fix: omit the agent "for" key when no party is named

go.mod

@@ -1,6 +1,6 @@
 module github.com/TransactionAuthorizationProtocol/tap-go
 
-go 1.26.2
+go 1.26.6
 
 require (
 	github.com/google/uuid v1.6.0

types.go

@@ -24,7 +24,7 @@ type Agent struct {
 	ID          string   `json:"@id"`
 	Type        string   `json:"@type,omitempty"`
 	Role        string   `json:"role,omitempty"`
-	For         ForField `json:"for,omitempty"`
+	For         ForField `json:"for,omitzero"`
 	Name        string   `json:"name,omitempty"`
 	NameHash    string   `json:"nameHash,omitempty"`
 	LEICode     string   `json:"lei:leiCode,omitempty"`
@@ -66,6 +66,11 @@ func (f ForField) IsEmpty() bool {
 	return len(f.values) == 0
 }
 
+// IsZero reports whether the ForField has no values; omitzero reads it.
+func (f ForField) IsZero() bool {
+	return len(f.values) == 0
+}
+
 // MarshalJSON marshals the ForField as a string (single value) or array (multiple values).
 func (f ForField) MarshalJSON() ([]byte, error) {
 	if len(f.values) == 0 {
@@ -79,6 +84,13 @@ func (f ForField) MarshalJSON() ([]byte, error) {
 
 // UnmarshalJSON unmarshals the ForField from a string or array of strings.
 func (f *ForField) UnmarshalJSON(data []byte) error {
+	// null into a non-pointer is a no-op returning no error, so the branch
+	// below would take it and produce [""].
+	if string(data) == "null" {
+		f.values = nil
+		return nil
+	}
+
 	// Try single string first
 	var single string
 	if err := json.Unmarshal(data, &single); err == nil {

types_test.go

@@ -224,3 +224,54 @@ func TestInvoice_JSONRoundTrip(t *testing.T) {
 		t.Errorf("LineItems mismatch: got %v", got.LineItems)
 	}
 }
+
+func TestAgent_OwnerlessOmitsForKey(t *testing.T) {
+	data, err := json.Marshal(Agent{ID: "did:pkh:eip155:1:0xabc", Role: "SettlementAddress"})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	if bytes.Contains(data, []byte(`"for"`)) {
+		t.Errorf("ownerless agent must omit the for key, got %s", data)
+	}
+
+	var got Agent
+	if err := json.Unmarshal(data, &got); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if !got.For.IsEmpty() {
+		t.Errorf("round-tripped ownerless agent declares an owner: %v", got.For.Values())
+	}
+}
+
+func TestForField_UnmarshalNull(t *testing.T) {
+	var f ForField
+	if err := json.Unmarshal([]byte("null"), &f); err != nil {
+		t.Fatalf("unmarshal null: %v", err)
+	}
+	if !f.IsEmpty() {
+		t.Errorf("null must decode to no values, got %v", f.Values())
+	}
+	if f.String() != "" {
+		t.Errorf("null must decode to the empty string, got %q", f.String())
+	}
+}
+
+func TestAgent_ForNullDecodesToNoOwner(t *testing.T) {
+	var got Agent
+	if err := json.Unmarshal([]byte(`{"@id":"did:pkh:eip155:1:0xabc","for":null}`), &got); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if !got.For.IsEmpty() {
+		t.Errorf(`"for": null must decode to no owner, got %v`, got.For.Values())
+	}
+}
+
+func TestForField_EmptySliceOmitsKey(t *testing.T) {
+	data, err := json.Marshal(Agent{ID: "did:eg:agent", For: NewForField()})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	if bytes.Contains(data, []byte(`"for"`)) {
+		t.Errorf("empty ForField must omit the for key, got %s", data)
+	}
+}