diff --git a/solver/llbsolver/provenance/buildconfig.go b/solver/llbsolver/provenance/buildconfig.go
index 8f903585be5c..362273029832 100644
--- a/solver/llbsolver/provenance/buildconfig.go
+++ b/solver/llbsolver/provenance/buildconfig.go
@@ -13,7 +13,7 @@ type BuildConfig struct {
 
 type BuildStep struct {
 	ID            string                  `json:"id,omitempty"`
-	Op            interface{}             `json:"op,omitempty"`
+	Op            pb.Op                   `json:"op,omitempty"`
 	Inputs        []string                `json:"inputs,omitempty"`
 	ResourceUsage *resourcestypes.Samples `json:"resourceUsage,omitempty"`
 }
diff --git a/solver/pb/json.go b/solver/pb/json.go
new file mode 100644
index 000000000000..8460ffc10f31
--- /dev/null
+++ b/solver/pb/json.go
@@ -0,0 +1,96 @@
+package pb
+
+import "encoding/json"
+
+func (m *Op) UnmarshalJSON(data []byte) error {
+	var v struct {
+		Inputs []*Input `json:"inputs,omitempty"`
+		Op     struct {
+			*Op_Exec
+			*Op_Source
+			*Op_File
+			*Op_Build
+			*Op_Merge
+			*Op_Diff
+		}
+		Platform    *Platform          `json:"platform,omitempty"`
+		Constraints *WorkerConstraints `json:"constraints,omitempty"`
+	}
+
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+
+	m.Inputs = v.Inputs
+	switch {
+	case v.Op.Op_Exec != nil:
+		m.Op = v.Op.Op_Exec
+	case v.Op.Op_Source != nil:
+		m.Op = v.Op.Op_Source
+	case v.Op.Op_File != nil:
+		m.Op = v.Op.Op_File
+	case v.Op.Op_Build != nil:
+		m.Op = v.Op.Op_Build
+	case v.Op.Op_Merge != nil:
+		m.Op = v.Op.Op_Merge
+	case v.Op.Op_Diff != nil:
+		m.Op = v.Op.Op_Diff
+	}
+	m.Platform = v.Platform
+	m.Constraints = v.Constraints
+	return nil
+}
+
+func (m *FileAction) UnmarshalJSON(data []byte) error {
+	var v struct {
+		Input          InputIndex  `json:"input"`
+		SecondaryInput InputIndex  `json:"secondaryInput"`
+		Output         OutputIndex `json:"output"`
+		Action         struct {
+			*FileAction_Copy
+			*FileAction_Mkfile
+			*FileAction_Mkdir
+			*FileAction_Rm
+		}
+	}
+
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+
+	m.Input = v.Input
+	m.SecondaryInput = v.SecondaryInput
+	m.Output = v.Output
+	switch {
+	case v.Action.FileAction_Copy != nil:
+		m.Action = v.Action.FileAction_Copy
+	case v.Action.FileAction_Mkfile != nil:
+		m.Action = v.Action.FileAction_Mkfile
+	case v.Action.FileAction_Mkdir != nil:
+		m.Action = v.Action.FileAction_Mkdir
+	case v.Action.FileAction_Rm != nil:
+		m.Action = v.Action.FileAction_Rm
+	}
+	return nil
+}
+
+func (m *UserOpt) UnmarshalJSON(data []byte) error {
+	var v struct {
+		User struct {
+			*UserOpt_ByName
+			*UserOpt_ByID
+		}
+	}
+
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+
+	switch {
+	case v.User.UserOpt_ByName != nil:
+		m.User = v.User.UserOpt_ByName
+	case v.User.UserOpt_ByID != nil:
+		m.User = v.User.UserOpt_ByID
+	}
+	return nil
+}
diff --git a/solver/pb/json_test.go b/solver/pb/json_test.go
new file mode 100644
index 000000000000..79976641a305
--- /dev/null
+++ b/solver/pb/json_test.go
@@ -0,0 +1,214 @@
+package pb
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestOp_UnmarshalJSON(t *testing.T) {
+	for _, tt := range []struct {
+		name string
+		op   *Op
+	}{
+		{
+			name: "exec",
+			op: &Op{
+				Op: &Op_Exec{
+					Exec: &ExecOp{
+						Meta: &Meta{
+							Args: []string{"echo", "Hello", "World"},
+						},
+						Mounts: []*Mount{
+							{Input: 0, Dest: "/", Readonly: true},
+						},
+					},
+				},
+			},
+		},
+		{
+			name: "source",
+			op: &Op{
+				Op: &Op_Source{
+					Source: &SourceOp{
+						Identifier: "local://context",
+					},
+				},
+				Constraints: &WorkerConstraints{},
+			},
+		},
+		{
+			name: "file",
+			op: &Op{
+				Op: &Op_File{
+					File: &FileOp{
+						Actions: []*FileAction{
+							{
+								Input:  1,
+								Output: 2,
+								Action: &FileAction_Copy{
+									Copy: &FileActionCopy{
+										Src:  "/foo",
+										Dest: "/bar",
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+		{
+			name: "build",
+			op: &Op{
+				Op: &Op_Build{
+					Build: &BuildOp{
+						Def: &Definition{},
+					},
+				},
+			},
+		},
+		{
+			name: "merge",
+			op: &Op{
+				Op: &Op_Merge{
+					Merge: &MergeOp{
+						Inputs: []*MergeInput{
+							{Input: 0},
+							{Input: 1},
+						},
+					},
+				},
+			},
+		},
+		{
+			name: "diff",
+			op: &Op{
+				Op: &Op_Diff{
+					Diff: &DiffOp{
+						Lower: &LowerDiffInput{Input: 0},
+						Upper: &UpperDiffInput{Input: 1},
+					},
+				},
+			},
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			out, err := json.Marshal(tt.op)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			exp, got := tt.op, &Op{}
+			if err := json.Unmarshal(out, got); err != nil {
+				t.Fatal(err)
+			}
+			require.Equal(t, exp, got)
+		})
+	}
+}
+
+func TestFileAction_UnmarshalJSON(t *testing.T) {
+	for _, tt := range []struct {
+		name       string
+		fileAction *FileAction
+	}{
+		{
+			name: "copy",
+			fileAction: &FileAction{
+				Action: &FileAction_Copy{
+					Copy: &FileActionCopy{
+						Src:  "/foo",
+						Dest: "/bar",
+					},
+				},
+			},
+		},
+		{
+			name: "mkfile",
+			fileAction: &FileAction{
+				Action: &FileAction_Mkfile{
+					Mkfile: &FileActionMkFile{
+						Path: "/foo",
+						Data: []byte("Hello, World!"),
+					},
+				},
+			},
+		},
+		{
+			name: "mkdir",
+			fileAction: &FileAction{
+				Action: &FileAction_Mkdir{
+					Mkdir: &FileActionMkDir{
+						Path:        "/foo/bar",
+						MakeParents: true,
+					},
+				},
+			},
+		},
+		{
+			name: "rm",
+			fileAction: &FileAction{
+				Action: &FileAction_Rm{
+					Rm: &FileActionRm{
+						Path:          "/foo",
+						AllowNotFound: true,
+					},
+				},
+			},
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			out, err := json.Marshal(tt.fileAction)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			exp, got := tt.fileAction, &FileAction{}
+			if err := json.Unmarshal(out, got); err != nil {
+				t.Fatal(err)
+			}
+			require.Equal(t, exp, got)
+		})
+	}
+}
+
+func TestUserOpt_UnmarshalJSON(t *testing.T) {
+	for _, tt := range []struct {
+		name    string
+		userOpt *UserOpt
+	}{
+		{
+			name: "byName",
+			userOpt: &UserOpt{
+				User: &UserOpt_ByName{
+					ByName: &NamedUserOpt{
+						Name: "foo",
+					},
+				},
+			},
+		},
+		{
+			name: "byId",
+			userOpt: &UserOpt{
+				User: &UserOpt_ByID{
+					ByID: 2,
+				},
+			},
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			out, err := json.Marshal(tt.userOpt)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			exp, got := tt.userOpt, &UserOpt{}
+			if err := json.Unmarshal(out, got); err != nil {
+				t.Fatal(err)
+			}
+			require.Equal(t, exp, got)
+		})
+	}
+}
diff --git a/solver/pb/ops.pb.go b/solver/pb/ops.pb.go
index aadff21b6474..d40a4a0c715b 100644
--- a/solver/pb/ops.pb.go
+++ b/solver/pb/ops.pb.go
@@ -151,6 +151,7 @@ func (CacheSharingOpt) EnumDescriptor() ([]byte, []int) {
 
 // Op represents a vertex of the LLB DAG.
 type Op struct {
+	// changes to this structure must be represented in json.go.
 	// inputs is a set of input edges.
 	Inputs []*Input `protobuf:"bytes,1,rep,name=inputs,proto3" json:"inputs,omitempty"`
 	// Types that are valid to be assigned to Op:
@@ -1961,6 +1962,7 @@ func (m *FileOp) GetActions() []*FileAction {
 }
 
 type FileAction struct {
+	// changes to this structure must be represented in json.go.
 	Input          InputIndex  `protobuf:"varint,1,opt,name=input,proto3,customtype=InputIndex" json:"input"`
 	SecondaryInput InputIndex  `protobuf:"varint,2,opt,name=secondaryInput,proto3,customtype=InputIndex" json:"secondaryInput"`
 	Output         OutputIndex `protobuf:"varint,3,opt,name=output,proto3,customtype=OutputIndex" json:"output"`
@@ -2482,6 +2484,8 @@ func (m *ChownOpt) GetGroup() *UserOpt {
 }
 
 type UserOpt struct {
+	// changes to this structure must be represented in json.go.
+	//
 	// Types that are valid to be assigned to User:
 	//
 	//	*UserOpt_ByName
diff --git a/solver/pb/ops.proto b/solver/pb/ops.proto
index 2f78628f42f9..4e934d85148f 100644
--- a/solver/pb/ops.proto
+++ b/solver/pb/ops.proto
@@ -10,6 +10,7 @@ option (gogoproto.stable_marshaler_all) = true;
 
 // Op represents a vertex of the LLB DAG.
 message Op {
+	// changes to this structure must be represented in json.go.
 	// inputs is a set of input edges.
 	repeated Input inputs = 1;
 	oneof op {
@@ -288,6 +289,7 @@ message FileOp {
 }
 
 message FileAction {
+	// changes to this structure must be represented in json.go.
 	int64 input = 1 [(gogoproto.customtype) = "InputIndex", (gogoproto.nullable) = false]; // could be real input or target (target index + max input index)
 	int64 secondaryInput = 2 [(gogoproto.customtype) = "InputIndex", (gogoproto.nullable) = false]; // --//--
 	int64 output = 3 [(gogoproto.customtype) = "OutputIndex", (gogoproto.nullable) = false];
@@ -373,6 +375,7 @@ message ChownOpt {
 }
 
 message UserOpt {
+	// changes to this structure must be represented in json.go.
 	oneof user {
 		NamedUserOpt byName = 1;
 		uint32 byID = 2;