diff --git a/api/server/router/build/build_routes.go b/api/server/router/build/build_routes.go
index 11062d69e0..d195e3d54c 100644
--- a/api/server/router/build/build_routes.go
+++ b/api/server/router/build/build_routes.go
@@ -1,6 +1,7 @@
 package build // import "github.com/docker/docker/api/server/router/build"
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"encoding/base64"
@@ -192,8 +193,19 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *
 
 	w.Header().Set("Content-Type", "application/json")
 
-	output := ioutils.NewWriteFlusher(w)
+	var output writeCloseFlusher = ioutils.NewWriteFlusher(w)
 	defer output.Close()
+
+	body := r.Body
+	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.
+		// this can be removed when
+		// https://github.com/golang/go/issues/15527
+		// https://github.com/golang/go/issues/22209
+		// has been fixed
+		body, output = wrapOutputBufferedUntilRequestRead(body, output)
+	}
 	errf := func(err error) error {
 		if httputils.BoolValue(r, "q") && notVerboseBuffer.Len() > 0 {
 			output.Write(notVerboseBuffer.Bytes())
@@ -235,7 +247,7 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *
 	wantAux := versions.GreaterThanOrEqualTo(version, "1.30")
 
 	imgID, err := br.backend.Build(ctx, backend.BuildConfig{
-		Source:         r.Body,
+		Source:         body,
 		Options:        buildOptions,
 		ProgressWriter: buildProgressWriter(out, wantAux, createProgressReader),
 	})
@@ -294,3 +306,92 @@ func buildProgressWriter(out io.Writer, wantAux bool, createProgressReader func(
 		ProgressReaderFunc: createProgressReader,
 	}
 }
+
+type writeCloseFlusher interface {
+	Flush()
+	Flushed() bool
+	io.WriteCloser
+}
+
+func wrapOutputBufferedUntilRequestRead(rc io.ReadCloser, out writeCloseFlusher) (io.ReadCloser, writeCloseFlusher) {
+	w := &wcf{
+		buf:               bytes.NewBuffer(nil),
+		writeCloseFlusher: out,
+	}
+	r := bufio.NewReader(rc)
+	_, err := r.Peek(1)
+	if err != nil {
+		return rc, out
+	}
+	rc = &rcNotifier{
+		Reader: r,
+		Closer: rc,
+		notify: w.notify,
+	}
+	return rc, w
+}
+
+type rcNotifier struct {
+	io.Reader
+	io.Closer
+	notify func()
+}
+
+func (r *rcNotifier) Read(b []byte) (int, error) {
+	n, err := r.Reader.Read(b)
+	if err != nil {
+		r.notify()
+	}
+	return n, err
+}
+
+type wcf struct {
+	writeCloseFlusher
+	mu      sync.Mutex
+	ready   bool
+	buf     *bytes.Buffer
+	flushed bool
+}
+
+func (w *wcf) Flush() {
+	w.mu.Lock()
+	w.flushed = true
+	if !w.ready {
+		w.mu.Unlock()
+		return
+	}
+	w.mu.Unlock()
+	w.writeCloseFlusher.Flush()
+}
+
+func (w *wcf) Flushed() bool {
+	w.mu.Lock()
+	b := w.flushed
+	w.mu.Unlock()
+	return b
+}
+
+func (w *wcf) Write(b []byte) (int, error) {
+	w.mu.Lock()
+	if !w.ready {
+		n, err := w.buf.Write(b)
+		w.mu.Unlock()
+		return n, err
+	}
+	w.mu.Unlock()
+	return w.writeCloseFlusher.Write(b)
+}
+
+func (w *wcf) notify() {
+	w.mu.Lock()
+	if !w.ready {
+		if w.buf.Len() > 0 {
+			io.Copy(w.writeCloseFlusher, w.buf)
+		}
+		if w.flushed {
+			w.writeCloseFlusher.Flush()
+		}
+		w.ready = true
+	}
+	w.mu.Unlock()
+}
diff --git a/builder/builder-next/reqbodyhandler.go b/builder/builder-next/reqbodyhandler.go
index 393e0cf33a..d4b5ead883 100644
--- a/builder/builder-next/reqbodyhandler.go
+++ b/builder/builder-next/reqbodyhandler.go
@@ -1,7 +1,6 @@
 package buildkit
 
 import (
-	"bufio"
 	"io"
 	"net/http"
 	"strings"
@@ -28,12 +27,9 @@ func newReqBodyHandler(rt http.RoundTripper) *reqBodyHandler {
 }
 
 func (h *reqBodyHandler) newRequest(rc io.ReadCloser) (string, func()) {
-	// handle expect-continue vs chunked output
-	r := bufio.NewReader(rc)
-	r.Peek(1)
 	id := identity.NewID()
 	h.mu.Lock()
-	h.requests[id] = &readCloser{Reader: r, Closer: rc}
+	h.requests[id] = rc
 	h.mu.Unlock()
 	return "http://" + urlPrefix + id, func() {
 		h.mu.Lock()
@@ -58,12 +54,14 @@ func (h *reqBodyHandler) RoundTrip(req *http.Request) (*http.Response, error) {
 			return nil, errors.Errorf("context not found")
 		}
 
-		return &http.Response{
+		resp := &http.Response{
 			Status:        "200 OK",
 			StatusCode:    200,
 			Body:          rc,
 			ContentLength: -1,
-		}, nil
+		}
+
+		return resp, nil
 	}
 	return h.rt.RoundTrip(req)
 }