diff --git a/load.go b/load.go
index 2be576713..0d6fc9148 100644
--- a/load.go
+++ b/load.go
@@ -210,36 +210,80 @@ func (s *Spec) SubstituteArgs(env map[string]string, opts ...SubstituteOpt) erro
 func LoadSpec(dt []byte) (*Spec, error) {
 	var spec Spec
 
-	dt, err := stripXFields(dt)
-	if err != nil {
-		return nil, fmt.Errorf("error stripping x-fields: %w", err)
-	}
-
 	if err := yaml.UnmarshalWithOptions(dt, &spec, yaml.Strict()); err != nil {
-		return nil, fmt.Errorf("error unmarshalling spec: %w", err)
+		return nil, errors.Wrap(err, "error unmarshalling spec")
 	}
 
 	if err := spec.Validate(); err != nil {
 		return nil, err
 	}
-	spec.FillDefaults()
 
+	spec.FillDefaults()
 	return &spec, nil
 }
 
-func stripXFields(dt []byte) ([]byte, error) {
+func (s *Spec) UnmarshalYAML(f func(interface{}) error) error {
 	var obj map[string]interface{}
-	if err := yaml.Unmarshal(dt, &obj); err != nil {
-		return nil, fmt.Errorf("error unmarshalling spec: %w", err)
+
+	if err := f(&obj); err != nil {
+		return err
+	}
+
+	var ext map[string]interface{}
+
+	addExt := func(k string, v interface{}) {
+		if ext == nil {
+			ext = make(map[string]interface{})
+		}
+		ext[k] = v
+	}
+	for k, v := range obj {
+		if !strings.HasPrefix(k, "x-") && !strings.HasPrefix(k, "X-") {
+			continue
+		}
+		addExt(strings.TrimPrefix(k, "x-"), v)
+		delete(obj, k)
+	}
+
+	type internalSpec Spec
+	var s2 internalSpec
+
+	dt, err := yaml.Marshal(obj)
+	if err != nil {
+		return err
+	}
+
+	if err := yaml.UnmarshalWithOptions(dt, &s2, yaml.Strict()); err != nil {
+		return err
+	}
+
+	*s = Spec(s2)
+	s.Ext = ext
+	return nil
+}
+
+func (s Spec) MarshalYAML() ([]byte, error) {
+	// We need to define a new type to avoid infinite recursion of MarshalYAML.
+	type internalSpec Spec
+
+	type specExt struct {
+		internalSpec `yaml:",inline"`
+		Ext          map[string]interface{} `yaml:",omitempty,inline"`
 	}
 
-	for k := range obj {
-		if strings.HasPrefix(k, "x-") || strings.HasPrefix(k, "X-") {
-			delete(obj, k)
+	var ext map[string]interface{}
+	if len(s.Ext) > 0 {
+		ext = maps.Clone(s.Ext)
+		for k, v := range ext {
+			delete(ext, k)
+			ext["x-"+k] = v
 		}
 	}
 
-	return yaml.Marshal(obj)
+	return yaml.Marshal(specExt{
+		internalSpec: internalSpec(s),
+		Ext:          ext,
+	})
 }
 
 func (s *BuildStep) processBuildArgs(lex *shell.Lex, args map[string]string, allowArg func(string) bool) error {
diff --git a/load_test.go b/load_test.go
index 4b8104586..d29fa0830 100644
--- a/load_test.go
+++ b/load_test.go
@@ -10,6 +10,7 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/goccy/go-yaml"
 	"github.com/moby/buildkit/frontend/dockerui"
 	"gotest.tools/v3/assert"
 	"gotest.tools/v3/assert/cmp"
@@ -1160,3 +1161,38 @@ func TestImage_validate(t *testing.T) {
 		})
 	}
 }
+
+func TestExtensionFieldMarshalUnmarshal(t *testing.T) {
+	dt := []byte(`
+name: test
+x-hello: world
+x-foo:
+- bar
+- baz
+`)
+
+	var spec Spec
+	err := yaml.Unmarshal(dt, &spec)
+	assert.NilError(t, err)
+
+	assert.Check(t, cmp.Equal(spec.Name, "test"), spec)
+	assert.Assert(t, spec.Ext != nil)
+	assert.Check(t, spec.Ext["hello"] != nil, "%+v", spec)
+	assert.Check(t, cmp.Equal(spec.Ext["hello"].(string), "world"), "%+v", spec.Ext)
+	assert.Check(t, cmp.DeepEqual(spec.Ext["foo"].([]interface{}), []interface{}{"bar", "baz"}), "%+v", spec.Ext)
+
+	// marshal and unmarshal to ensure the extension fields are preserved
+
+	dt, err = yaml.Marshal(spec)
+	assert.NilError(t, err)
+
+	var spec2 Spec
+	err = yaml.Unmarshal(dt, &spec2)
+	assert.NilError(t, err)
+
+	assert.Check(t, cmp.Equal(spec2.Name, "test"), spec2)
+	assert.Assert(t, spec2.Ext != nil, string(dt))
+	assert.Check(t, spec2.Ext["hello"] != nil, "%+v", spec2.Ext)
+	assert.Check(t, cmp.Equal(spec2.Ext["hello"].(string), "world"), "%+v", spec2.Ext)
+	assert.Check(t, cmp.DeepEqual(spec2.Ext["foo"].([]interface{}), []interface{}{"bar", "baz"}), "%+v", spec2.Ext)
+}
diff --git a/spec.go b/spec.go
index da4251fca..7f90d8763 100644
--- a/spec.go
+++ b/spec.go
@@ -94,6 +94,8 @@ type Spec struct {
 	// Each item in this list is run with a separate rootfs and cannot interact with other tests.
 	// Each [TestSpec] is run with a separate rootfs, asynchronously from other [TestSpec].
 	Tests []*TestSpec `yaml:"tests,omitempty" json:"tests,omitempty"`
+
+	Ext map[string]interface{} `yaml:"-" json:"-"`
 }
 
 // PatchSpec is used to apply a patch to a source with a given set of options.