feat(35-03): add subgraph extraction and scoped context to chat API

- Import extractSubgraph from subgraphExtractor
- Parse selectedNodeIds from request body
- Call extractSubgraph before building workflow context
- Pass subgraph.selectedNodes and selectedEdges to buildWorkflowContext
- Update buildEditSystemPrompt to accept optional restSummary parameter
- Add WORKFLOW CONTEXT (SELECTED SUBSET) section when scoped
- Include rest-of-workflow summary with type breakdown and boundary connections
- Add metadata placeholder note to system prompt
- Enhance error handling to return 413 for oversized payloads

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/api/chat/route.ts

@@ -2,6 +2,7 @@ import { streamText, convertToModelMessages, UIMessage, stepCountIs } from 'ai';
 import { createGoogleGenerativeAI } from '@ai-sdk/google';
 import { createChatTools, buildEditSystemPrompt } from '@/lib/chat/tools';
 import { buildWorkflowContext } from '@/lib/chat/contextBuilder';
+import { extractSubgraph } from '@/lib/chat/subgraphExtractor';
 import { WorkflowNode } from '@/types';
 import { WorkflowEdge } from '@/types/workflow';
 
@@ -9,9 +10,10 @@ export const maxDuration = 60; // 1 minute timeout
 
 export async function POST(request: Request) {
   try {
-    const { messages, workflowState } = await request.json() as {
+    const { messages, workflowState, selectedNodeIds } = await request.json() as {
       messages: UIMessage[];
       workflowState?: { nodes: WorkflowNode[]; edges: WorkflowEdge[] };
+      selectedNodeIds?: string[];
     };
 
     // Get API key from environment
@@ -20,14 +22,21 @@ export async function POST(request: Request) {
       return new Response('GEMINI_API_KEY not configured', { status: 500 });
     }
 
-    // Build workflow context from client-provided state
-    const context = buildWorkflowContext(
+    // Extract subgraph if nodes are selected, otherwise use full workflow
+    const subgraph = extractSubgraph(
       workflowState?.nodes || [],
-      workflowState?.edges || []
+      workflowState?.edges || [],
+      selectedNodeIds || []
+    );
+
+    // Build workflow context from selected subgraph
+    const context = buildWorkflowContext(
+      subgraph.selectedNodes,
+      subgraph.selectedEdges
     );
 
-    // Build context-aware system prompt
-    const systemPrompt = buildEditSystemPrompt(context);
+    // Build context-aware system prompt with optional rest summary
+    const systemPrompt = buildEditSystemPrompt(context, subgraph.restSummary);
 
     // Extract node IDs for tool validation
     const nodeIds = (workflowState?.nodes || []).map(n => n.id);
@@ -60,6 +69,14 @@ export async function POST(request: Request) {
       return new Response('Rate limit reached. Please wait and try again.', { status: 429 });
     }
 
+    // Check for token/size errors and return 413
+    if (error instanceof Error) {
+      const errorMsg = error.message.toLowerCase();
+      if (errorMsg.includes('too large') || errorMsg.includes('token limit') || errorMsg.includes('payload') || errorMsg.includes('request entity too large')) {
+        return new Response('This workflow is too large for the AI to process. Try selecting fewer nodes.', { status: 413 });
+      }
+    }
+
     return new Response(
       error instanceof Error ? error.message : 'Chat request failed',
       { status: 500 }

src/lib/chat/tools.ts

@@ -2,6 +2,7 @@ import { tool } from "ai";
 import { z } from "zod";
 import { EditOperation } from "./editOperations";
 import { WorkflowContext, formatContextForPrompt } from "./contextBuilder";
+import { SubgraphResult } from "./subgraphExtractor";
 import { NodeType } from "@/types";
 
 /**
@@ -23,9 +24,13 @@ const VALID_NODE_TYPES: NodeType[] = [
  * Builds the enhanced system prompt with current workflow context and tool usage rules.
  *
  * @param workflowContext - Current workflow state summary
+ * @param restSummary - Optional summary of unselected nodes (when selection scoped)
  * @returns Complete system prompt with context and rules
  */
-export function buildEditSystemPrompt(workflowContext: WorkflowContext): string {
+export function buildEditSystemPrompt(
+  workflowContext: WorkflowContext,
+  restSummary?: SubgraphResult['restSummary']
+): string {
   // Base domain expertise from existing SYSTEM_PROMPT
   const baseDomainExpertise = `You are a workflow expert for Node Banana, a visual node-based AI image generation tool. Be concise and direct — short bullet points, no fluff. Use the same language the user sees in the UI. Never expose internal property names, JSON structure, or code.
 
@@ -92,12 +97,39 @@ Displays the final generated image or video. Connect any image or video output h
 - Ask one clarifying question at a time if goal is unclear`;
 
   // Current workflow context
-  const contextSection = `
+  let contextSection = `
 
 ## CURRENT WORKFLOW
 
 ${formatContextForPrompt(workflowContext)}`;
 
+  // Add subgraph summary if scoped to selected nodes
+  if (restSummary && restSummary.nodeCount > 0) {
+    const typeBreakdown = Object.entries(restSummary.typeBreakdown)
+      .map(([type, count]) => `${count} ${type}`)
+      .join(', ');
+
+    const boundaryInfo = restSummary.boundaryConnections.length > 0
+      ? `\nConnections to selected nodes: ${restSummary.boundaryConnections.map(bc =>
+          `${bc.direction === 'incoming' ? 'Input from' : 'Output to'} ${bc.otherNodeId} (${bc.handleType})`
+        ).join(', ')}`
+      : '';
+
+    contextSection += `
+
+## WORKFLOW CONTEXT (SELECTED SUBSET)
+
+You are focused on the selected nodes. The rest of the workflow:
+- ${restSummary.nodeCount} other node(s): ${typeBreakdown}${boundaryInfo}
+
+Note: Binary data (images, videos) has been replaced with metadata descriptions like [image: 1024x768, 245KB]. These are not editable - they represent existing content that will be preserved.`;
+  } else if (!restSummary) {
+    // Full workflow - add metadata note
+    contextSection += `
+
+Note: Binary data (images, videos) has been replaced with metadata descriptions like [image: 1024x768, 245KB]. These are not editable - they represent existing content that will be preserved.`;
+  }
+
   // Tool usage rules
   const toolUsageRules = `