angular/angular-cli · error · Error

Watch mode execution (serve target or watch option) is not y

Error message

Watch mode execution (serve target or watch option) is not yet supported by 'run_target'. Please use the legacy 'devserver_start' / 'devserver_wait_for_build' tools instead.

What it means

The generic run_target strategy refuses to execute targets that require watch mode: the 'serve' target or any target invoked with watch: true. Watch mode is a long-running process the run_target tool does not support yet, so it throws and points users to the legacy devserver_start / devserver_wait_for_build MCP tools.

Source

Thrown at packages/angular/cli/src/commands/mcp/tools/run-target/generic-target-strategy.ts:35

  'test',
  'e2e',
  'serve',
  'deploy',
  'extract-i18n',
  'lint',
]);

export class GenericTargetStrategy implements TargetStrategy {
  canHandle(targetName: string, builder?: string): boolean {
    return true; // Universal fallback strategy
  }

  async execute(
    input: StrategyExecutionContext,
    context: McpToolContext,
  ): Promise<RunTargetOutput> {
    if (input.targetName === 'serve' || input.options?.['watch'] === true) {
      throw new Error(
        `Watch mode execution (serve target or watch option) is not yet supported by 'run_target'. ` +
          `Please use the legacy 'devserver_start' / 'devserver_wait_for_build' tools instead.`,
      );
    }

    const args: string[] = [];
    if (BUILT_IN_COMMANDS.has(input.targetName)) {
      args.push(input.targetName, input.projectName);
    } else {
      args.push('run', `${input.projectName}:${input.targetName}`);
    }

    if (input.configuration) {
      args.push(`--configuration=${input.configuration}`);
    }

    let options = input.options;
    if (input.targetName === 'test') {

View on GitHub (pinned to bb72145f9a)

Solutions

1. Use the legacy devserver_start tool followed by devserver_wait_for_build for serve/watch scenarios
2. For run_target, drop options.watch (set it to false) and target a non-serve target such as build or test
3. Restrict the agent/tooling to call run_target only for one-shot builds and fall back to devserver tools for interactive serving

Example fix

// before
runTarget({ target: 'serve' });
// after
devserverStart({});
const result = await devserverWaitForBuild({});

if (input.targetName === 'serve' || input.options?.['watch'] === true) {
  // route to legacy tools instead
  devserverStart();
}

function isWatchable(input: { targetName: string; options?: Record<string, unknown> }): boolean {
  return input.targetName === 'serve' || input.options?.['watch'] === true;
}

try {
  await runTarget(input);
} catch (e) {
  if ((e as Error).message.includes("not yet supported by 'run_target'")) {
    devserverStart();
    await devserverWaitForBuild();
  } else throw e;
}

When it happens

Trigger: Calling the run_target MCP tool with targetName === 'serve', or with options containing watch: true (e.g. { target: 'serve' } or { target: 'build', options: { watch: true } }) — the guard in execute() at generic-target-strategy.ts:35 throws.

Common situations: Migrating an agent workflow that previously used devserver_start to the new run_target tool; passing watch: true inherited from an angular.json serve configuration; assuming run_target supports hot-reload dev servers.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Workspace path is outside the allowed MCP roots: ${workspace
• Workspace path is outside the allowed MCP roots: ${workspace
• The current directory resolves to a workspace outside the al
• Failed to update roots on notification: ${e instanceof Error
• Failed to initialize roots on connection: ${e instanceof Err

AI-assisted analysis of angular/angular-cli@bb72145f9a (2026-08-30). Data as JSON: /api/errors/429e3c13c6dbf76c. Report an issue: GitHub.