fix(ai-assist): prevent HTML-as-JSON crash; add safeJsonFetch + JSON-only API errors

- Created lib/safeJsonFetch.ts helper to safely parse JSON responses
- Updated AIAssist.tsx to use safeJsonFetch with proper Content-Type header
- Improved app/api/ai-assist/route.ts with requestId tracking and safe body parsing
- All API responses now return JSON with proper error messages

app/api/ai-assist/route.ts

@@ -1,18 +1,19 @@
 import { NextRequest, NextResponse } from "next/server";
+import { randomUUID } from "crypto";
 import { z } from "zod";
 
-// We'll use the environment variables for provider routing
+// Schema validation
 const schema = z.object({
-    request: z.string().min(10),
-    step: z.enum(["plan", "generate", "preview"]).default("plan"),
-    plan: z.any().optional(),
-    code: z.string().optional(),
-    provider: z.string().optional(),
-    model: z.string().optional()
+  request: z.string().min(1),
+  step: z.enum(["plan", "generate", "preview"]).default("plan"),
+  plan: z.any().optional(),
+  code: z.string().optional(),
+  provider: z.string().optional(),
+  model: z.string().optional()
 });
 
 const STEPS = {
-    plan: `You are an expert software architect. Create a DETAILED DEVELOPMENT PLAN for the following request: "{request}"
+  plan: `You are an expert software architect. Create a DETAILED DEVELOPMENT PLAN for the following request: "{request}"
 
 Output ONLY a JSON object:
 {
@@ -27,7 +28,7 @@ Output ONLY a JSON object:
   "risks": ["Potential blockers"]
 }`,
 
-    generate: `You are a Senior Vibe Coder. Execute the following approved plan:
+  generate: `You are a Senior Vibe Coder. Execute the following approved plan:
 Plan: {plan}
 
 Generate COMPLETE, PRODUCTION-READY code for all files. 
@@ -42,7 +43,7 @@ Output ONLY a JSON object:
   "explanation": "How it works"
 }`,
 
-    preview: `Convert the following code into a single-file interactive HTML preview (Standalone).
+  preview: `Convert the following code into a single-file interactive HTML preview (Standalone).
 Use Tailwind CDN. 
 
 Code: {code}
@@ -51,25 +52,57 @@ Output ONLY valid HTML.`
 };
 
 export async function POST(req: NextRequest) {
-    try {
-        const body = await req.json();
-        const { request, step, plan, code } = schema.parse(body);
+  const requestId = randomUUID();
 
-        let prompt = STEPS[step];
-        prompt = prompt.replace("{request}", request);
-        if (plan) prompt = prompt.replace("{plan}", JSON.stringify(plan));
-        if (code) prompt = prompt.replace("{code}", code);
+  try {
+    // Safe body parsing
+    const body = await req.json().catch(() => null);
 
-        // In a real scenario, this would call the ModelAdapter/Service
-        // For now, we'll return a structure that the frontend can handle,
-        // instructing it to use the existing streaming adapter for the heavy lifting.
-
-        return NextResponse.json({
-            prompt,
-            step,
-            success: true
-        });
-    } catch (error: any) {
-        return NextResponse.json({ success: false, error: error.message }, { status: 400 });
+    if (!body) {
+      return NextResponse.json(
+        { error: "Invalid JSON body", requestId, success: false },
+        { status: 400 }
+      );
     }
+
+    // Validate schema
+    const parseResult = schema.safeParse(body);
+    if (!parseResult.success) {
+      return NextResponse.json(
+        {
+          error: "Invalid request body",
+          details: parseResult.error.flatten(),
+          requestId,
+          success: false
+        },
+        { status: 400 }
+      );
+    }
+
+    const { request, step, plan, code } = parseResult.data;
+
+    let prompt = STEPS[step];
+    prompt = prompt.replace("{request}", request);
+    if (plan) prompt = prompt.replace("{plan}", JSON.stringify(plan));
+    if (code) prompt = prompt.replace("{code}", code);
+
+    // Return the prompt for the frontend to use with the streaming adapter
+    return NextResponse.json({
+      prompt,
+      step,
+      requestId,
+      success: true
+    });
+  } catch (err: any) {
+    console.error(`[ai-assist] requestId=${requestId}`, err);
+
+    return NextResponse.json(
+      {
+        error: err?.message ?? "AI Assist failed",
+        requestId,
+        success: false
+      },
+      { status: 500 }
+    );
+  }
 }