@@ -17,11 +17,11 @@ func (cli *Client) ImageHistory(ctx context.Context, imageID string, opts image.

 			return nil, err

 		}

-		p, err := json.Marshal(*opts.Platform)

+		p, err := encodePlatform(opts.Platform)

 		if err != nil {

 			return nil, fmt.Errorf("invalid platform: %v", err)

 		}

-		query.Set("platform", string(p))

+		query.Set("platform", p)

 	}

 	var history []image.HistoryResponseItem

@@ -2,7 +2,6 @@ package client // import "github.com/docker/docker/client"

 import (

 	"context"

-	"encoding/json"

 	"io"

 	"net/http"

 	"net/url"

@@ -28,11 +27,11 @@ func (cli *Client) ImageLoad(ctx context.Context, input io.Reader, opts image.Lo

 			return image.LoadResponse{}, err

 		}

-		p, err := json.Marshal(*opts.Platform)

+		p, err := encodePlatform(opts.Platform)

 		if err != nil {

 			return image.LoadResponse{}, err

 		}

-		query.Set("platform", string(p))

+		query.Set("platform", p)

 	}

 	resp, err := cli.postRaw(ctx, "/images/load", query, input, http.Header{

@@ -2,8 +2,6 @@ package client // import "github.com/docker/docker/client"

 import (

 	"context"

-	"encoding/json"

-	"fmt"

 	"io"

 	"net/url"

@@ -22,11 +20,11 @@ func (cli *Client) ImageSave(ctx context.Context, imageIDs []string, opts image.

 			return nil, err

 		}

-		p, err := json.Marshal(*opts.Platform)

+		p, err := encodePlatform(opts.Platform)

 		if err != nil {

-			return nil, fmt.Errorf("invalid platform: %v", err)

+			return nil, err

 		}

-		query.Set("platform", string(p))

+		query.Set("platform", p)

 	}

 	resp, err := cli.get(ctx, "/images/get", query, nil)

@@ -1,10 +1,14 @@

 package client // import "github.com/docker/docker/client"

 import (

+	"encoding/json"

+	"fmt"

 	"net/url"

 	"regexp"

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

+	"github.com/docker/docker/errdefs"

+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"

 )

 var headerRegexp = regexp.MustCompile(`\ADocker/.+\s\((.+)\)\z`)

@@ -32,3 +36,43 @@ func getFiltersQuery(f filters.Args) (url.Values, error) {

 	}

 	return query, nil

 }

+

+// encodePlatforms marshals the given platform(s) to JSON format, to

+// be used for query-parameters for filtering / selecting platforms.

+func encodePlatforms(platform ...ocispec.Platform) ([]string, error) {

+	if len(platform) == 0 {

+		return []string{}, nil

+	}

+	if len(platform) == 1 {

+		p, err := encodePlatform(&platform[0])

+		if err != nil {

+			return nil, err

+		}

+		return []string{p}, nil

+	}

+

+	seen := make(map[string]struct{}, len(platform))

+	out := make([]string, 0, len(platform))

+	for i := range platform {

+		p, err := encodePlatform(&platform[i])

+		if err != nil {

+			return nil, err

+		}

+		if _, ok := seen[p]; !ok {

+			out = append(out, p)

+			seen[p] = struct{}{}

+		}

+	}

+	return out, nil

+}

+

+// encodePlatforms marshals the given platform to JSON format, to

+// be used for query-parameters for filtering / selecting platforms. It

+// is used as a helper for encodePlatforms,

+func encodePlatform(platform *ocispec.Platform) (string, error) {

+	p, err := json.Marshal(platform)

+	if err != nil {

+		return "", errdefs.InvalidParameter(fmt.Errorf("invalid platform: %v", err))

+	}

+	return string(p), nil

+}

new file mode 100644

@@ -0,0 +1,56 @@

+package client

+

+import (

+	"testing"

+

+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"

+	"gotest.tools/v3/assert"

+)

+

+func TestEncodePlatforms(t *testing.T) {

+	tests := []struct {

+		doc       string

+		platforms []ocispec.Platform

+		expected  []string

+	}{

+		{

+			doc: "single platform",

+			platforms: []ocispec.Platform{

+				{Architecture: "arm64", OS: "windows", Variant: "v8", OSVersion: "99.99.99"},

+			},

+			expected: []string{

+				`{"architecture":"arm64","os":"windows","os.version":"99.99.99","variant":"v8"}`,

+			},

+		},

+		{

+			doc: "multiple platforms",

+			platforms: []ocispec.Platform{

+				{Architecture: "arm64", OS: "linux", Variant: "v8"},

+				{Architecture: "arm64", OS: "windows", Variant: "v8", OSVersion: "99.99.99"},

+			},

+			expected: []string{

+				`{"architecture":"arm64","os":"linux","variant":"v8"}`,

+				`{"architecture":"arm64","os":"windows","os.version":"99.99.99","variant":"v8"}`,

+			},

+		},

+		{

+			doc: "multiple platforms with duplicates",

+			platforms: []ocispec.Platform{

+				{Architecture: "arm64", OS: "linux", Variant: "v8"},

+				{Architecture: "arm64", OS: "windows", Variant: "v8", OSVersion: "99.99.99"},

+				{Architecture: "arm64", OS: "linux", Variant: "v8"},

+			},

+			expected: []string{

+				`{"architecture":"arm64","os":"linux","variant":"v8"}`,

+				`{"architecture":"arm64","os":"windows","os.version":"99.99.99","variant":"v8"}`,

+			},

+		},

+	}

+	for _, tc := range tests {

+		t.Run(tc.doc, func(t *testing.T) {

+			out, err := encodePlatforms(tc.platforms...)

+			assert.NilError(t, err)

+			assert.DeepEqual(t, out, tc.expected)

+		})

+	}

+}