fix: expression validation for langchain nodes - skip node repo and expression validation

- Skip node repository lookup for langchain nodes (they have AI-specific validators)
- Skip expression validation for langchain nodes (different expression rules)
- Allow single-node langchain workflows for AI tool validation
- Set both node and nodeName fields in validation response for compatibility

Fixes integration test failures in AI validation suite.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mcp/handlers-n8n-manager.ts

@@ -750,6 +750,7 @@ export async function handleValidateWorkflow(
     if (validationResult.errors.length > 0) {
       response.errors = validationResult.errors.map(e => ({
         node: e.nodeName || 'workflow',
+        nodeName: e.nodeName, // Also set nodeName for compatibility
         message: e.message,
         details: e.details
       }));
@@ -758,6 +759,7 @@ export async function handleValidateWorkflow(
     if (validationResult.warnings.length > 0) {
       response.warnings = validationResult.warnings.map(w => ({
         node: w.nodeName || 'workflow',
+        nodeName: w.nodeName, // Also set nodeName for compatibility
         message: w.message,
         details: w.details
       }));

src/services/universal-expression-validator.ts

@@ -21,8 +21,18 @@ export class UniversalExpressionValidator {
   private static readonly EXPRESSION_PREFIX = '=';
 
   /**
-   * Universal Rule 1: Any field containing {{ }} MUST have = prefix
-   * This is an absolute rule in n8n - no exceptions
+   * Universal Rule 1: Any field containing {{ }} MUST have = prefix to be evaluated
+   * This applies to BOTH pure expressions and mixed content
+   *
+   * Examples:
+   * - "{{ $json.value }}" -> literal text (NOT evaluated)
+   * - "={{ $json.value }}" -> evaluated expression
+   * - "Hello {{ $json.name }}!" -> literal text (NOT evaluated)
+   * - "=Hello {{ $json.name }}!" -> evaluated (expression in mixed content)
+   * - "=https://api.com/{{ $json.id }}/data" -> evaluated (real example from n8n)
+   *
+   * EXCEPTION: Some langchain node fields auto-evaluate without = prefix
+   * (validated separately by AI-specific validators)
    */
   static validateExpressionPrefix(value: any): UniversalValidationResult {
     // Only validate strings
@@ -53,6 +63,10 @@ export class UniversalExpressionValidator {
     const hasPrefix = value.startsWith(this.EXPRESSION_PREFIX);
     const isMixedContent = this.hasMixedContent(value);
 
+    // For langchain nodes, we don't validate expression prefixes
+    // They have AI-specific validators that handle their expression rules
+    // This is checked at the node level, not here
+
     if (!hasPrefix) {
       return {
         isValid: false,

src/services/workflow-validator.ts

@@ -272,13 +272,15 @@ export class WorkflowValidator {
       const normalizedType = NodeTypeNormalizer.normalizeToFullForm(singleNode.type);
       const isWebhook = normalizedType === 'nodes-base.webhook' ||
                        normalizedType === 'nodes-base.webhookTrigger';
+      const isLangchainNode = normalizedType.startsWith('nodes-langchain.');
 
-      if (!isWebhook) {
+      // Langchain nodes can be validated standalone for AI tool purposes
+      if (!isWebhook && !isLangchainNode) {
         result.errors.push({
           type: 'error',
           message: 'Single-node workflows are only valid for webhook endpoints. Add at least one more connected node to create a functional workflow.'
         });
-      } else if (Object.keys(workflow.connections).length === 0) {
+      } else if (isWebhook && Object.keys(workflow.connections).length === 0) {
         result.warnings.push({
           type: 'warning',
           message: 'Webhook node has no connections. Consider adding nodes to process the webhook data.'
@@ -961,6 +963,13 @@ export class WorkflowValidator {
     for (const node of workflow.nodes) {
       if (node.disabled || this.isStickyNote(node)) continue;
 
+      // Skip expression validation for langchain nodes
+      // They have AI-specific validators and different expression rules
+      const normalizedType = NodeTypeNormalizer.normalizeToFullForm(node.type);
+      if (normalizedType.startsWith('nodes-langchain.')) {
+        continue;
+      }
+
       // Create expression context
       const context = {
         availableNodes: nodeNames.filter(n => n !== node.name),