@@ -1285,9 +1285,6 @@ func (s *Server) postBuild(eng *engine.Engine, version version.Version, w http.R

 		}

 	}

-	stdout := engine.NewOutput()

-	stdout.Set(utils.NewWriteFlusher(w))

-

 	if version.GreaterThanOrEqualTo("1.8") {

 		w.Header().Set("Content-Type", "application/json")

 		buildConfig.JSONFormat = true

@@ -1304,7 +1301,8 @@ func (s *Server) postBuild(eng *engine.Engine, version version.Version, w http.R

 		buildConfig.Pull = true

 	}

-	buildConfig.Stdout = stdout

+	output := utils.NewWriteFlusher(w)

+	buildConfig.Stdout = output

 	buildConfig.Context = r.Body

 	buildConfig.RemoteURL = r.FormValue("remote")

@@ -1336,7 +1334,9 @@ func (s *Server) postBuild(eng *engine.Engine, version version.Version, w http.R

 	}

 	if err := builder.Build(s.daemon, eng, buildConfig); err != nil {

-		if !stdout.Used() {

+		// 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 interal error.

+		if !output.Flushed() {

 			return err

 		}

 		sf := streamformatter.NewStreamFormatter(version.GreaterThanOrEqualTo("1.8"))

@@ -128,12 +128,14 @@ type WriteFlusher struct {

 	sync.Mutex

 	w       io.Writer

 	flusher http.Flusher

+	flushed bool

 }

 func (wf *WriteFlusher) Write(b []byte) (n int, err error) {

 	wf.Lock()

 	defer wf.Unlock()

 	n, err = wf.w.Write(b)

+	wf.flushed = true

 	wf.flusher.Flush()

 	return n, err

 }

@@ -142,9 +144,16 @@ func (wf *WriteFlusher) Write(b []byte) (n int, err error) {

 func (wf *WriteFlusher) Flush() {

 	wf.Lock()

 	defer wf.Unlock()

+	wf.flushed = true

 	wf.flusher.Flush()

 }

+func (wf *WriteFlusher) Flushed() bool {

+	wf.Lock()

+	defer wf.Unlock()

+	return wf.flushed

+}

+

 func NewWriteFlusher(w io.Writer) *WriteFlusher {

 	var flusher http.Flusher

 	if f, ok := w.(http.Flusher); ok {