fix(dev-ui): handle EADDRINUSE by prompting to kill existing instance

When the dev server port is already in use, prompt the user to kill the
existing process instead of crashing with an unhandled error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Askir

packages/core/src/dev-ui/dev-server.ts

@@ -1,8 +1,10 @@
 import { createServer as createHttpServer } from "node:http";
+import { execSync } from "node:child_process";
 import { existsSync, readFileSync } from "node:fs";
 import { readFile } from "node:fs/promises";
 import { resolve, dirname, extname } from "node:path";
 import { fileURLToPath } from "node:url";
+import * as p from "@clack/prompts";
 import { createWSServer, type WSClientMessage } from "./ws.js";
 import { createWatcher } from "./watcher.js";
 import type { PtyManager } from "./pty.js";
@@ -12,6 +14,22 @@ import { getSchemaName } from "../dbos.js";
 import { getAppSchema } from "../cli/app.js";
 import pg from "pg";
 
+async function killPortHolder(port: number): Promise<boolean> {
+  try {
+    const pids = execSync(`lsof -ti :${port}`, { encoding: "utf-8" }).trim();
+    if (pids) {
+      for (const pid of pids.split("\n")) {
+        process.kill(parseInt(pid, 10), "SIGTERM");
+      }
+      await new Promise((r) => setTimeout(r, 500));
+      return true;
+    }
+  } catch {
+    // no process on port or kill failed
+  }
+  return false;
+}
+
 const MIME: Record<string, string> = {
   ".html": "text/html; charset=utf-8",
   ".js": "text/javascript; charset=utf-8",
@@ -136,6 +154,7 @@ export async function startDevServer(options: DevServerOptions) {
   };
 
   const { broadcast, sendTo, wss } = createWSServer(httpServer, onClientMessage);
+  wss.on("error", () => {}); // errors handled on httpServer
 
   try {
     const { createPtyManager } = await import("./pty.js");
@@ -193,10 +212,21 @@ export async function startDevServer(options: DevServerOptions) {
 
   const hostname = host ? "0.0.0.0" : "localhost";
 
-  // Try the requested port; fall back to OS-assigned port if taken
+  // Try the requested port; offer to kill existing instance if taken
   const actualPort = await new Promise<number>((resolvePromise, rejectPromise) => {
-    httpServer.once("error", (err: NodeJS.ErrnoException) => {
+    httpServer.once("error", async (err: NodeJS.ErrnoException) => {
       if (err.code === "EADDRINUSE") {
+        const shouldKill = await p.confirm({
+          message: "0pflow is likely already running. Kill it and restart?",
+        });
+        if (!p.isCancel(shouldKill) && shouldKill) {
+          const killed = await killPortHolder(port);
+          if (killed) {
+            httpServer.listen(port, hostname, () => resolvePromise(port));
+            return;
+          }
+        }
+        // Fall back to OS-assigned port
         httpServer.listen(0, hostname, () => {
           const addr = httpServer.address();
           resolvePromise(typeof addr === "object" && addr ? addr.port : 0);