gofiber/fiber · error

failed to close multipart writer: %w

Error message

failed to close multipart writer: %w

What it means

Returned in parserRequestBodyFile (client/hooks.go:250) when multipart.Writer.Close fails. Close writes the trailing boundary marker that terminates a multipart body; if the underlying writer (req.RawRequest.BodyWriter()) errors while flushing that marker, the body is incomplete and the error is surfaced rather than sending a malformed request.

Source

Thrown at client/hooks.go:250

	return nil
}

// parserRequestBodyFile handles the case where the request contains files to be uploaded.
func parserRequestBodyFile(req *Request) error {
	mw := multipart.NewWriter(req.RawRequest.BodyWriter())
	if err := mw.SetBoundary(req.boundary); err != nil {
		return fmt.Errorf("set boundary error: %w", err)
	}

	if err := writeMultipartBody(mw, req); err != nil {
		mw.Close() //nolint:errcheck // the body write already failed; surface that error instead
		return err
	}

	// Close writes the trailing boundary; if it fails the multipart body is
	// incomplete, so surface the error instead of sending a malformed request.
	if err := mw.Close(); err != nil {
		return fmt.Errorf("failed to close multipart writer: %w", err)
	}

	return nil
}

// writeMultipartBody writes the form fields and files of req to mw.
func writeMultipartBody(mw *multipart.Writer, req *Request) error {
	// Add form data.
	var err error
	for key, value := range req.formData.All() {
		err = mw.WriteField(utils.UnsafeString(key), utils.UnsafeString(value))
		if err != nil {
			break
		}
	}
	if err != nil {
		return fmt.Errorf("write formdata error: %w", err)
	}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Verify the Request is not being concurrently released/reused while the upload is being built.
2. Check whether the total multipart body size exceeds any configured max body size and raise it or stream instead.
3. Retry the request — if the underlying cause was a transient transport teardown, a fresh request may succeed.
4. Inspect the wrapped error to distinguish a size-limit failure from a connection-level failure.

Example fix

// before — reusing/releasing the request concurrently
resp, err := core.execute(ctx, client, req)
ReleaseRequest(req) // may race the close

// after — keep the request alive until execute returns, then release
resp, err := core.execute(ctx, client, req)
if resp != nil { ReleaseResponse(resp) }
ReleaseRequest(req)

resp, err := core.execute(ctx, client, req)
if err != nil && strings.Contains(err.Error(), "failed to close multipart writer") {
    // likely a transient buffer/transport issue — rebuild and retry once
    req = rebuildUploadRequest()
    resp, err = core.execute(ctx, client, req)
}

When it happens

Trigger: The request's body writer returns an error while the closing boundary is written — e.g. an in-memory buffer that hit a size cap, a streaming body destination whose connection was already torn down, or a fasthttp request body buffer overflow. Occurs only on multipart file-upload requests.

Common situations: Uploading a very large multipart body that exceeds an internal buffer/size limit; a body-stream target that was closed early; concurrent release/reuse of the Request mid-upload; a transport reset between writing files and closing the writer.

Related errors

• write formdata error: %w
• create file error: %w
• file: file header is nil
• file: failed to read file
• rand.Read failed: %w

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/fd9b61a6587423ae.json. Report an issue: GitHub.