What causes "cannot parse content-length: %w" in valyala/fasthttp?

Send a plain decimal integer as Content-Length (e.g. 'Content-Length: 42').

cannot parse content-length: %w

Error message

cannot parse content-length: %w

What it means

parseContentLength failed because parseUintBuf could not read any valid unsigned integer from the Content-Length bytes (e.g. empty or starting with a non-digit). The underlying parse error is wrapped with this message.

Source

Thrown at header.go:3345

	for i, n := 0, len(h.h); i < n; i++ {
		kv := &h.h[i]
		if caseInsensitiveCompare(kv.key, strCookie) {
			h.cookies = parseRequestCookies(h.cookies, kv.value)
			tmp := *kv
			copy(h.h[i:], h.h[i+1:])
			n--
			i--
			h.h[n] = tmp
			h.h = h.h[:n]
		}
	}
	h.cookiesCollected = true
}

func parseContentLength(b []byte) (int, error) {
	v, n, err := parseUintBuf(b)
	if err != nil {
		return -1, fmt.Errorf("cannot parse content-length: %w", err)
	}
	if n != len(b) {
		return -1, fmt.Errorf("cannot parse content-length: %w", ErrNonNumericChars)
	}
	return v, nil
}

type headerValueScanner struct {
	b     []byte
	value []byte
}

func (s *headerValueScanner) next() bool {
	b := s.b
	if len(b) == 0 {
		return false
	}
	before, after, ok := bytes.Cut(b, []byte{','})

View on GitHub (pinned to c96f600972)

Solutions

Send a plain decimal integer as Content-Length (e.g. 'Content-Length: 42').
Use chunked transfer-encoding instead if the length is unknown.
Fix the client/proxy that mangles the Content-Length value.
Log the raw header at an early middleware to identify the offending producer.

Example fix

// before
req.Header.Set("Content-Length", "12 bytes")
// after
req.Header.Set("Content-Length", strconv.Itoa(len(body)))

Defensive patterns

Strategy: validation

Validate before calling

func validContentLength(cl string) bool {
    if cl == "" { return false }
    for i := 0; i < len(cl); i++ { if cl[i] < '0' || cl[i] > '9' { return false } }
    return true
}

Try / catch

_, err := parseContentLength(b)
if err != nil && strings.Contains(err.Error(), "cannot parse content-length") {
    // return 400 Bad Request with framing error
}

Prevention

Set Content-Length programmatically via strconv.Itoa(len(body))
Never hard-code Content-Length strings
Prefer chunked encoding when length is unknown

When it happens

Trigger: A Content-Length header whose value is empty or begins with a non-numeric character, e.g. 'Content-Length: ' or 'Content-Length: abc' or 'Content-Length: +12'.

Common situations: Hand-crafted or scripted HTTP requests with bad framing; proxy rewriting bugs truncating the value; fuzzing/scanning tools sending garbage framing headers.

Related errors

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/e12297db1903156b. Report an issue: GitHub.