fix: add warnings for If/Switch node connection parameters (issue #360)

Implemented a warning system to guide users toward using smart parameters
(branch="true"/"false" for If nodes, case=N for Switch nodes) instead of
sourceIndex, which can lead to incorrect branch routing.

Changes:
- Added warnings property to WorkflowDiffResult interface
- Warnings generated when sourceIndex used with If/Switch nodes
- Enhanced tool documentation with CRITICAL pitfalls
- Added regression tests reproducing issue #360
- Version bump to 2.22.1

The branch parameter functionality works correctly - this fix adds helpful
warnings to prevent users from accidentally using the less intuitive
sourceIndex parameter.

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

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

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

src/mcp/tool-docs/workflow_management/n8n-update-partial-workflow.ts

@@ -272,6 +272,8 @@ Please choose a different name.
       'Use "updates" property for updateNode operations: {type: "updateNode", updates: {...}}',
       'Smart parameters (branch, case) only work with IF and Switch nodes - ignored for other node types',
       'Explicit sourceIndex overrides smart parameters (branch, case) if both provided',
+      '**CRITICAL**: For If nodes, ALWAYS use branch="true"/"false" instead of sourceIndex. Using sourceIndex=0 for multiple connections will put them ALL on the TRUE branch (main[0]), breaking your workflow logic!',
+      '**CRITICAL**: For Switch nodes, ALWAYS use case=N instead of sourceIndex. Using same sourceIndex for multiple connections will put them on the same case output.',
       'cleanStaleConnections removes ALL broken connections - cannot be selective',
       'replaceConnections overwrites entire connections object - all previous connections lost',
       '**Auto-sanitization behavior**: Binary operators (equals, contains) automatically have singleValue removed; unary operators (isEmpty, isNotEmpty) automatically get singleValue:true added',

src/services/workflow-diff-engine.ts

@@ -38,6 +38,8 @@ const logger = new Logger({ prefix: '[WorkflowDiffEngine]' });
 export class WorkflowDiffEngine {
   // Track node name changes during operations for connection reference updates
   private renameMap: Map<string, string> = new Map();
+  // Track warnings during operation processing
+  private warnings: WorkflowDiffValidationError[] = [];
 
   /**
    * Apply diff operations to a workflow
@@ -47,8 +49,9 @@ export class WorkflowDiffEngine {
     request: WorkflowDiffRequest
   ): Promise<WorkflowDiffResult> {
     try {
-      // Reset rename tracking for this diff operation
+      // Reset tracking for this diff operation
       this.renameMap.clear();
+      this.warnings = [];
 
       // Clone workflow to avoid modifying original
       const workflowCopy = JSON.parse(JSON.stringify(workflow));
@@ -114,6 +117,7 @@ export class WorkflowDiffEngine {
               ? 'Validation successful. All operations are valid.'
               : `Validation completed with ${errors.length} errors.`,
             errors: errors.length > 0 ? errors : undefined,
+            warnings: this.warnings.length > 0 ? this.warnings : undefined,
             applied: appliedIndices,
             failed: failedIndices
           };
@@ -126,6 +130,7 @@ export class WorkflowDiffEngine {
           operationsApplied: appliedIndices.length,
           message: `Applied ${appliedIndices.length} operations, ${failedIndices.length} failed (continueOnError mode)`,
           errors: errors.length > 0 ? errors : undefined,
+          warnings: this.warnings.length > 0 ? this.warnings : undefined,
           applied: appliedIndices,
           failed: failedIndices
         };
@@ -213,7 +218,8 @@ export class WorkflowDiffEngine {
           success: true,
           workflow: workflowCopy,
           operationsApplied,
-          message: `Successfully applied ${operationsApplied} operations (${nodeOperations.length} node ops, ${otherOperations.length} other ops)`
+          message: `Successfully applied ${operationsApplied} operations (${nodeOperations.length} node ops, ${otherOperations.length} other ops)`,
+          warnings: this.warnings.length > 0 ? this.warnings : undefined
         };
       }
     } catch (error) {
@@ -685,6 +691,24 @@ export class WorkflowDiffEngine {
       sourceIndex = operation.case;
     }
 
+    // Validation: Warn if using sourceIndex with If/Switch nodes without smart parameters
+    if (sourceNode && operation.sourceIndex !== undefined && operation.branch === undefined && operation.case === undefined) {
+      if (sourceNode.type === 'n8n-nodes-base.if') {
+        this.warnings.push({
+          operation: -1,  // Not tied to specific operation index in request
+          message: `Connection to If node "${operation.source}" uses sourceIndex=${operation.sourceIndex}. ` +
+            `Consider using branch="true" or branch="false" for better clarity. ` +
+            `If node outputs: main[0]=TRUE branch, main[1]=FALSE branch.`
+        });
+      } else if (sourceNode.type === 'n8n-nodes-base.switch') {
+        this.warnings.push({
+          operation: -1,  // Not tied to specific operation index in request
+          message: `Connection to Switch node "${operation.source}" uses sourceIndex=${operation.sourceIndex}. ` +
+            `Consider using case=N for better clarity (case=0 for first output, case=1 for second, etc.).`
+        });
+      }
+    }
+
     return { sourceOutput, sourceIndex };
   }
 

src/types/workflow-diff.ts

@@ -170,6 +170,7 @@ export interface WorkflowDiffResult {
   success: boolean;
   workflow?: any; // Updated workflow if successful
   errors?: WorkflowDiffValidationError[];
+  warnings?: WorkflowDiffValidationError[]; // Non-blocking warnings (e.g., parameter suggestions)
   operationsApplied?: number;
   message?: string;
   applied?: number[]; // Indices of successfully applied operations (when continueOnError is true)