fix: disable duplicate trigger nodes in workflow builder

AddNodePanel: disable trigger buttons and show tooltip when a trigger
already exists on the canvas, using bySpecName to identify trigger-
category specs from the live node list.
useWorkflowState: preflight in saveWorkflow rejects saves with multiple
trigger nodes via a sonner toast before the network request is made.
text_chat_session_service: include the original exception message in
TextChatSessionExecutionError so the HTTP 500 detail surfaces the root
cause without DB inspection.

Closes #378

api/services/workflow/text_chat_session_service.py

@@ -207,7 +207,7 @@ async def execute_pending_text_chat_turn(
             error_message=str(e),
         )
         raise TextChatSessionExecutionError(
-            "Failed to execute text chat assistant turn"
+            f"Failed to execute text chat assistant turn: {e}"
         ) from e
 
     completed_session_data = normalize_text_chat_session_data(text_session.session_data)

api/tests/test_text_chat_session_service.py

@@ -1,4 +1,4 @@
-from unittest.mock import AsyncMock
+from unittest.mock import AsyncMock, patch
 
 import pytest
 
@@ -9,6 +9,7 @@ from api.services.workflow.text_chat_session_service import (
     TextChatTurnNotFoundError,
     _reload_text_chat_session,
     build_pending_text_chat_turn,
+    execute_pending_text_chat_turn,
     truncate_text_chat_future_turns,
     validate_text_chat_turn_cursor,
 )
@@ -77,6 +78,31 @@ async def test_reload_text_chat_session_uses_run_id_to_resolve_organization(
     get_text_session.assert_awaited_once_with(123, organization_id=77)
 
 
+@pytest.mark.asyncio
+async def test_execute_pending_turn_surfaces_original_exception_message(monkeypatch):
+    session = WorkflowRunTextSessionModel(workflow_run_id=42)
+    session.session_data = {"turns": [{"id": "turn-1", "status": "pending"}], "cursor_turn_id": "turn-1"}
+    session.checkpoint = None
+
+    monkeypatch.setattr(
+        text_chat_session_service,
+        "execute_text_chat_pending_turn",
+        AsyncMock(side_effect=RuntimeError("Workflow has 2 start nodes")),
+    )
+    monkeypatch.setattr(
+        text_chat_session_service,
+        "_mark_pending_turn_failed",
+        AsyncMock(),
+    )
+
+    with pytest.raises(TextChatSessionExecutionError, match="Workflow has 2 start nodes"):
+        await execute_pending_text_chat_turn(
+            workflow_id=1,
+            run_id=42,
+            text_session=session,
+        )
+
+
 @pytest.mark.asyncio
 async def test_reload_text_chat_session_raises_when_run_organization_is_missing(
     monkeypatch,

ui/src/app/workflow/[workflowId]/RenderWorkflow.tsx

@@ -690,6 +690,7 @@ function RenderWorkflow({
                     isOpen={isAddNodePanelOpen}
                     onNodeSelect={handleNodeSelect}
                     onClose={() => setIsAddNodePanelOpen(false)}
+                    nodes={nodes}
                 />
 
                 <VersionHistoryPanel

ui/src/app/workflow/[workflowId]/hooks/useWorkflowState.ts

@@ -10,6 +10,7 @@ import { EdgeChange, NodeChange } from "@xyflow/system";
 import { useRouter } from "next/navigation";
 import posthog from "posthog-js";
 import { useCallback, useEffect, useRef } from "react";
+import { toast } from "sonner";
 
 import { useWorkflowStore } from "@/app/workflow/[workflowId]/stores/workflowStore";
 import {
@@ -306,6 +307,13 @@ export const useWorkflowState = ({
         // This avoids a race condition where rfInstance.toObject() may return
         // stale node data if React hasn't re-rendered yet after a store update.
         const { nodes: currentNodes, edges: currentEdges } = useWorkflowStore.getState();
+        const triggerCount = currentNodes.filter(
+            (n) => bySpecName.get(n.type)?.category === 'trigger',
+        ).length;
+        if (triggerCount > 1) {
+            toast.error('A workflow can only have one trigger. Remove the extra trigger node before saving.');
+            return;
+        }
         const viewport = rfInstance.current.getViewport();
         const flow = { nodes: currentNodes, edges: currentEdges, viewport };
         let result: { versionNumber?: number; versionStatus?: string } | undefined;
@@ -372,6 +380,7 @@ export const useWorkflowState = ({
         user,
         validateWorkflow,
         applyWorkflowErrors,
+        bySpecName,
     ]);
 
     // Set up keyboard shortcut for save (Cmd/Ctrl + S)

ui/src/components/flow/AddNodePanel.tsx

@@ -6,12 +6,13 @@ import type { NodeSpec } from '@/client/types.gen';
 import { useNodeSpecs } from '@/components/flow/renderer';
 import { Button } from '@/components/ui/button';
 
-import { NodeType } from './types';
+import { FlowNode, NodeType } from './types';
 
 type AddNodePanelProps = {
     isOpen: boolean;
     onClose: () => void;
     onNodeSelect: (nodeType: NodeType) => void;
+    nodes: FlowNode[];
 };
 
 // Section ordering and labels. Drives both the category → section title
@@ -32,10 +33,12 @@ function NodeSection({
     title,
     specs,
     onNodeSelect,
+    disableTriggers,
 }: {
     title: string;
     specs: NodeSpec[];
     onNodeSelect: (nodeType: NodeType) => void;
+    disableTriggers: boolean;
 }) {
     if (specs.length === 0) return null;
     return (
@@ -46,12 +49,15 @@ function NodeSection({
             <div className="space-y-2">
                 {specs.map((spec) => {
                     const Icon = resolveIcon(spec.icon);
+                    const disabled = disableTriggers && spec.category === 'trigger';
                     return (
                         <Button
                             key={spec.name}
                             variant="outline"
                             className="w-full justify-start p-4 h-auto hover:bg-accent/50 transition-colors"
                             onClick={() => onNodeSelect(spec.name as NodeType)}
+                            disabled={disabled}
+                            title={disabled ? 'A trigger already exists in this workflow' : undefined}
                         >
                             <div className="flex items-center">
                                 <div className="bg-muted p-2 rounded-lg mr-3 border border-border">
@@ -74,8 +80,8 @@ function NodeSection({
     );
 }
 
-export default function AddNodePanel({ isOpen, onNodeSelect, onClose }: AddNodePanelProps) {
-    const { specs } = useNodeSpecs();
+export default function AddNodePanel({ isOpen, onNodeSelect, onClose, nodes }: AddNodePanelProps) {
+    const { specs, bySpecName } = useNodeSpecs();
 
     // Group registered specs by category, preserving the SECTION_ORDER.
     // Adding a new node type with a new spec.category just shows up here.
@@ -86,6 +92,11 @@ export default function AddNodePanel({ isOpen, onNodeSelect, onClose }: AddNodeP
         }));
     }, [specs]);
 
+    const hasTrigger = useMemo(
+        () => nodes.some((n) => bySpecName.get(n.type)?.category === 'trigger'),
+        [nodes, bySpecName],
+    );
+
     useEffect(() => {
         const handleKeyDown = (event: KeyboardEvent) => {
             if (event.key === 'Escape' && isOpen) {
@@ -128,6 +139,7 @@ export default function AddNodePanel({ isOpen, onNodeSelect, onClose }: AddNodeP
                             title={title}
                             specs={specs}
                             onNodeSelect={onNodeSelect}
+                            disableTriggers={hasTrigger}
                         />
                     ))}
                 </div>