diff --git a/api/server/backend/build/backend.go b/api/server/backend/build/backend.go
index 546ad5f86d81d..5e04e837a1bde 100644
--- a/api/server/backend/build/backend.go
+++ b/api/server/backend/build/backend.go
@@ -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
 			}
 		}
diff --git a/api/server/router/build/build_routes.go b/api/server/router/build/build_routes.go
index 071402ae70a9b..c4699f3d865aa 100644
--- a/api/server/router/build/build_routes.go
+++ b/api/server/router/build/build_routes.go
@@ -14,7 +14,6 @@ import (
 	"strings"
 	"sync"
 
-	"github.com/containerd/containerd/platforms"
 	"github.com/docker/docker/api/server/httputils"
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/backend"
@@ -24,8 +23,7 @@ import (
 	"github.com/docker/docker/pkg/ioutils"
 	"github.com/docker/docker/pkg/progress"
 	"github.com/docker/docker/pkg/streamformatter"
-	"github.com/docker/docker/pkg/system"
-	"github.com/docker/go-units"
+	units "github.com/docker/go-units"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 )
@@ -72,17 +70,7 @@ func newImageBuildOptions(ctx context.Context, r *http.Request) (*types.ImageBui
 	options.Target = r.FormValue("target")
 	options.RemoteContext = r.FormValue("remote")
 	if versions.GreaterThanOrEqualTo(version, "1.32") {
-		apiPlatform := r.FormValue("platform")
-		if apiPlatform != "" {
-			sp, err := platforms.Parse(apiPlatform)
-			if err != nil {
-				return nil, err
-			}
-			if err := system.ValidatePlatform(sp); err != nil {
-				return nil, err
-			}
-			options.Platform = &sp
-		}
+		options.Platform = r.FormValue("platform")
 	}
 
 	if r.Form.Get("shmsize") != "" {
@@ -243,6 +231,10 @@ 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 +247,6 @@ 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{
diff --git a/api/types/client.go b/api/types/client.go
index 33bc98e0bb401..3b698c2c240d0 100644
--- a/api/types/client.go
+++ b/api/types/client.go
@@ -7,8 +7,7 @@ import (
 
 	"github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/api/types/filters"
-	"github.com/docker/go-units"
-	specs "github.com/opencontainers/image-spec/specs-go/v1"
+	units "github.com/docker/go-units"
 )
 
 // CheckpointCreateOptions holds parameters to create a checkpoint from a container
@@ -181,7 +180,7 @@ type ImageBuildOptions struct {
 	ExtraHosts  []string // List of extra hosts
 	Target      string
 	SessionID   string
-	Platform    *specs.Platform
+	Platform    string
 	// Version specifies the version of the unerlying builder to use
 	Version BuilderVersion
 	// BuildID is an optional identifier that can be passed together with the
diff --git a/builder/builder-next/builder.go b/builder/builder-next/builder.go
index 5a82cddf444ea..b1d31a5225635 100644
--- a/builder/builder-next/builder.go
+++ b/builder/builder-next/builder.go
@@ -2,7 +2,6 @@ package buildkit
 
 import (
 	"context"
-	"encoding/json"
 	"io"
 	"strings"
 	"sync"
@@ -14,7 +13,8 @@ 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"
+	"github.com/docker/docker/pkg/system"
 	controlapi "github.com/moby/buildkit/api/services/control"
 	"github.com/moby/buildkit/control"
 	"github.com/moby/buildkit/identity"
@@ -209,8 +209,17 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.
 		frontendAttrs["no-cache"] = ""
 	}
 
-	if opt.Options.Platform != nil {
-		frontendAttrs["platform"] = platforms.Format(*opt.Options.Platform)
+	if opt.Options.Platform != "" {
+		// same as in newBuilder in builder/dockerfile.builder.go
+		// TODO: remove once opt.Options.Platform is of type specs.Platform
+		sp, err := platforms.Parse(opt.Options.Platform)
+		if err != nil {
+			return nil, err
+		}
+		if err := system.ValidatePlatform(sp); err != nil {
+			return nil, err
+		}
+		frontendAttrs["platform"] = opt.Options.Platform
 	}
 
 	exporterAttrs := map[string]string{}
@@ -228,6 +237,8 @@ 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 +251,7 @@ 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 +269,9 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.
 			if err != nil {
 				return err
 			}
-
-			auxJSONBytes, err := json.Marshal(dt)
-			if err != nil {
-				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 {
+			if err := aux.Emit("moby.buildkit.trace", dt); err != nil {
 				return err
 			}
-			if n != len(msgJSON) {
-				return io.ErrShortWrite
-			}
 		}
 		return nil
 	})
diff --git a/builder/dockerfile/builder.go b/builder/dockerfile/builder.go
index b58534707916d..ee95f236892fe 100644
--- a/builder/dockerfile/builder.go
+++ b/builder/dockerfile/builder.go
@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/containerd/containerd/platforms"
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/backend"
 	"github.com/docker/docker/api/types/container"
@@ -25,6 +26,7 @@ import (
 	"github.com/moby/buildkit/frontend/dockerfile/parser"
 	"github.com/moby/buildkit/frontend/dockerfile/shell"
 	"github.com/moby/buildkit/session"
+	specs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sync/syncmap"
@@ -111,7 +113,11 @@ func (bm *BuildManager) Build(ctx context.Context, config backend.BuildConfig) (
 		PathCache:      bm.pathCache,
 		IDMappings:     bm.idMappings,
 	}
-	return newBuilder(ctx, builderOptions).build(source, dockerfile)
+	b, err := newBuilder(ctx, builderOptions)
+	if err != nil {
+		return nil, err
+	}
+	return b.build(source, dockerfile)
 }
 
 func (bm *BuildManager) initializeClientSession(ctx context.Context, cancel func(), options *types.ImageBuildOptions) (builder.Source, error) {
@@ -175,10 +181,11 @@ type Builder struct {
 	pathCache        pathCache
 	containerManager *containerManager
 	imageProber      ImageProber
+	platform         *specs.Platform
 }
 
 // newBuilder creates a new Dockerfile builder from an optional dockerfile and a Options.
-func newBuilder(clientCtx context.Context, options builderOptions) *Builder {
+func newBuilder(clientCtx context.Context, options builderOptions) (*Builder, error) {
 	config := options.Options
 	if config == nil {
 		config = new(types.ImageBuildOptions)
@@ -199,7 +206,20 @@ func newBuilder(clientCtx context.Context, options builderOptions) *Builder {
 		containerManager: newContainerManager(options.Backend),
 	}
 
-	return b
+	// same as in Builder.Build in builder/builder-next/builder.go
+	// TODO: remove once config.Platform is of type specs.Platform
+	if config.Platform != "" {
+		sp, err := platforms.Parse(config.Platform)
+		if err != nil {
+			return nil, err
+		}
+		if err := system.ValidatePlatform(sp); err != nil {
+			return nil, err
+		}
+		b.platform = &sp
+	}
+
+	return b, nil
 }
 
 // Build 'LABEL' command(s) from '--label' options and add to the last stage
@@ -257,7 +277,7 @@ 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 {
@@ -365,9 +385,12 @@ func BuildFromConfig(config *container.Config, changes []string, os string) (*co
 		return nil, errdefs.InvalidParameter(err)
 	}
 
-	b := newBuilder(context.Background(), builderOptions{
+	b, err := newBuilder(context.Background(), builderOptions{
 		Options: &types.ImageBuildOptions{NoCache: true},
 	})
+	if err != nil {
+		return nil, err
+	}
 
 	// ensure that the commands are valid
 	for _, n := range dockerfile.AST.Children {
diff --git a/builder/dockerfile/copy.go b/builder/dockerfile/copy.go
index 7e9dc6036a460..74e245bdc40b9 100644
--- a/builder/dockerfile/copy.go
+++ b/builder/dockerfile/copy.go
@@ -87,7 +87,7 @@ func copierFromDispatchRequest(req dispatchRequest, download sourceDownloader, i
 		pathCache:   req.builder.pathCache,
 		download:    download,
 		imageSource: imageSource,
-		platform:    req.builder.options.Platform,
+		platform:    req.builder.platform,
 	}
 }
 
diff --git a/builder/dockerfile/dispatchers.go b/builder/dockerfile/dispatchers.go
index 1032c6cdada89..5d72237cdb429 100644
--- a/builder/dockerfile/dispatchers.go
+++ b/builder/dockerfile/dispatchers.go
@@ -146,7 +146,7 @@ func (d *dispatchRequest) getImageMount(imageRefOrID string) (*imageMount, error
 		imageRefOrID = stage.Image
 		localOnly = true
 	}
-	return d.builder.imageSources.Get(imageRefOrID, localOnly, d.builder.options.Platform)
+	return d.builder.imageSources.Get(imageRefOrID, localOnly, d.builder.platform)
 }
 
 // FROM [--platform=platform] imagename[:tag | @digest] [AS build-stage-name]
@@ -238,7 +238,7 @@ func (d *dispatchRequest) getImageOrStage(name string, platform *specs.Platform)
 	}
 
 	if platform == nil {
-		platform = d.builder.options.Platform
+		platform = d.builder.platform
 	}
 
 	// Windows cannot support a container with no base image unless it is LCOW.
diff --git a/builder/dockerfile/dispatchers_test.go b/builder/dockerfile/dispatchers_test.go
index 047a8742e8300..c61a45b03a1d6 100644
--- a/builder/dockerfile/dispatchers_test.go
+++ b/builder/dockerfile/dispatchers_test.go
@@ -6,7 +6,6 @@ import (
 	"runtime"
 	"testing"
 
-	"github.com/containerd/containerd/platforms"
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/backend"
 	"github.com/docker/docker/api/types/container"
@@ -23,8 +22,7 @@ import (
 
 func newBuilderWithMockBackend() *Builder {
 	mockBackend := &MockBackend{}
-	defaultPlatform := platforms.DefaultSpec()
-	opts := &types.ImageBuildOptions{Platform: &defaultPlatform}
+	opts := &types.ImageBuildOptions{}
 	ctx := context.Background()
 	b := &Builder{
 		options:       opts,
@@ -116,7 +114,7 @@ func TestFromScratch(t *testing.T) {
 	err := initializeStage(sb, cmd)
 
 	if runtime.GOOS == "windows" && !system.LCOWSupported() {
-		assert.Check(t, is.Error(err, "Windows does not support FROM scratch"))
+		assert.Check(t, is.Error(err, "Linux containers are not supported on this system"))
 		return
 	}
 
diff --git a/builder/dockerfile/internals.go b/builder/dockerfile/internals.go
index 5e2c286d75453..1b3a5b0f03ac2 100644
--- a/builder/dockerfile/internals.go
+++ b/builder/dockerfile/internals.go
@@ -169,7 +169,7 @@ func (b *Builder) performCopy(req dispatchRequest, inst copyInstruction) error {
 		return err
 	}
 
-	imageMount, err := b.imageSources.Get(state.imageID, true, req.builder.options.Platform)
+	imageMount, err := b.imageSources.Get(state.imageID, true, req.builder.platform)
 	if err != nil {
 		return errors.Wrapf(err, "failed to get destination image %q", state.imageID)
 	}
@@ -416,7 +416,9 @@ func (b *Builder) probeAndCreate(dispatchState *dispatchState, runConfig *contai
 
 func (b *Builder) create(runConfig *container.Config) (string, error) {
 	logrus.Debugf("[BUILDER] Command to be executed: %v", runConfig.Cmd)
-	hostConfig := hostConfigFromOptions(b.options)
+
+	isWCOW := runtime.GOOS == "windows" && b.platform != nil && b.platform.OS == "windows"
+	hostConfig := hostConfigFromOptions(b.options, isWCOW)
 	container, err := b.containerManager.Create(runConfig, hostConfig)
 	if err != nil {
 		return "", err
@@ -429,7 +431,7 @@ func (b *Builder) create(runConfig *container.Config) (string, error) {
 	return container.ID, nil
 }
 
-func hostConfigFromOptions(options *types.ImageBuildOptions) *container.HostConfig {
+func hostConfigFromOptions(options *types.ImageBuildOptions, isWCOW bool) *container.HostConfig {
 	resources := container.Resources{
 		CgroupParent: options.CgroupParent,
 		CPUShares:    options.CPUShares,
@@ -457,7 +459,7 @@ func hostConfigFromOptions(options *types.ImageBuildOptions) *container.HostConf
 	// is too small for builder scenarios where many users are
 	// using RUN statements to install large amounts of data.
 	// Use 127GB as that's the default size of a VHD in Hyper-V.
-	if runtime.GOOS == "windows" && options.Platform != nil && options.Platform.OS == "windows" {
+	if isWCOW {
 		hc.StorageOpt = make(map[string]string)
 		hc.StorageOpt["size"] = "127GB"
 	}
diff --git a/client/image_build.go b/client/image_build.go
index e5013176a2f96..9add3c10b348b 100644
--- a/client/image_build.go
+++ b/client/image_build.go
@@ -8,8 +8,8 @@ import (
 	"net/http"
 	"net/url"
 	"strconv"
+	"strings"
 
-	"github.com/containerd/containerd/platforms"
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/container"
 )
@@ -30,12 +30,6 @@ func (cli *Client) ImageBuild(ctx context.Context, buildContext io.Reader, optio
 	}
 	headers.Add("X-Registry-Config", base64.URLEncoding.EncodeToString(buf))
 
-	if options.Platform != nil {
-		if err := cli.NewVersionError("1.32", "platform"); err != nil {
-			return types.ImageBuildResponse{}, err
-		}
-		query.Set("platform", platforms.Format(*options.Platform))
-	}
 	headers.Set("Content-Type", "application/x-tar")
 
 	serverResp, err := cli.postRaw(ctx, "/build", query, buildContext, headers)
@@ -130,8 +124,11 @@ func (cli *Client) imageBuildOptionsToQuery(options types.ImageBuildOptions) (ur
 	if options.SessionID != "" {
 		query.Set("session", options.SessionID)
 	}
-	if options.Platform != nil {
-		query.Set("platform", platforms.Format(*options.Platform))
+	if options.Platform != "" {
+		if err := cli.NewVersionError("1.32", "platform"); err != nil {
+			return query, err
+		}
+		query.Set("platform", strings.ToLower(options.Platform))
 	}
 	if options.BuildID != "" {
 		query.Set("buildid", options.BuildID)
diff --git a/pkg/streamformatter/streamformatter.go b/pkg/streamformatter/streamformatter.go
index 2b5e713040eb4..04917d49ab29d 100644
--- a/pkg/streamformatter/streamformatter.go
+++ b/pkg/streamformatter/streamformatter.go
@@ -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
 	}
diff --git a/pkg/streamformatter/streamformatter_test.go b/pkg/streamformatter/streamformatter_test.go
index 4399a6509b444..f630699d73e11 100644
--- a/pkg/streamformatter/streamformatter_test.go
+++ b/pkg/streamformatter/streamformatter_test.go
@@ -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()))
 }