gofiber/fiber · error

set boundary error

Error message

set boundary error: %w

What it means

parserRequestBodyFile creates a multipart.Writer over the request's body writer and immediately calls SetBoundary with req.boundary. This error wraps mime/multipart's rejection: the boundary string contains invalid characters or is too short/long (RFC 2046 limits it to 1–70 chars of allowed runes).

Solutions

1. Use a boundary made only of letters, digits, and '-', 1–70 characters long.
2. Let the client auto-generate the boundary (omit SetBoundary) unless you have a specific reason.
3. Validate the boundary with a regex like ^[A-Za-z0-9'()+_,\-./:=?]{1,70}$ before setting it.

Example fix

// before
req.SetBoundary("my boundary with spaces")

// after
req.SetBoundary("GoFiberBoundary7MA4YWxk")

var boundaryRE = regexp.MustCompile(`^[A-Za-z0-9'()+_,\-./:=?]{1,70}$`)
if !boundaryRE.MatchString(b) {
    return fmt.Errorf("invalid multipart boundary: %q", b)
}
req.SetBoundary(b)

When it happens

Trigger: Calling req.SetBoundary with a custom string containing whitespace, control chars, special RFC characters, or exceeding 70 characters; a boundary that begins/ends with spaces.

Common situations: Hard-coding a boundary copied from a browser/cURL capture that includes quotes or whitespace; programmatically building a boundary from user input without sanitizing; a copy-paste that includes trailing newlines.

Related errors

• boundary generation
• failed to close multipart writer
• file: file header is nil
• rand.Read failed
• the file should have a name

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/9303257bd3c31df9. Report an issue: GitHub.

Appendix: source

Thrown at client/hooks.go:239

		if body, ok := req.body.([]byte); ok { //nolint:revive // ignore simplicity
			req.RawRequest.SetBody(body)
		} else {
			return ErrBodyType
		}
	case noBody:
		// No body to set.
		return nil
	default:
		return ErrBodyTypeNotSupported
	}
	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 {

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