slimtoolkit/slim · error

fcgi: invalid header version

Error message

fcgi: invalid header version

What it means

The internal FastCGI client reads response records whose header carries a protocol version byte; per the FastCGI spec it must be FCGI_VERSION_1. Any other version byte means the stream is not a valid FastCGI response, so the reader rejects it.

Source

Thrown at pkg/app/master/probe/http/internal/client.go:159

func (h *header) init(recType uint8, reqID uint16, contentLength int) {
	h.Version = 1
	h.Type = recType
	h.ID = reqID
	h.ContentLength = uint16(contentLength)
	h.PaddingLength = uint8(-contentLength & 7)
}

type record struct {
	h    header
	rbuf []byte
}

func (rec *record) read(r io.Reader) (buf []byte, err error) {
	if err = binary.Read(r, binary.BigEndian, &rec.h); err != nil {
		return
	}
	if rec.h.Version != 1 {
		err = errors.New("fcgi: invalid header version")
		return
	}
	if rec.h.Type == EndRequest {
		err = io.EOF
		return
	}
	n := int(rec.h.ContentLength) + int(rec.h.PaddingLength)
	if len(rec.rbuf) < n {
		rec.rbuf = make([]byte, n)
	}
	if _, err = io.ReadFull(r, rec.rbuf[:n]); err != nil {
		return
	}
	buf = rec.rbuf[:int(rec.h.ContentLength)]

	return
}

View on GitHub (pinned to 81940d17fa)

Solutions

1. Confirm the target address/port speaks FastCGI (e.g. php-fpm on 9000), not HTTP
2. Check what the server actually returned (error page, redirect, TLS handshake bytes)
3. Fix the probe URL/scheme so it doesn't hit an HTTP-only endpoint

Example fix

// before
probeURL = "http://svc:80/status"   // plain HTTP
// after
probeURL = "fcgi://svc:9000/status" // FastCGI endpoint

// verify the endpoint speaks FastCGI before probing
u, _ := url.Parse(probeURL)
if u.Scheme != "fcgi" { return fmt.Errorf("probe target must be fcgi://, got %s", u.Scheme) }

body, err := client.Read()
if err != nil && strings.Contains(err.Error(), "invalid header version") {
  return fmt.Errorf("endpoint did not return FastCGI records; check scheme/port: %w", err)
}

When it happens

Trigger: Reading a record whose header version byte != 1 — typically because the endpoint returned plain HTTP/HTML (e.g. an error page) or a server speaking a different/incompatible protocol instead of FastCGI.

Common situations: Pointing the probe at a non-FastCGI port; a reverse proxy or web server responding with an HTTP error page; garbage/corrupted response bytes.

Related errors

• building environment: %v
• dialing backend: %v
• setting read timeout: %v
• setting write timeout: %v
• invalid ws proto - %s

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/c150e45d6675b336. Report an issue: GitHub.