fix(terminal): prevent child block mixing across loop iterations for workflow blocks

Author: waleedlatif1

apps/sim/app/api/workflows/[id]/execute/route.ts

@@ -787,6 +787,11 @@ export async function POST(req: NextRequest, { params }: { params: Promise<{ id:
                 }
               : {}
 
+            // Extract per-invocation instance ID and strip from user-visible output
+            const childWorkflowInstanceId: string | undefined =
+              callbackData.output?._childWorkflowInstanceId
+            const instanceData = childWorkflowInstanceId ? { childWorkflowInstanceId } : {}
+
             if (hasError) {
               logger.info(`[${requestId}] ✗ onBlockComplete (error) called:`, {
                 blockId,
@@ -816,6 +821,7 @@ export async function POST(req: NextRequest, { params }: { params: Promise<{ id:
                     iterationContainerId: iterationContext.iterationContainerId,
                   }),
                   ...childWorkflowData,
+                  ...instanceData,
                 },
               })
             } else {
@@ -824,6 +830,8 @@ export async function POST(req: NextRequest, { params }: { params: Promise<{ id:
                 blockName,
                 blockType,
               })
+              const { _childWorkflowInstanceId: _stripped, ...strippedOutput } =
+                callbackData.output ?? {}
               sendEvent({
                 type: 'block:completed',
                 timestamp: new Date().toISOString(),
@@ -834,7 +842,7 @@ export async function POST(req: NextRequest, { params }: { params: Promise<{ id:
                   blockName,
                   blockType,
                   input: callbackData.input,
-                  output: callbackData.output,
+                  output: strippedOutput,
                   durationMs: callbackData.executionTime || 0,
                   startedAt: callbackData.startedAt,
                   executionOrder: callbackData.executionOrder,
@@ -846,6 +854,7 @@ export async function POST(req: NextRequest, { params }: { params: Promise<{ id:
                     iterationContainerId: iterationContext.iterationContainerId,
                   }),
                   ...childWorkflowData,
+                  ...instanceData,
                 },
               })
             }

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/terminal/utils.ts

@@ -198,17 +198,25 @@ interface IterationGroup {
  * enabling correct tree construction for deeply-nested child workflows.
  */
 function collectWorkflowDescendants(
-  workflowBlockId: string,
+  instanceKey: string,
   workflowChildGroups: Map<string, ConsoleEntry[]>,
   visited: Set<string> = new Set()
 ): ConsoleEntry[] {
-  if (visited.has(workflowBlockId)) return []
-  visited.add(workflowBlockId)
-  const direct = workflowChildGroups.get(workflowBlockId) ?? []
+  if (visited.has(instanceKey)) return []
+  visited.add(instanceKey)
+  const direct = workflowChildGroups.get(instanceKey) ?? []
   const result = [...direct]
   for (const entry of direct) {
     if (isWorkflowBlockType(entry.blockType)) {
-      result.push(...collectWorkflowDescendants(entry.blockId, workflowChildGroups, visited))
+      // Use childWorkflowInstanceId when available (unique per-invocation) to correctly
+      // separate children across loop iterations of the same workflow block.
+      result.push(
+        ...collectWorkflowDescendants(
+          entry.childWorkflowInstanceId ?? entry.blockId,
+          workflowChildGroups,
+          visited
+        )
+      )
     }
   }
   return result
@@ -387,11 +395,12 @@ function buildEntryTree(entries: ConsoleEntry[]): EntryNode[] {
       // Block nodes within this iteration — workflow blocks get their full subtree
       const blockNodes: EntryNode[] = iterBlocks.map((block) => {
         if (isWorkflowBlockType(block.blockType)) {
-          const allDescendants = collectWorkflowDescendants(block.blockId, workflowChildGroups)
+          const instanceKey = block.childWorkflowInstanceId ?? block.blockId
+          const allDescendants = collectWorkflowDescendants(instanceKey, workflowChildGroups)
           const rawChildren = allDescendants.map((c) => ({
             ...c,
             childWorkflowBlockId:
-              c.childWorkflowBlockId === block.blockId ? undefined : c.childWorkflowBlockId,
+              c.childWorkflowBlockId === instanceKey ? undefined : c.childWorkflowBlockId,
           }))
           return {
             entry: block,
@@ -426,11 +435,12 @@ function buildEntryTree(entries: ConsoleEntry[]): EntryNode[] {
 
   for (const block of regularBlocks) {
     if (isWorkflowBlockType(block.blockType)) {
-      const allDescendants = collectWorkflowDescendants(block.blockId, workflowChildGroups)
+      const instanceKey = block.childWorkflowInstanceId ?? block.blockId
+      const allDescendants = collectWorkflowDescendants(instanceKey, workflowChildGroups)
       const rawChildren = allDescendants.map((c) => ({
         ...c,
         childWorkflowBlockId:
-          c.childWorkflowBlockId === block.blockId ? undefined : c.childWorkflowBlockId,
+          c.childWorkflowBlockId === instanceKey ? undefined : c.childWorkflowBlockId,
       }))
       const children = buildEntryTree(rawChildren)
       workflowNodes.push({ entry: block, children, nodeType: 'workflow' as const })

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/hooks/use-workflow-execution.ts

@@ -385,6 +385,7 @@ export function useWorkflowExecution() {
           iterationContainerId: data.iterationContainerId,
           childWorkflowBlockId: data.childWorkflowBlockId,
           childWorkflowName: data.childWorkflowName,
+          childWorkflowInstanceId: data.childWorkflowInstanceId,
         })
       }
 
@@ -410,6 +411,7 @@ export function useWorkflowExecution() {
           iterationContainerId: data.iterationContainerId,
           childWorkflowBlockId: data.childWorkflowBlockId,
           childWorkflowName: data.childWorkflowName,
+          childWorkflowInstanceId: data.childWorkflowInstanceId,
         })
       }
 
@@ -431,6 +433,7 @@ export function useWorkflowExecution() {
             iterationContainerId: data.iterationContainerId,
             childWorkflowBlockId: data.childWorkflowBlockId,
             childWorkflowName: data.childWorkflowName,
+            childWorkflowInstanceId: data.childWorkflowInstanceId,
           },
           executionIdRef.current
         )
@@ -455,6 +458,7 @@ export function useWorkflowExecution() {
             iterationContainerId: data.iterationContainerId,
             childWorkflowBlockId: data.childWorkflowBlockId,
             childWorkflowName: data.childWorkflowName,
+            childWorkflowInstanceId: data.childWorkflowInstanceId,
           },
           executionIdRef.current
         )

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/utils/workflow-execution-utils.ts

@@ -175,6 +175,7 @@ export async function executeWorkflowWithFullLogging(
                 iterationContainerId: event.data.iterationContainerId,
                 childWorkflowBlockId: event.data.childWorkflowBlockId,
                 childWorkflowName: event.data.childWorkflowName,
+                childWorkflowInstanceId: event.data.childWorkflowInstanceId,
               })
 
               if (options.onBlockComplete) {
@@ -214,6 +215,7 @@ export async function executeWorkflowWithFullLogging(
                 iterationContainerId: event.data.iterationContainerId,
                 childWorkflowBlockId: event.data.childWorkflowBlockId,
                 childWorkflowName: event.data.childWorkflowName,
+                childWorkflowInstanceId: event.data.childWorkflowInstanceId,
               })
               break
             }

apps/sim/executor/errors/child-workflow-error.ts

@@ -7,6 +7,7 @@ interface ChildWorkflowErrorOptions {
   childTraceSpans?: TraceSpan[]
   executionResult?: ExecutionResult
   childWorkflowSnapshotId?: string
+  childWorkflowInstanceId?: string
   cause?: Error
 }
 
@@ -18,6 +19,8 @@ export class ChildWorkflowError extends Error {
   readonly childWorkflowName: string
   readonly executionResult?: ExecutionResult
   readonly childWorkflowSnapshotId?: string
+  /** Per-invocation unique ID used to correlate child block events with this workflow block. */
+  readonly childWorkflowInstanceId?: string
 
   constructor(options: ChildWorkflowErrorOptions) {
     super(options.message, { cause: options.cause })
@@ -26,6 +29,7 @@ export class ChildWorkflowError extends Error {
     this.childTraceSpans = options.childTraceSpans ?? []
     this.executionResult = options.executionResult
     this.childWorkflowSnapshotId = options.childWorkflowSnapshotId
+    this.childWorkflowInstanceId = options.childWorkflowInstanceId
   }
 
   static isChildWorkflowError(error: unknown): error is ChildWorkflowError {

apps/sim/executor/execution/block-executor.ts

@@ -249,6 +249,9 @@ export class BlockExecutor {
       if (error.childWorkflowSnapshotId) {
         errorOutput.childWorkflowSnapshotId = error.childWorkflowSnapshotId
       }
+      if (error.childWorkflowInstanceId) {
+        errorOutput._childWorkflowInstanceId = error.childWorkflowInstanceId
+      }
     }
 
     this.state.setBlockOutput(node.id, errorOutput, duration)

apps/sim/executor/handlers/workflow/workflow-handler.ts

@@ -58,6 +58,10 @@ export class WorkflowBlockHandler implements BlockHandler {
     const workflowMetadata = workflows[workflowId]
     let childWorkflowName = workflowMetadata?.name || workflowId
 
+    // Unique ID per invocation — used to correlate child block events with this specific
+    // workflow block execution, preventing cross-iteration child mixing in loop contexts.
+    const instanceId = crypto.randomUUID()
+
     let childWorkflowSnapshotId: string | undefined
     try {
       const currentDepth = (ctx.workflowId?.split('_sub_').length || 1) - 1
@@ -135,7 +139,7 @@ export class WorkflowBlockHandler implements BlockHandler {
             onBlockComplete: ctx.onBlockComplete,
             onStream: ctx.onStream as ((streamingExecution: unknown) => Promise<void>) | undefined,
             childWorkflowContext: {
-              parentBlockId: block.id,
+              parentBlockId: instanceId,
               workflowName: childWorkflowName,
               workflowId,
               depth: childDepth,
@@ -162,6 +166,7 @@ export class WorkflowBlockHandler implements BlockHandler {
         workflowId,
         childWorkflowName,
         duration,
+        instanceId,
         childTraceSpans,
         childWorkflowSnapshotId
       )
@@ -197,6 +202,7 @@ export class WorkflowBlockHandler implements BlockHandler {
         childTraceSpans,
         executionResult,
         childWorkflowSnapshotId,
+        childWorkflowInstanceId: instanceId,
         cause: error instanceof Error ? error : undefined,
       })
     }
@@ -539,6 +545,7 @@ export class WorkflowBlockHandler implements BlockHandler {
     childWorkflowId: string,
     childWorkflowName: string,
     duration: number,
+    instanceId: string,
     childTraceSpans?: WorkflowTraceSpan[],
     childWorkflowSnapshotId?: string
   ): BlockOutput {
@@ -552,6 +559,7 @@ export class WorkflowBlockHandler implements BlockHandler {
         childWorkflowName,
         childTraceSpans: childTraceSpans || [],
         childWorkflowSnapshotId,
+        childWorkflowInstanceId: instanceId,
       })
     }
 
@@ -562,6 +570,7 @@ export class WorkflowBlockHandler implements BlockHandler {
       ...(childWorkflowSnapshotId ? { childWorkflowSnapshotId } : {}),
       result,
       childTraceSpans: childTraceSpans || [],
+      _childWorkflowInstanceId: instanceId,
     } as Record<string, any>
   }
 }

apps/sim/lib/workflows/executor/execution-events.ts

@@ -109,6 +109,8 @@ export interface BlockCompletedEvent extends BaseExecutionEvent {
     iterationContainerId?: string
     childWorkflowBlockId?: string
     childWorkflowName?: string
+    /** Per-invocation unique ID for correlating child block events with this workflow block. */
+    childWorkflowInstanceId?: string
   }
 }
 
@@ -134,6 +136,8 @@ export interface BlockErrorEvent extends BaseExecutionEvent {
     iterationContainerId?: string
     childWorkflowBlockId?: string
     childWorkflowName?: string
+    /** Per-invocation unique ID for correlating child block events with this workflow block. */
+    childWorkflowInstanceId?: string
   }
 }
 
@@ -287,6 +291,11 @@ export function createSSECallbacks(options: SSECallbackOptions) {
         }
       : {}
 
+    // Extract per-invocation instance ID and strip from user-visible output
+    const childWorkflowInstanceId: string | undefined =
+      callbackData.output?._childWorkflowInstanceId
+    const instanceData = childWorkflowInstanceId ? { childWorkflowInstanceId } : {}
+
     if (hasError) {
       sendEvent({
         type: 'block:error',
@@ -305,9 +314,11 @@ export function createSSECallbacks(options: SSECallbackOptions) {
           endedAt: callbackData.endedAt,
           ...iterationData,
           ...childWorkflowData,
+          ...instanceData,
         },
       })
     } else {
+      const { _childWorkflowInstanceId: _stripped, ...strippedOutput } = callbackData.output ?? {}
       sendEvent({
         type: 'block:completed',
         timestamp: new Date().toISOString(),
@@ -318,13 +329,14 @@ export function createSSECallbacks(options: SSECallbackOptions) {
           blockName,
           blockType,
           input: callbackData.input,
-          output: callbackData.output,
+          output: strippedOutput,
           durationMs: callbackData.executionTime || 0,
           startedAt: callbackData.startedAt,
           executionOrder: callbackData.executionOrder,
           endedAt: callbackData.endedAt,
           ...iterationData,
           ...childWorkflowData,
+          ...instanceData,
         },
       })
     }

apps/sim/stores/terminal/console/store.ts

@@ -428,6 +428,10 @@ export const useTerminalConsoleStore = create<ConsoleStore>()(
                 updatedEntry.childWorkflowName = update.childWorkflowName
               }
 
+              if (update.childWorkflowInstanceId !== undefined) {
+                updatedEntry.childWorkflowInstanceId = update.childWorkflowInstanceId
+              }
+
               return updatedEntry
             })
 

apps/sim/stores/terminal/console/types.ts

@@ -28,6 +28,8 @@ export interface ConsoleEntry {
   childWorkflowBlockId?: string
   /** Display name of the child workflow this block belongs to */
   childWorkflowName?: string
+  /** Per-invocation unique ID linking this workflow block to its child block events */
+  childWorkflowInstanceId?: string
 }
 
 export interface ConsoleUpdate {
@@ -50,6 +52,7 @@ export interface ConsoleUpdate {
   iterationContainerId?: string
   childWorkflowBlockId?: string
   childWorkflowName?: string
+  childWorkflowInstanceId?: string
 }
 
 export interface ConsoleStore {