Output directory must not be the project root or contain the

Error message

Output directory must not be the project root or contain the Mastra source directory

What it means

buildExperimentWorker validates the output directory before bundling: it must not be the project root itself, and the Mastra source directory (src/mastra by default, or --dir) must not be inside it. Otherwise the bundler would recursively consume the source it is compiling (and destroy/overwrite files in place), producing a broken or self-clobbering artifact. The build aborts with this error.

Source

Thrown at packages/cli/src/commands/experiment/build.ts:35

}) {
  const rootDir = resolve(root || process.cwd());
  const mastraDir = dir ? (isAbsolute(dir) ? dir : join(rootDir, dir)) : join(rootDir, 'src', 'mastra');
  const outputDirectory = outputDir
    ? isAbsolute(outputDir)
      ? resolve(outputDir)
      : resolve(rootDir, outputDir)
    : join(rootDir, '.mastra', 'experiment-worker');
  const logger = createLogger(debug ?? false);

  try {
    const outputFromRoot = relative(resolve(rootDir), outputDirectory);
    const mastraFromOutput = relative(outputDirectory, resolve(mastraDir));
    if (
      outputFromRoot === '' ||
      mastraFromOutput === '' ||
      (!mastraFromOutput.startsWith('..') && !isAbsolute(mastraFromOutput))
    ) {
      throw new Error('Output directory must not be the project root or contain the Mastra source directory');
    }

    const fs = new FileService();
    const mastraEntryFile = fs.getFirstExistingFile([join(mastraDir, 'index.ts'), join(mastraDir, 'index.js')]);
    const bundler = new ExperimentBundler();
    bundler.__setLogger(logger);
    await bundler.prepare(outputDirectory);
    await bundler.bundle(mastraEntryFile, outputDirectory, { toolsPaths: [], projectRoot: rootDir });
    await bundler.writeArtifactManifest(outputDirectory, pkgJson.version);
    logger.info(`Experiment worker build complete: ${outputDirectory}`);
  } catch (error) {
    const message = error instanceof Error ? error.message : String(error);
    logger.error(`Experiment worker build failed: ${message}`, {
      ...(error instanceof Error ? { stack: error.stack } : {}),
    });
    process.exitCode = 1;
  }
}

View on GitHub (pinned to 75dd419e61)

Solutions

Use the default output directory (.mastra/experiment-worker) by omitting --output.
Pass a dedicated output path outside the source tree, e.g. `--output .mastra/experiment-worker` or `--output dist/experiment-worker`.
If --dir points at a custom mastra folder, ensure the output directory does not contain that folder.
Fix scripts/CI that interpolate an empty or root value into --output.

Example fix

// before
mastra experiment build --output .
// error: Output directory must not be the project root...
// after
mastra experiment build --output .mastra/experiment-worker

Defensive patterns

Strategy: validation

Validate before calling

import { isAbsolute, join, relative, resolve } from 'node:path';
function validateOutputDir(root: string, mastraDir: string, outputDirectory: string) {
  const outputFromRoot = relative(resolve(root), outputDirectory);
  const mastraFromOutput = relative(outputDirectory, resolve(mastraDir));
  if (outputFromRoot === '' || mastraFromOutput === '' || (!mastraFromOutput.startsWith('..') && !isAbsolute(mastraFromOutput)))
    throw new Error('Choose an output dir outside the project root and not containing src/mastra');
}

Type guard

null

Try / catch

try {
  await buildExperimentWorker({ dir, root, outputDir });
} catch (e) {
  if (String(e?.message).includes('must not be the project root')) {
    console.error('Re-run with --output .mastra/experiment-worker');
  } else throw e;
}

Prevention

Omit --output and use the default .mastra/experiment-worker
Never set outputDir to the repo root or any ancestor of src/mastra
Validate interpolated --output values in CI scripts
Keep mastra source under src/mastra and outputs under .mastra or dist

When it happens

Trigger: Running the experiment worker build with `--output` equal to the project root (relative(outputDirectory, mastraDir) === '' case or outputFromRoot === ''), or with an output directory that contains the mastra `dir` (mastraFromOutput does not start with '..' and is not absolute).

Common situations: Passing `--output .` or `--output /path/to/project`; setting outputDir to a parent folder like `--output ./src` which contains src/mastra; a config that computes outputDir = rootDir.

Related errors

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/1319d7c13fe7cefd. Report an issue: GitHub.