🐛 Bugfix: What caused the persistent 'VITTO WebSocket error: Unexpecte...

Date: 2026-02-06
Time: 21:02:49 UTC
Session: editor2-97.7
Agent: backend
Confidence: 100%
Status: ✅ Documented

📋 Decision Context

Question:

What caused the persistent 'VITTO WebSocket error: Unexpected server response: 400' on ws://localhost:3001/ws?

Task: Fix VITTO WebSocket 400 error on backend-ai-api

🎯 Decision Made

Chosen:

Node.js inspector port conflict (--port flag in @nx/js:node executor)

Options Considered:

1. engine.io destroyUpgrade conflict between multiple Socket.IO instances
2. Node.js inspector port conflict (--port flag in @nx/js:node executor)
3. NativeWsGateway lifecycle timing issue (OnApplicationBootstrap vs explicit call)
4. CORS or origin rejection at HTTP level

💭 Reasoning

The true root cause was in tools/scripts/dev-serve.js which passed --port=3001 to npx nx serve backend-ai-api. For backend apps using @nx/js:node executor, --port sets the Node.js INSPECTOR port (like --inspect=3001), NOT the application port. The app port comes from process.env.AI_API_PORT. This caused the Node.js debugger to bind to localhost:3001 on the loopback interface BEFORE the NestJS app could bind to *:3001. When VITTO connected to ws://localhost:3001/ws, DNS resolved to the loopback address first, hitting the inspector which responded with HTTP 400 'WebSockets request was expected' (inspector expects Chrome DevTools Protocol, not app WebSocket). The fix: add backend: true to APP_PORT_MAP entries for backend apps and skip the --port flag for them. The inspector now defaults to port 9229.

📁 Files Involved

• tools/scripts/dev-serve.js
• apis/backend-ai-api/src/main.ts
• apis/backend-ai-api/src/app/mcp-gateway/native-ws.gateway.ts

🔗 Related

• Decision ID: dev_1770411769170_0cepyet
• Session: 97
• Workspace: editor2

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