diff --git a/api/server/router/build/build_routes.go b/api/server/router/build/build_routes.go
index 775ed55d7e..2d73e9b1f3 100644
--- a/api/server/router/build/build_routes.go
+++ b/api/server/router/build/build_routes.go
@@ -193,10 +193,8 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *
 
 	w.Header().Set("Content-Type", "application/json")
 
-	var output writeCloseFlusher = ioutils.NewWriteFlusher(w)
-	defer output.Close()
-
 	body := r.Body
+	var ww io.Writer = w
 	if body != nil {
 		// there is a possibility that output is written before request body
 		// has been fully read so we need to protect against it.
@@ -204,18 +202,25 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *
 		// https://github.com/golang/go/issues/15527
 		// https://github.com/golang/go/issues/22209
 		// has been fixed
-		body, output = wrapOutputBufferedUntilRequestRead(body, output)
+		body, ww = wrapOutputBufferedUntilRequestRead(body, ww)
 	}
+
+	output := ioutils.NewWriteFlusher(ww)
+	defer output.Close()
+
 	errf := func(err error) error {
+
 		if httputils.BoolValue(r, "q") && notVerboseBuffer.Len() > 0 {
 			output.Write(notVerboseBuffer.Bytes())
 		}
+
+		logrus.Debugf("isflushed %v", output.Flushed())
 		// Do not write the error in the http output if it's still empty.
 		// This prevents from writing a 200(OK) when there is an internal error.
 		if !output.Flushed() {
 			return err
 		}
-		_, err = w.Write(streamformatter.FormatError(err))
+		_, err = output.Write(streamformatter.FormatError(err))
 		if err != nil {
 			logrus.Warnf("could not write error response: %v", err)
 		}
@@ -311,16 +316,20 @@ func buildProgressWriter(out io.Writer, wantAux bool, createProgressReader func(
 	}
 }
 
-type writeCloseFlusher interface {
+type flusher interface {
 	Flush()
-	Flushed() bool
-	io.WriteCloser
 }
 
-func wrapOutputBufferedUntilRequestRead(rc io.ReadCloser, out writeCloseFlusher) (io.ReadCloser, writeCloseFlusher) {
+func wrapOutputBufferedUntilRequestRead(rc io.ReadCloser, out io.Writer) (io.ReadCloser, io.Writer) {
+	var fl flusher = &ioutils.NopFlusher{}
+	if f, ok := out.(flusher); ok {
+		fl = f
+	}
+
 	w := &wcf{
-		buf:               bytes.NewBuffer(nil),
-		writeCloseFlusher: out,
+		buf:     bytes.NewBuffer(nil),
+		Writer:  out,
+		flusher: fl,
 	}
 	r := bufio.NewReader(rc)
 	_, err := r.Peek(1)
@@ -355,7 +364,8 @@ func (r *rcNotifier) Close() error {
 }
 
 type wcf struct {
-	writeCloseFlusher
+	io.Writer
+	flusher
 	mu      sync.Mutex
 	ready   bool
 	buf     *bytes.Buffer
@@ -370,7 +380,7 @@ func (w *wcf) Flush() {
 		return
 	}
 	w.mu.Unlock()
-	w.writeCloseFlusher.Flush()
+	w.flusher.Flush()
 }
 
 func (w *wcf) Flushed() bool {
@@ -388,17 +398,17 @@ func (w *wcf) Write(b []byte) (int, error) {
 		return n, err
 	}
 	w.mu.Unlock()
-	return w.writeCloseFlusher.Write(b)
+	return w.Writer.Write(b)
 }
 
 func (w *wcf) notify() {
 	w.mu.Lock()
 	if !w.ready {
 		if w.buf.Len() > 0 {
-			io.Copy(w.writeCloseFlusher, w.buf)
+			io.Copy(w.Writer, w.buf)
 		}
 		if w.flushed {
-			w.writeCloseFlusher.Flush()
+			w.flusher.Flush()
 		}
 		w.ready = true
 	}