improvement(response): only allow singleton

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/components/workflow-block/components/action-bar/action-bar.tsx

@@ -87,8 +87,8 @@ export const ActionBar = memo(
 
     const userPermissions = useUserPermissionsContext()
 
-    // Check for start_trigger (unified start block) - prevent duplication but allow deletion
     const isStartBlock = blockType === 'starter' || blockType === 'start_trigger'
+    const isResponseBlock = blockType === 'response'
     const isNoteBlock = blockType === 'note'
 
     /**
@@ -140,7 +140,7 @@ export const ActionBar = memo(
           </Tooltip.Root>
         )}
 
-        {!isStartBlock && (
+        {!isStartBlock && !isResponseBlock && (
           <Tooltip.Root>
             <Tooltip.Trigger asChild>
               <Button

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

@@ -23,7 +23,8 @@ interface TriggerValidationResult {
 }
 
 /**
- * Validates that pasting/duplicating trigger blocks won't violate constraints.
+ * Validates that pasting/duplicating blocks won't violate constraints.
+ * Checks both trigger constraints and single-instance block constraints.
  * Returns validation result with error message if invalid.
  */
 export function validateTriggerPaste(
@@ -43,6 +44,13 @@ export function validateTriggerPaste(
         return { isValid: false, message }
       }
     }
+
+    const singleInstanceIssue = TriggerUtils.getSingleInstanceBlockIssue(existingBlocks, block.type)
+    if (singleInstanceIssue) {
+      const actionText = action === 'paste' ? 'paste' : 'duplicate'
+      const message = `A workflow can only have one ${singleInstanceIssue.blockName} block. ${action === 'paste' ? 'Please remove the existing one before pasting.' : `Cannot ${actionText}.`}`
+      return { isValid: false, message }
+    }
   }
   return { isValid: true }
 }

apps/sim/app/workspace/[workspaceId]/w/[workflowId]/workflow.tsx

@@ -1129,24 +1129,36 @@ const WorkflowContent = React.memo(() => {
   )
 
   /**
-   * Checks if adding a trigger block would violate constraints and shows notification if so.
+   * Checks if adding a block would violate constraints (triggers or single-instance blocks)
+   * and shows notification if so.
    * @returns true if validation failed (caller should return early), false if ok to proceed
    */
   const checkTriggerConstraints = useCallback(
     (blockType: string): boolean => {
-      const issue = TriggerUtils.getTriggerAdditionIssue(blocks, blockType)
-      if (issue) {
+      const triggerIssue = TriggerUtils.getTriggerAdditionIssue(blocks, blockType)
+      if (triggerIssue) {
         const message =
-          issue.issue === 'legacy'
+          triggerIssue.issue === 'legacy'
             ? 'Cannot add new trigger blocks when a legacy Start block exists. Available in newer workflows.'
-            : `A workflow can only have one ${issue.triggerName} trigger block. Please remove the existing one before adding a new one.`
+            : `A workflow can only have one ${triggerIssue.triggerName} trigger block. Please remove the existing one before adding a new one.`
         addNotification({
           level: 'error',
           message,
           workflowId: activeWorkflowId || undefined,
         })
         return true
       }
+
+      const singleInstanceIssue = TriggerUtils.getSingleInstanceBlockIssue(blocks, blockType)
+      if (singleInstanceIssue) {
+        addNotification({
+          level: 'error',
+          message: `A workflow can only have one ${singleInstanceIssue.blockName} block. Please remove the existing one before adding a new one.`,
+          workflowId: activeWorkflowId || undefined,
+        })
+        return true
+      }
+
       return false
     },
     [blocks, addNotification, activeWorkflowId]

apps/sim/blocks/blocks/response.ts

@@ -17,6 +17,7 @@ export const ResponseBlock: BlockConfig<ResponseBlockOutput> = {
   category: 'blocks',
   bgColor: '#2F55FF',
   icon: ResponseIcon,
+  singleInstance: true,
   subBlocks: [
     {
       id: 'dataMode',

apps/sim/blocks/types.ts

@@ -320,6 +320,7 @@ export interface BlockConfig<T extends ToolResponse = ToolResponse> {
   subBlocks: SubBlockConfig[]
   triggerAllowed?: boolean
   authMode?: AuthMode
+  singleInstance?: boolean
   tools: {
     access: string[]
     config?: {

apps/sim/lib/copilot/tools/server/workflow/edit-workflow.ts

@@ -11,6 +11,7 @@ import { extractAndPersistCustomTools } from '@/lib/workflows/persistence/custom
 import { loadWorkflowFromNormalizedTables } from '@/lib/workflows/persistence/utils'
 import { isValidKey } from '@/lib/workflows/sanitization/key-validation'
 import { validateWorkflowState } from '@/lib/workflows/sanitization/validation'
+import { TriggerUtils } from '@/lib/workflows/triggers/triggers'
 import { getAllBlocks, getBlock } from '@/blocks/registry'
 import type { SubBlockConfig } from '@/blocks/types'
 import { EDGE, normalizeName } from '@/executor/constants'
@@ -62,6 +63,8 @@ type SkippedItemType =
   | 'invalid_subflow_parent'
   | 'nested_subflow_not_allowed'
   | 'duplicate_block_name'
+  | 'duplicate_trigger'
+  | 'duplicate_single_instance_block'
 
 /**
  * Represents an item that was skipped during operation application
@@ -1775,6 +1778,34 @@ function applyOperationsToWorkflowState(
           break
         }
 
+        const triggerIssue = TriggerUtils.getTriggerAdditionIssue(modifiedState.blocks, params.type)
+        if (triggerIssue) {
+          logSkippedItem(skippedItems, {
+            type: 'duplicate_trigger',
+            operationType: 'add',
+            blockId: block_id,
+            reason: `Cannot add ${triggerIssue.triggerName} - a workflow can only have one`,
+            details: { requestedType: params.type, issue: triggerIssue.issue },
+          })
+          break
+        }
+
+        // Check single-instance block constraints (e.g., Response block)
+        const singleInstanceIssue = TriggerUtils.getSingleInstanceBlockIssue(
+          modifiedState.blocks,
+          params.type
+        )
+        if (singleInstanceIssue) {
+          logSkippedItem(skippedItems, {
+            type: 'duplicate_single_instance_block',
+            operationType: 'add',
+            blockId: block_id,
+            reason: `Cannot add ${singleInstanceIssue.blockName} - a workflow can only have one`,
+            details: { requestedType: params.type },
+          })
+          break
+        }
+
         // Create new block with proper structure
         const newBlock = createBlockFromParams(
           block_id,

apps/sim/lib/workflows/triggers/triggers.ts

@@ -592,4 +592,34 @@ export class TriggerUtils {
     const parentWithType = parent as T & { type?: string }
     return parentWithType.type === 'loop' || parentWithType.type === 'parallel'
   }
+
+  static isSingleInstanceBlockType(blockType: string): boolean {
+    const blockConfig = getBlock(blockType)
+    return blockConfig?.singleInstance === true
+  }
+
+  static wouldViolateSingleInstanceBlock<T extends { type: string }>(
+    blocks: T[] | Record<string, T>,
+    blockType: string
+  ): boolean {
+    if (!TriggerUtils.isSingleInstanceBlockType(blockType)) {
+      return false
+    }
+
+    const blockArray = Array.isArray(blocks) ? blocks : Object.values(blocks)
+    return blockArray.some((block) => block.type === blockType)
+  }
+
+  static getSingleInstanceBlockIssue<T extends { type: string }>(
+    blocks: T[] | Record<string, T>,
+    blockType: string
+  ): { issue: 'duplicate'; blockName: string } | null {
+    if (!TriggerUtils.wouldViolateSingleInstanceBlock(blocks, blockType)) {
+      return null
+    }
+
+    const blockConfig = getBlock(blockType)
+    const blockName = blockConfig?.name || blockType
+    return { issue: 'duplicate', blockName }
+  }
 }

apps/sim/stores/workflows/utils.ts

@@ -41,6 +41,10 @@ export function getUniqueBlockName(baseName: string, existingBlocks: Record<stri
     return 'Start'
   }
 
+  if (normalizedBaseName === 'response') {
+    return 'Response'
+  }
+
   const baseNameMatch = baseName.match(/^(.*?)(\s+\d+)?$/)
   const namePrefix = baseNameMatch ? baseNameMatch[1].trim() : baseName