René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[BUG] Variable references not injected into execution context, causing ReferenceError","articleBody":"## Bug Description\n\nWhen function/condition blocks reference undefined block variables (e.g., `\u003cwebhook1.conversation.id\u003e` when webhook block hasn't executed), the variable reference is substituted into the code but the actual variable is never injected into the execution context, causing \"ReferenceError: variable is not defined\" at runtime.\n\n**This issue was discovered as a consequence of fixing #2794** - while the nullish coalescing fix addressed the substitution syntax, it revealed this deeper IPC serialization bug.\n\n## Root Cause\n\nThe variable substitution process has a multi-layer issue:\n\n1. **Code Substitution Layer**: `\u003cwebhook1.conversation.id\u003e` is correctly replaced with `__tag_webhook1_conversation_id` in the code\n2. **Context Variable Storage**: The variable IS added to `contextVariables` map with `undefined` value\n3. **IPC Serialization Layer**: When `contextVariables` is serialized via IPC to the worker thread, `JSON.stringify()` **strips keys with undefined values**, causing those variables to not be transmitted\n4. **Execution Layer**: The worker receives an empty `contextVariables` object and can't inject the missing variables into the VM\n\nResult: Code references `__tag_webhook1_conversation_id` but it's never defined in the execution context.\n\n## Error Manifestation\n\n```\nReferenceError: __tag_webhook1_conversation_id is not defined\n```\n\nThis occurs in:\n- E2B JavaScript execution path\n- E2B Python execution path\n- Isolated-VM execution path\n\n## Reproduction Steps\n\n1. Create a workflow with Start trigger and Webhook trigger\n2. Add Extract Context function block with: `const x = \u003cwebhook1.optional_field\u003e ?? \"default\";`\n3. Execute workflow using **Start trigger** (webhook doesn't execute)\n4. Function block fails with \"ReferenceError: __tag_webhook1_optional_field is not defined\"\n\n## Root Issue: IPC Serialization\n\nThe key problem occurs in `apps/sim/lib/execution/isolated-vm.ts`:\n\n```javascript\n// BROKEN - undefined values are stripped during JSON serialization in IPC\nworker!.send({ type: 'execute', executionId, request: req })\n```\n\nWhen `req.contextVariables` contains `{ __tag_webhook_id: undefined }`, the IPC serialization strips it, resulting in an empty object on the worker side.\n\n## Solution Applied\n\n### File 1: `apps/sim/lib/execution/isolated-vm.ts`\n\nAdd explicit sanitization BEFORE IPC transmission:\n\n```diff\n+  // Convert undefined values to null in contextVariables before IPC serialization.\n+  // JSON.stringify() strips keys with undefined values, which would cause ReferenceErrors\n+  // when user code references variables that weren't properly injected into the VM context.\n+  const sanitizedContextVariables: Record\u003cstring, unknown\u003e = {}\n+  for (const [key, value] of Object.entries(req.contextVariables)) {\n+    sanitizedContextVariables[key] = value === undefined ? null : value\n+  }\n+  const sanitizedReq = { ...req, contextVariables: sanitizedContextVariables }\n\n   return new Promise((resolve) =\u003e {\n     // ...\n     try {\n-      worker!.send({ type: 'execute', executionId, request: req })\n+      worker!.send({ type: 'execute', executionId, request: sanitizedReq })\n     } catch {\n```\n\n### File 2: `apps/sim/app/api/function/execute/route.ts`\n\nFix E2B JavaScript and Python paths:\n\n```diff\n // E2B JavaScript path (around line 745)\n for (const [k, v] of Object.entries(contextVariables)) {\n-  prologue += `const ${k} = JSON.parse(${JSON.stringify(JSON.stringify(v))});\\n`\n+  // Convert undefined to null so JSON.stringify produces valid output\n+  const safeValue = v === undefined ? null : v\n+  prologue += `const ${k} = JSON.parse(${JSON.stringify(JSON.stringify(safeValue))});\\n`\n   prologueLineCount++\n }\n\n // E2B Python path (around line 817)\n for (const [k, v] of Object.entries(contextVariables)) {\n-  prologue += `${k} = json.loads(${JSON.stringify(JSON.stringify(v))})\\n`\n+  // Convert undefined to null so JSON.stringify produces valid output\n+  const safeValue = v === undefined ? null : v\n+  prologue += `${k} = json.loads(${JSON.stringify(JSON.stringify(safeValue))})\\n`\n   prologueLineCount++\n }\n```\n\n### File 3: `apps/sim/lib/execution/isolated-vm-worker.cjs`\n\nAdd defensive handling in worker:\n\n```diff\n for (const [key, value] of Object.entries(contextVariables)) {\n-  await jail.set(key, new ivm.ExternalCopy(value).copyInto())\n+  // Convert undefined to null to ensure the variable is properly set in the VM\n+  const safeValue = value === undefined ? null : value\n+  await jail.set(key, new ivm.ExternalCopy(safeValue).copyInto())\n }\n```\n\n## Why This Fix Works\n\nThis ensures:\n- All variables are transmitted via IPC (null is serializable, undefined is not)\n- Worker receives all variables and can inject them into the VM context\n- User code can use nullish checks (`??`) on the received null values\n\n## Related Issues\n\n- #2794: Variable substitution breaks nullish coalescing operator - **this bug was discovered as a follow-up to that fix**\n\n## Testing\n\nVerify with:\n1. Start trigger + Extract Context referencing non-existent webhook fields\n2. Confirm `??` operator fallback works correctly\n3. Test all three execution paths (E2B JS, E2B Python, Isolated-VM)","author":{"url":"https://github.com/PlaneInABottle","@type":"Person","name":"PlaneInABottle"},"datePublished":"2026-01-13T21:31:31.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/2798/sim/issues/2798"}