multer throws "File too large" error if file size of exact "limits.fileSize" bytes provided.

[ho94949]

Multer throws "File too large" error if file size of exat "limits.fileSize" bytes provided. It is non-intuitive behavior. (Or it might be a corner-case.)

It seems that busboy emits event "limit" when limits.fileSize bytes is hit. However, this means that busboy will not further process files, and it might be truncated.

multer/lib/make-middleware.js

Lines 153 to 156 in 2e07e77

	fileStream.on('limit', function () {
	aborting = true
	abortWithCode('LIMIT_FILE_SIZE', fieldname)
	})

Relevant lines in busboy: https://github.com/mscdex/busboy/blob/6b3dcf69d38c1a8d53a0b3e4c88ba296f6c91525/lib/types/multipart.js#L476-L482

Current Workaround

increase limits.fileSize by 1.

Environment information

Version:
To find the installed version of a package, you can check the package.json file in the root directory of your project. The version will be listed under dependencies or devDependencies, like this:

"dependencies": {
    // ...
    "express": "^5.1.0",
    "multer": "^2.0.2",
    // ...
}

Platform:
Linux NX-281041 6.6.87.2-microsoft-standard-WSL2 #1 SMP PREEMPT_DYNAMIC Thu Jun 5 18:30:46 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

Node.js version:
v22.16.0

[mahmoodhamdi]

This is a busboy behavior - it fires the 'limit' event when the stream reaches exactly the configured limit. From busboy's perspective, it can't tell whether the file is exactly that size or larger, so it treats it as hitting the limit.

One possible fix at the multer level would be to track bytes consumed and only treat it as an error if the actual bytes written exceed the limit:

// In make-middleware.js, instead of immediately rejecting on 'limit':
file.stream.on('limit', function () {
  // Only reject if we actually exceeded the limit
  // (busboy fires 'limit' at exactly fileSize bytes too)
  pendingWrites.push(
    new Promise((resolve) => {
      file.stream.on('end', () => {
        // Check if more data was available after the limit
        if (file.stream.truncated) {
          // File was actually larger than the limit
          reject(new MulterError('LIMIT_FILE_SIZE', fieldname))
        }
        resolve()
      })
    })
  )
})

Though this might need more thought around whether busboy's truncated flag is reliable for this check.