffer, TextEncoder, TextDecoder, atob, btoa })
+`,
+  },
   {
     name: 'pdf-lib',
     outFile: 'pdf-lib.cjs',
@@ -106,31 +121,52 @@ async function main(): Promise<void> {
     const entryPath = join(ENTRIES_DIR, `${spec.name}.entry.ts`)
     writeFileSync(entryPath, spec.entry, 'utf-8')
 
-    const result = await Bun.build({
-      entrypoints: [entryPath],
-      target: 'browser',
-      format: 'iife',
-      minify: true,
-      sourcemap: 'none',
-      root: APP_SIM_ROOT,
-    })
-
-    if (!result.success) {
-      for (const log of result.logs) {
-        logger.error(String(log))
+    let code: string
+    if (spec.name === 'function-globals') {
+      const result = await build({
+        entryPoints: [entryPath],
+        platform: 'neutral',
+        mainFields: ['module', 'main'],
+        format: 'iife',
+        bundle: true,
+        minify: true,
+        write: false,
+        legalComments: 'eof',
+      })
+      if (result.outputFiles.length !== 1) {
+        throw new Error('Expected one Function globals bundle')
       }
-      throw new Error(`Failed to build sandbox bundle: ${spec.name}`)
-    }
+      code = result.outputFiles[0].text
+    } else {
+      const result = await Bun.build({
+        entrypoints: [entryPath],
+        target: 'browser',
+        format: 'iife',
+        minify: true,
+        sourcemap: 'none',
+        root: APP_SIM_ROOT,
+      })
 
-    if (result.outputs.length === 0) {
-      throw new Error(`No output produced for sandbox bundle: ${spec.name}`)
+      if (!result.success) {
+        for (const log of result.logs) {
+          logger.error(String(log))
+        }
+        throw new Error(`Failed to build sandbox bundle: ${spec.name}`)
+      }
+      if (result.outputs.length === 0) {
+        throw new Error(`No output produced for sandbox bundle: ${spec.name}`)
+      }
+      code = await result.outputs[0].text()
     }
 
-    const code = await result.outputs[0].text()
-    const banner = `// sandbox bundle: ${spec.name}\n// generated by apps/sim/lib/execution/sandbox/bundles/build.ts\n// do not edit by hand. run \`bun run build:sandbox-bundles\` to regenerate.\n`
+    const banner = `/**\n * Sandbox bundle: ${spec.name}\n * Generated by apps/sim/lib/execution/sandbox/bundles/build.ts.\n * Do not edit by hand. Run \`bun run build:sandbox-bundles\` to regenerate.\n */\n`
     const output = banner + code
     try {
-      evaluateSandboxBundle(output, spec.name)
+      if (spec.name === 'function-globals') {
+        evaluateFunctionGlobals(output)
+      } else {
+        evaluateSandboxBundle(output, spec.name)
+      }
     } catch (error) {
       throw new Error(
         `Sandbox bundle ${spec.name} does not evaluate in a bare isolate context: ${String(error)}`