[Bug] Watch mode breaks after code reload when `app.mcpTool(...)` is registered (`No script host available`)

[maaaNu]

Title: Watch mode breaks after code reload when app.mcpTool(...) is registered (No script host available)

Description
When using the Node.js worker in watch mode, a simple HTTP trigger reloads fine on file changes. As soon as an MCP tool is registered via app.mcpTool(...), the first start works, but after the first file change triggers a reload, the host becomes unhealthy and the function app stops responding with:

[Tag='azure.functions.script_host.lifecycle']: {status: Unhealthy, description: No script host available}

Cold-restarting func start makes it work again until the next watch reload.

Steps

1. Create a new Functions app (TypeScript).
2. Install deps and run in watch mode (commands below).
3. Start with a single HTTP endpoint → reloads fine.
4. Add an MCP tool registration (see code below).
5. Save any file to trigger watch reload → host becomes unhealthy and HTTP endpoint stops working.

Expected behavior

• After a file change, the script host should reload and continue serving both the HTTP trigger and the MCP tool without going unhealthy.

Actual behavior

• First run works.
• On first watch-triggered reload, the host reports unhealthy and no longer serves requests until the process is restarted.

Logs

[12:26:44 PM] Found 0 errors. Watching for file changes.

[2025-11-25T11:26:49.013Z] [Tag=''] Process reporting unhealthy: Unhealthy. Health check entries are azure.functions.web_host.lifecycle: {status: Healthy, description: }, azure.functions.script_host.lifecycle: {status: Unhealthy, description: No script host available}
[2025-11-25T11:26:49.014Z] [Tag='azure.functions.readiness'] Process reporting unhealthy: Unhealthy. Health check entries are azure.functions.script_host.lifecycle: {status: Unhealthy, description: No script host available}

Minimal repro (TypeScript)

src/functions/test.ts (HTTP trigger)

import { app, HttpHandler } from "@azure/functions";

const handler: HttpHandler = async (_req, _ctx) => {
  return { status: 200, body: JSON.stringify("Hello World!") };
};

app.http("test", {
  methods: ["GET"],
  authLevel: "anonymous",
  route: "test",
  handler,
});

src/functions/mcp.ts (MCP tool)

import util from "node:util";
import { app, InvocationContext, arg } from "@azure/functions";


export async function mcpToolHello(_toolArguments: unknown, context: InvocationContext): Promise<string> {
  console.log(util.inspect(_toolArguments, { depth: null, colors: true }));
  if (!context.triggerMetadata) {
    throw new Error("triggerMetadata is undefined");
  }
  const mcptoolargs = context.triggerMetadata.mcptoolargs as { name?: string };
  const name = mcptoolargs?.name;
  console.info(`Hello ${name}, I am MCP Tool!`);
  return `Hello ${name || "World"}, I am MCP Tool!!!`;
}

// Register the hello tool (adding this causes watch reload to fail)
app.mcpTool("hello world", {
  toolName: "hello world",
  description: "says hello!",
  toolProperties: {
    name: arg.string().describe("Required property to identify the caller.").optional(),
  },
  handler: mcpToolHello,
});

Commands used

{
    "build": "tsc --project tsconfig.json && tsc-alias -p tsconfig.json",
    "prestart": "npm run build && func extensions install",
    "start:host": "func start --typescript",
    "start": "npm-run-all --parallel start:host watch",
    "watch": "tsc -w",
    "test": "jest"
}

Environment

• OS: macOS 15.7.2
• Node.js: 20.18.1
• Azure Functions Core Tools: 4.5.0
• @azure/functions: 4.9.0
• Language: TypeScript 5.9.3

Notes / Hypothesis

• The issue only manifests after the worker/host tries to reload on file change. Without app.mcpTool(...), watch reload is stable.
• Looks like the MCP tool registration may not be disposed/re-registered cleanly on hot reload, leaving the script host unavailable.
• A full process restart is a reliable workaround for now.