@@ -73,7 +73,7 @@ func (b *Backend) Build(ctx context.Context, config backend.BuildConfig) (string

 			return "", err

 		}

 		if config.ProgressWriter.AuxFormatter != nil {

-			if err = config.ProgressWriter.AuxFormatter.Emit(types.BuildResult{ID: imageID}); err != nil {

+			if err = config.ProgressWriter.AuxFormatter.Emit("moby.image.id", types.BuildResult{ID: imageID}); err != nil {

 				return "", err

 			}

 		}

@@ -243,6 +243,10 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

 		return errdefs.InvalidParameter(errors.New("squash is only supported with experimental mode"))

 	}

+	if buildOptions.Version == types.BuilderBuildKit && !br.daemon.HasExperimental() {

+		return errdefs.InvalidParameter(errors.New("buildkit is only supported with experimental mode"))

+	}

+

 	out := io.Writer(output)

 	if buildOptions.SuppressOutput {

 		out = notVerboseBuffer

@@ -255,10 +259,6 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

 		return progress.NewProgressReader(in, progressOutput, r.ContentLength, "Downloading context", buildOptions.RemoteContext)

 	}

-	if buildOptions.Version == types.BuilderBuildKit && !br.daemon.HasExperimental() {

-		return errdefs.InvalidParameter(errors.New("buildkit is only supported with experimental mode"))

-	}

-

 	wantAux := versions.GreaterThanOrEqualTo(version, "1.30")

 	imgID, err := br.backend.Build(ctx, backend.BuildConfig{

@@ -2,7 +2,6 @@ package buildkit

 import (

 	"context"

-	"encoding/json"

 	"io"

 	"strings"

 	"sync"

@@ -14,7 +13,7 @@ import (

 	"github.com/docker/docker/api/types/backend"

 	"github.com/docker/docker/builder"

 	"github.com/docker/docker/daemon/images"

-	"github.com/docker/docker/pkg/jsonmessage"

+	"github.com/docker/docker/pkg/streamformatter"

 	controlapi "github.com/moby/buildkit/api/services/control"

 	"github.com/moby/buildkit/control"

 	"github.com/moby/buildkit/identity"

@@ -228,6 +227,8 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.

 		Session:       opt.Options.SessionID,

 	}

+	aux := streamformatter.AuxFormatter{opt.ProgressWriter.Output}

+

 	eg, ctx := errgroup.WithContext(ctx)

 	eg.Go(func() error {

@@ -240,7 +241,7 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.

 			return errors.Errorf("missing image id")

 		}

 		out.ImageID = id

-		return nil

+		return aux.Emit("moby.image.id", types.BuildResult{ID: id})

 	})

 	ch := make(chan *controlapi.StatusResponse)

@@ -258,25 +259,9 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.

 			if err != nil {

 				return err

 			}

-

-			auxJSONBytes, err := json.Marshal(dt)

-			if err != nil {

+			if err := aux.Emit("moby.buildkit.trace", dt); err != nil {

 				return err

 			}

-			auxJSON := new(json.RawMessage)

-			*auxJSON = auxJSONBytes

-			msgJSON, err := json.Marshal(&jsonmessage.JSONMessage{ID: "moby.buildkit.trace", Aux: auxJSON})

-			if err != nil {

-				return err

-			}

-			msgJSON = append(msgJSON, []byte("\r\n")...)

-			n, err := opt.ProgressWriter.Output.Write(msgJSON)

-			if err != nil {

-				return err

-			}

-			if n != len(msgJSON) {

-				return io.ErrShortWrite

-			}

 		}

 		return nil

 	})

@@ -257,7 +257,7 @@ func emitImageID(aux *streamformatter.AuxFormatter, state *dispatchState) error

 	if aux == nil || state.imageID == "" {

 		return nil

 	}

-	return aux.Emit(types.BuildResult{ID: state.imageID})

+	return aux.Emit("", types.BuildResult{ID: state.imageID})

 }

 func processMetaArg(meta instructions.ArgCommand, shlex *shell.Lex, args *BuildArgs) error {

@@ -139,14 +139,14 @@ type AuxFormatter struct {

 }

 // Emit emits the given interface as an aux progress message

-func (sf *AuxFormatter) Emit(aux interface{}) error {

+func (sf *AuxFormatter) Emit(id string, aux interface{}) error {

 	auxJSONBytes, err := json.Marshal(aux)

 	if err != nil {

 		return err

 	}

 	auxJSON := new(json.RawMessage)

 	*auxJSON = auxJSONBytes

-	msgJSON, err := json.Marshal(&jsonmessage.JSONMessage{Aux: auxJSON})

+	msgJSON, err := json.Marshal(&jsonmessage.JSONMessage{ID: id, Aux: auxJSON})

 	if err != nil {

 		return err

 	}

@@ -106,7 +106,7 @@ func TestAuxFormatterEmit(t *testing.T) {

 	sampleAux := &struct {

 		Data string

 	}{"Additional data"}

-	err := aux.Emit(sampleAux)

+	err := aux.Emit("", sampleAux)

 	assert.NilError(t, err)

 	assert.Check(t, is.Equal(`{"aux":{"Data":"Additional data"}}`+streamNewline, b.String()))

 }