valyala/fasthttp · error

fasthttp: unexpected char at the end of status code

Error message

fasthttp: unexpected char at the end of status code

What it means

While parsing a response status line, fasthttp reads the 3-digit status code and expects it to be followed by a space, CR/LF, or end of line. If an unexpected character appears where the status code should end, ErrUnexpectedStatusCodeChar is returned. It indicates the peer sent a syntactically invalid status line.

Solutions

1. Log the raw first line (wrap the conn or use curl -v against the same URL) to see what the peer actually returns.
2. Fix the upstream server/proxy to emit RFC-compliant status lines like 'HTTP/1.1 200 OK'.
3. Ensure the https:// scheme and TLSClientConfig are set for TLS servers so ciphertext is not parsed as plain HTTP.
4. Retry against a healthy endpoint; fasthttp closes the connection after this parse error so the pooled conn is discarded.

Example fix

// before
req.SetRequestURI("http://svc.internal:8443/x") // TLS server, plain HTTP request
// after
req.SetRequestURI("https://svc.internal:8443/x")
client.TLSConfig = &tls.Config{ InsecureSkipVerify: true } // trust setup as needed

// Probe the raw status line once at startup:
// curl -sv URL | head -1 must match '^HTTP/1\.[01] [0-9]{3} '

func isBadStatusLine(err error) bool {
    return err == fasthttp.ErrUnexpectedStatusCodeChar
}

err := client.Do(req, resp)
if err == fasthttp.ErrUnexpectedStatusCodeChar {
    client.CloseIdleConnections() // discard possibly-corrupted pooled conns
    return retryable(fmt.Errorf("malformed status line: %w", err))
}

When it happens

Trigger: Response.Read / Client.Do / HostClient.Do on a response whose first line is like 'HTTP/1.1 20x OK' or 'HTTP/1.1 1234abc' — any non-digit/non-separator char after the status digits (see fasthttputil/ParseUint usage in parseFirstLine).

Common situations: Talking to a misbehaving or nonconformant server/proxy, an adversarial or fuzzed peer, or a port returning proprietary protocol data instead of HTTP. Also common when TLS is missing so raw handshake bytes are parsed as HTTP.

Related errors

• fasthttp: cannot find whitespace in the first line of…
• cannot find whitespace in the first line of response
• error when reading request headers
• fasthttp: error when reading response headers
• invalid response status code

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/c43d9d2b259a26a8. Report an issue: GitHub.

Appendix: source

Thrown at header.go:473

// 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.
//
// Trailers are only supported with chunked transfer.
// Trailers allow the sender to include additional headers at the end of chunked messages.

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