WI-003: #18 ProblemDetails has custom MarshalJSON but no UnmarshalJSON — problem responses can't round-trip

problem_details.go

@@ -47,6 +47,42 @@ func (p ProblemDetails) MarshalJSON() ([]byte, error) {
 	return json.Marshal(payload)
 }
 
+// UnmarshalJSON reads the standard RFC 9457 members and collects every other
+// top-level member into Extensions, mirroring MarshalJSON.
+func (p *ProblemDetails) UnmarshalJSON(data []byte) error {
+	var known struct {
+		Type     string `json:"type"`
+		Title    string `json:"title"`
+		Status   int    `json:"status"`
+		Detail   string `json:"detail"`
+		Instance string `json:"instance"`
+	}
+	if err := json.Unmarshal(data, &known); err != nil {
+		return err
+	}
+
+	var members map[string]any
+	if err := json.Unmarshal(data, &members); err != nil {
+		return err
+	}
+	for _, key := range []string{"type", "title", "status", "detail", "instance"} {
+		delete(members, key)
+	}
+
+	decoded := ProblemDetails{
+		Type:     known.Type,
+		Title:    known.Title,
+		Status:   known.Status,
+		Detail:   known.Detail,
+		Instance: known.Instance,
+	}
+	if len(members) > 0 {
+		decoded.Extensions = members
+	}
+	*p = decoded
+	return nil
+}
+
 // NewProblemDetails creates a ProblemDetails instance with standard fields.
 func NewProblemDetails(status int, problemType, title, detail string) *ProblemDetails {
 	if status < 100 || status > 599 {

problem_details_test.go

@@ -2,6 +2,7 @@ package httpsuite
 
 import (
 	"encoding/json"
+	"reflect"
 	"testing"
 )
 
@@ -20,6 +21,60 @@ func TestNewProblemDetailsDefaults(t *testing.T) {
 	}
 }
 
+func TestProblemDetailsJSONRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	original := ProblemDetails{
+		Type:     "https://example.com/errors/bad",
+		Title:    "Bad Request",
+		Status:   400,
+		Detail:   "broken",
+		Instance: "/things/1",
+		Extensions: map[string]interface{}{
+			"messageKey":    "errors.bad",
+			"correlationId": "abc-123",
+		},
+	}
+
+	body, err := json.Marshal(original)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+
+	var decoded ProblemDetails
+	if err := json.Unmarshal(body, &decoded); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+
+	if !reflect.DeepEqual(original, decoded) {
+		t.Fatalf("round trip mismatch:\nwant %#v\ngot  %#v", original, decoded)
+	}
+}
+
+func TestProblemDetailsUnmarshalJSONNoExtensions(t *testing.T) {
+	t.Parallel()
+
+	decoded := ProblemDetails{Extensions: map[string]interface{}{"stale": true}}
+	if err := json.Unmarshal([]byte(`{"type":"about:blank","title":"T","status":404}`), &decoded); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if decoded.Extensions != nil {
+		t.Fatalf("expected nil extensions, got %#v", decoded.Extensions)
+	}
+	if decoded.Status != 404 || decoded.Title != "T" {
+		t.Fatalf("unexpected decode: %#v", decoded)
+	}
+}
+
+func TestProblemDetailsUnmarshalJSONInvalid(t *testing.T) {
+	t.Parallel()
+
+	var decoded ProblemDetails
+	if err := json.Unmarshal([]byte(`{"status":"nope"}`), &decoded); err == nil {
+		t.Fatal("expected error for wrong status type")
+	}
+}
+
 func TestProblemDetailsMarshalJSONFlattensExtensions(t *testing.T) {
 	t.Parallel()