valyala/fasthttp · error

fasthttp: request content-type has bad boundary or is not mu

Error message

fasthttp: request content-type has bad boundary or is not multipart/form-data

What it means

Request.MultipartForm() parses the body as multipart/form-data, which requires a Content-Type of multipart/form-data with a valid boundary parameter. If the Content-Type is missing, is another type, or the boundary is bad, fasthttp returns ErrNoMultipartForm.

Source

Thrown at http.go:1120

	req.parsePostArgs()
	return &req.postArgs
}

func (req *Request) parsePostArgs() {
	if req.parsedPostArgs {
		return
	}
	req.parsedPostArgs = true

	if !bytes.HasPrefix(req.Header.ContentType(), strPostArgsContentType) {
		return
	}
	req.postArgs.ParseBytes(req.bodyBytes())
}

// ErrNoMultipartForm means that the request's Content-Type
// isn't 'multipart/form-data'.
var ErrNoMultipartForm = errors.New("fasthttp: request content-type has bad boundary or is not multipart/form-data")

// MultipartForm returns request's multipart form.
//
// Returns ErrNoMultipartForm if request's Content-Type
// isn't 'multipart/form-data'.
//
// This method is equivalent to MultipartFormWithLimit(0), i.e. no body size
// limit is applied during multipart parsing.
//
// RemoveMultipartFormFiles must be called after returned multipart form
// is processed.
func (req *Request) MultipartForm() (*multipart.Form, error) {
	return req.MultipartFormWithLimit(0)
}

// MultipartFormWithLimit returns request's multipart form and limits the
// read multipart body size to maxBodySize bytes.
//

View on GitHub (pinned to c96f600972)

Solutions

1. Check req.Header.ContentType() (or strings.HasPrefix) for multipart/form-data before calling MultipartForm, and handle urlencoded via req.PostArgs otherwise.
2. Fix the client/form to send enctype="multipart/form-data" with a proper boundary.
3. For non-multipart uploads, read the body directly (req.Body()) instead of forcing multipart parsing.
4. Reject unsupported content types with 415 Unsupported Media Type in your handler.

Example fix

// before
form, err := req.MultipartForm()
// after
if !strings.HasPrefix(string(req.Header.ContentType()), "multipart/form-data") {
    // handle urlencoded/plain body instead
    req.PostArgs()
    return
}
form, err := req.MultipartForm()

mediaType, params, _ := mime.ParseMediaType(string(req.Header.ContentType()))
isMultipart := mediaType == "multipart/form-data" && params["boundary"] != ""

form, err := req.MultipartForm()
if errors.Is(err, fasthttp.ErrNoMultipartForm) {
    // respond 415 or parse urlencoded body instead
}

When it happens

Trigger: Calling req.MultipartForm() when the request Content-Type isn't 'multipart/form-data; boundary=...' — e.g. a JSON POST, application/x-www-form-urlencoded POST, or a multipart body with a malformed/missing boundary.

Common situations: Endpoint assumed multipart but clients send JSON; file-upload forms missing the enctype='multipart/form-data' attribute; hand-rolled multipart bodies with wrong boundary; callers not checking IsPost/Content-Type first.

Related errors

• form boundary cannot be empty
• fasthttp: contain forbidden trailer
• fasthttp: error when reading response headers
• fasthttp: error when reading response trailer
• fasthttp: cannot find whitespace in the first line of respon

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/c49784b61aa7db2f. Report an issue: GitHub.