valyala/fasthttp · error

fasthttp: error when reading response headers

Error message

fasthttp: error when reading response headers

What it means

ErrReadingResponseHeaders indicates fasthttp failed while parsing an HTTP response's header section from the wire — malformed start line, invalid header syntax, or truncated input. It is an exported sentinel from header.go used (possibly wrapped) when header parsing errors occur.

Source

Thrown at header.go:470

// Trailers are only supported with chunked transfer.
// Trailers allow the sender to include additional headers at the end of chunked messages.
//
// The following trailers are forbidden:
// 1. necessary for message framing (e.g., Transfer-Encoding and Content-Length),
// 2. routing (e.g., Host),
// 3. request modifiers (e.g., controls and conditionals in Section 5 of [RFC7231]),
// 4. authentication (e.g., see [RFC7235] and [RFC6265]),
// 5. response control data (e.g., see Section 7.1 of [RFC7231]),
// 6. determining how to process the payload (e.g., Content-Encoding, Content-Type, Content-Range, and Trailer)
//
// Return ErrBadTrailer if contain any forbidden trailers.
func (h *header) AddTrailer(trailer string) error {
	return h.AddTrailerBytes(s2b(trailer))
}

var (
	ErrBadTrailer                    = errors.New("fasthttp: contain forbidden trailer")
	ErrReadingResponseHeaders        = errors.New("fasthttp: error when reading response headers")
	ErrReadingResponseTrailer        = errors.New("fasthttp: error when reading response trailer")
	ErrResponseFirstLineMissingSpace = errors.New("fasthttp: cannot find whitespace in the first line of response")
	ErrUnexpectedStatusCodeChar      = errors.New("fasthttp: unexpected char at the end of status code")
	ErrMissingRequestMethod          = errors.New("fasthttp: cannot find http request method")
	ErrUnsupportedRequestMethod      = errors.New("fasthttp: unsupported http request method")
	ErrExtraWhitespaceInRequestLine  = errors.New("fasthttp: extra whitespace in request line")
	ErrEmptyRequestURI               = errors.New("fasthttp: requesturi cannot be empty")
	ErrDuplicateContentLength        = errors.New("fasthttp: duplicate content-length header")
	ErrUnsupportedTransferEncoding   = errors.New("fasthttp: unsupported transfer-encoding")
	ErrNonNumericChars               = errors.New("fasthttp: non-numeric chars found")
	ErrNeedMore                      = errors.New("fasthttp: need more data: cannot find trailing lf")
	ErrSmallReadBuffer               = errors.New("fasthttp: small read buffer. increase readbuffersize")
)

// AddTrailerBytes add Trailer header value for chunked response
// to indicate which headers will be sent after the body.
//
// Use Set to set the trailer header later.

View on GitHub (pinned to c96f600972)

Solutions

  1. Verify the endpoint scheme/port (https:// vs http://) matches the server
  2. Inspect the raw response (curl -v) to see what the server actually sends
  3. Check for intercepting proxies/VPN/CDN mangling responses and set proper Proxy settings
  4. Increase ReadTimeout/retry, or upgrade fasthttp if parsing a valid but unusual header fails
Defensive patterns

Strategy: try-catch

Validate before calling

// verify endpoint scheme/port before requesting
u, _ := url.Parse(target)
if u.Scheme == "https" && !clientTLS { return errors.New("client not configured for TLS") }

Try / catch

err := client.Do(req, resp)
if err != nil {
    if errors.Is(err, fasthttp.ErrReadingResponseHeaders) {
        // log raw bytes, check scheme/proxy, retry once
    }
}

Prevention

When it happens

Trigger: Client calls (Do/DoTimeout/DoRedirect) receiving a response whose header bytes are malformed: missing space in status line, non-ASCII/garbage bytes, connection closed mid-headers, or a non-HTTP proxy response.

Common situations: Target server is actually speaking TLS to a plain-HTTP client (or vice versa); intermediate proxies/CDNs returning HTML error pages; UDP/TCP misrouting; server sending HTTP/2 to an HTTP/1.1-only client.

Related errors

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/b26b88489ce03bfe. Report an issue: GitHub.