Prettier experimental-cli parallel mode returns undefined rejection reason

[nicksrandall]

What version of Bun is running?

1.3.3

What platform is your computer?

Darwin arm64 (macOS 26.2)

What steps can reproduce the bug?

1. Create a directory with some TypeScript/JavaScript files
2. Run prettier's experimental CLI with the --bun flag in parallel mode (default):

bunx --bun prettier --experimental-cli --check .

What is the expected behavior?

Prettier should check all files and report formatting status.

What do you see instead?

A TypeError crash in prettier's experimental CLI:

12466 |     } else {
12467 |       const error = fileResult.reason;
12468 |       if (error.name === "UndefinedParserError") {
                  ^
TypeError: undefined is not an object (evaluating 'error.name')
      at runGlobs (/path/to/node_modules/.bun/[email protected]/node_modules/prettier/internal/experimental-cli.mjs:12468:11)

The issue is that fileResult.reason is undefined when it should be an Error object.

Workarounds that confirm this is Bun-specific

All of these work correctly:

Command	Result
bunx --bun prettier --experimental-cli --check .	❌ Crashes
bunx prettier --experimental-cli --check . (Node runtime)	✅ Works
bunx --bun prettier --experimental-cli --check . --no-parallel	✅ Works
bunx --bun prettier --check . (regular CLI)	✅ Works

Analysis

The experimental CLI uses Promise.allSettled() for parallel file processing. When a promise is rejected, the reason property should contain the error. Under Bun's runtime, some rejected promises have reason: undefined instead of the actual error object.

This suggests a bug in how Bun handles Promise.allSettled() with rejected promises in worker threads or parallel execution contexts.

Additional information

• Prettier version: 3.7.3
• Node version (for comparison): v22.19.0
• The bug is reproducible consistently

[github-actions[bot]]

Found 1 possible duplicate issue:

1. https://github.com/oven-sh/bun/issues/21901

This issue will be automatically closed as a duplicate in 3 days.

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