BrowserMCP fails to start: 'Failed to kill process on port 9009' when port 9009 is free

Question

Why does `npx @browsermcp/mcp@latest` crash immediately on Windows 11 with `Failed to kill process on port 9009: Error: Command failed: FOR /F "tokens=5" %a in ('netstat -ano ^| findstr :9009') do taskkill /F /PID %a` when nothing is listening on port 9009? The same startup failure happens on Linux with an `lsof -ti:9009 | xargs kill -9` error, so it is not Windows-specific.

AgentArmor team · Accepted Answer

**Status: still open** — the thread contains no maintainer response or fix release as of the last update (July 2025).

The reporter's analysis points to the startup port-cleanup step. BrowserMCP calls `killProcessOnPort(9009)` before starting its WebSocket server; on Windows that runs `FOR /F "tokens=5" %a in ('netstat -ano ^| findstr :9009') do taskkill /F /PID %a` and on Linux `lsof -ti:9009 | xargs kill -9`. When port 9009 is free, the netstat/lsof output is empty, the kill command exits with status 1, and startup aborts instead of continuing.

Workarounds that worked in the thread:

1. **Occupy port 9009 first (Windows).** Start any simple Node server that listens on TCP 9009 before launching `npx @browsermcp/mcp@latest`; the startup routine finds a PID, kills it, and proceeds.

2. **Patch the installed `dist/index.js` (Linux Mint).** In `~/.npm/_npx/<hash>/node_modules/@browsermcp/mcp/dist/index.js`, guard the cleanup so it only runs when the port is in use (source is `src/ws.ts` in the repo):

```js
async function createWebSocketServer(port = mcpConfig.defaultWsPort) {
  if (await isPortInUse(port)) {
    killProcessOnPort(port);
    while (await isPortInUse(port)) {
      await wait(100);
    }
  }
  return new WebSocketServer({ port });
}
```

As general advice: this patch lives in the npx cache and is lost when the package is re-fetched, so you may need to reapply it after updates. One commenter also suggested reinstalling Node.js/npm, but did not report a result. Another shimmed `lsof` on Linux to return the expected output; details were not given.

Security note: startup force-kills whatever process is listening on port 9009, so don't launch it while an unrelated service is bound to that port.

AgentArmor team · Answer

**What @reas24 tried on GitHub** (Worked)

Started a simple Node server listening on port 9009 outside VS Code, then reopened VS Code so BrowserMCP's startup port-kill finds a PID to kill instead of failing.

Source: https://github.com/BrowserMCP/mcp/issues/14#issuecomment-2838597296

AgentArmor team · Answer

**What @furas tried on GitHub** (Worked)

Edited the installed `@browsermcp/mcp/dist/index.js` on Linux Mint to wrap the kill/wait logic in `if (await isPortInUse(port))`, so nothing is killed when port 9009 is free.

Source: https://github.com/BrowserMCP/mcp/issues/14#issuecomment-3064466293

AgentArmor team · Answer

**What @eriknelson tried on GitHub** (Outcome unclear)

On Linux, shimmed the `lsof` command to return what it expects so the `lsof -ti:9009 | xargs kill -9` step succeeds; no details were given.

Source: https://github.com/BrowserMCP/mcp/issues/14#issuecomment-2838820223

AgentArmor team · Answer

**What @mydvdf tried on GitHub** (Outcome unclear)

Suggested reinstalling Node.js and npm to resolve the recurring `FOR /F ... taskkill` failure, but did not report whether it fixed anything.

Source: https://github.com/BrowserMCP/mcp/issues/14#issuecomment-2888873019

BrowserMCP fails to start: 'Failed to kill process on port 9009' when port 9009 is free

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BrowserMCP fails to start: 'Failed to kill process on port 9009' when port 9009 is free

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