What causes "Dev server already started." in angular/angular-cli?

Call devserver.stop() (or the devserver-stop MCP tool) before starting again.

angular/angular-cli · error · Error

Dev server already started.

Error message

Dev server already started.

What it means

The MCP devserver manager in @angular/cli throws this plain Error from start() when a Devserver instance already has a child process running. It guards against launching a second `ng serve` process from the same Devserver object, since each instance owns exactly one dev-server process.

Source

Thrown at packages/angular/cli/src/commands/mcp/devserver.ts:111

    host,
    port,
    workspacePath,
    project,
  }: {
    host: Host;
    port: number;
    workspacePath: string;
    project: string;
  }) {
    this.host = host;
    this.port = port;
    this.workspacePath = workspacePath;
    this.project = project;
  }

  start() {
    if (this.devserverProcess) {
      throw Error('Dev server already started.');
    }

    const args = ['serve'];
    if (this.project) {
      args.push(this.project);
    }

    args.push(`--port=${this.port}`);

    this.devserverProcess = this.host.startNgProcess(args, {
      stdio: 'pipe',
      cwd: this.workspacePath,
    });
    processStreamLines(this.devserverProcess.stdout, (line) => this.addLog(line));
    processStreamLines(this.devserverProcess.stderr, (line) => this.addLog(line));

    this.devserverProcess.on('close', () => {
      this.stop();

View on GitHub (pinned to bb72145f9a)

Solutions

Call devserver.stop() (or the devserver-stop MCP tool) before starting again.
Check whether a server is already running (list current devservers / track the instance) and reuse it instead of calling start().
Wrap start() in a check or try-catch and treat this message as an idempotent 'already running' signal.
Create a fresh Devserver instance for a different workspace/project instead of reusing the existing one.

Example fix

// before
const server = new Devserver({ workspacePath, project });
server.start();
server.start(); // throws 'Dev server already started.'
// after
const server = new Devserver({ workspacePath, project });
if (!server.isRunning()) server.start();

Defensive patterns

Strategy: try-catch

Validate before calling

if (!server.isRunning?.()) {
  server.start();
}

Type guard

function isAlreadyStartedError(err: unknown): err is Error {
  return err instanceof Error && err.message === 'Dev server already started.';
}

Try / catch

try {
  server.start();
} catch (err) {
  if (!isAlreadyStartedError(err)) throw err;
  // already running: reuse the existing devserver
}

Prevention

Track whether a Devserver instance was already started before invoking start().
Treat devserver-start as idempotent: on this error, reuse the running server.
Always pair start with stop in teardown logic.
Use the MCP devserver-stop tool before re-running devserver-start.

When it happens

Trigger: Calling Devserver.start() twice on the same instance without calling stop() in between — e.g. the MCP client invokes the devserver-start tool twice, or start() is retried after a first successful launch while this.devserverProcess is still set.

Common situations: An MCP agent re-issues a devserver-start tool call because it assumed the previous one failed; automation scripts starting the server on each request; forgetting that the server persists across tool calls within one MCP session.

Related errors

AI-assisted analysis of angular/angular-cli@bb72145f9a (2026-08-30). Data as JSON: /api/errors/3e831a8d5de82db9. Report an issue: GitHub.